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The effect of water motion on short-term rates of photosynthesis by marine phytoplankton
trends in plant science Reviews
The effect of water motion on short-term rates of photosynthesis by marine phytoplankton Hugh L. MacIntyre, Todd M. Kana and Richard J. Geider Phytoplankton respond to variations in light intensity as they are mixed through the water column. Changes in pigment content are characteristic of the relatively slow response of ‘sun–shade’ photoacclimation that occurs on timescales typical of mixing in the open ocean. In estuaries, the variations are much faster and induce correspondingly rapid changes in the activity (rather than abundance) of different components of the photosynthetic apparatus. These components modulate light harvesting and Calvin cycle activity, or protect the pigment bed from excess energy absorption. When the protective capacity is exceeded, photoinhibition occurs. All these mechanisms modulate the rate of photosynthesis in situ.
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lanktonic microalgae (phytoplankton) live in diverse and highly variable environments. Their photosynthetic apparatus is subject to significant stresses because of rapid changes or imbalances in irradiance and nutrient supply imposed by the physics and chemistry of natural water bodies. The ways in which the photosynthetic apparatus adjusts to these temporally variable and complex environments are of interest for both practical and fundamental reasons, because phytoplankton photosynthesis is responsible for ~50% of global productivity. Modern techniques for assessing productivity on global scales rely on remotely sensing plankton’s optical properties, using the signature of chlorophyll as an index of abundance (Fig. 1). The emphasis on chlorophyll is inevitable given chlorophyll’s distinctive optical characteristics, which enables plant material to be distinguished from other suspended matter. The relationship between chlorophyll and organic carbon (the desired currency for productivity models) is highly plastic, varying with growth, irradiance, nutrient availability and temperature. However, the variability is ordered, not random, and knowledge about the effects of environmental variables on the efficiency of light absorption and its conversion to biomass during photosynthesis allows estimates of chlorophyll abundance to be translated into carbon equivalents.
Understanding the effect of varying environmental conditions on photosynthetic rates can be considered in terms of the regulation of the amounts and the specific activities of components of the photosynthetic apparatus. Regulation can be accomplished by variations in the relative abundance of the constituents [e.g. chlorophyll and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)] or, on a shorter timescale, by varying the efficiency of their coupling and activity (e.g. the activation state of Rubisco). The distinction is blurred at some levels but serves to distinguish between those means of photosynthetic regulation that depend on synthesis or turnover (usually of chlorophyll) in photoacclimation, and those that depend on more rapid changes in activation states or efficiencies, independent of turnover. Work in this area is complicated by the fact that phytoplankton include representatives of four kingdoms1 and are highly variable in their molecular, structural and optical properties. Whereas physiological studies have focused largely on taxa that are easy to maintain in culture, particularly chlorophytes and diatoms. It has been challenging to bring qualitative information on photosynthetic mechanisms together with quantitative kinetic information on rates of response to understand cellular photosynthesis in the natural environment. Here we review recent developments in our understanding of how phytoplankton photosynthesis adjusts to naturally variable environments. The principal environmental factors that affect phytoplankton photosynthesis are light, nutrient availability and temperature. In nature, these factors operate independently on timescales that match photosynthetic physiology, presenting a complex and unpredictable environment to the cells. Phytoplankton respond to such stochastic environments using a variety of physiological processes that affect light Fig. 1. Composite false-color image of mean annual ocean chlorophyll concentrations, as detected by satellite remote-sensing. Chlorophyll is used as an index of biomass in seawater because, unlike other organic compounds, harvesting efficiency and it is unique to phytoplankton and common to all autotrophs. Note the higher concentrations near the land photosynthetic capacity. Two key issues are at the forefront of ecophysiological research:
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trends in plant science Reviews • What are the kinetic constraints on processes that modulate photosynthesis in rapidly fluctuating environments? • What are the mechanisms by which cells integrate multiple environmental factors in regulating their physiological responses? These questions are related: the photosynthetic mechanisms that modulate energy and material flow through the cell are common to phytoplankton in spite of their diversity. Significant conceptual advances have been made recently that tie whole-cell physiology with photosynthetic regulatory processes operating at the biochemical and biophysical levels. Environmental control of photosynthetic processes
By definition, phytoplankton are incapable of sustained directional movement and therefore are subject to the environmental conditions in their parent body of water. Photosynthesis is responsive to changes in nutrient availability on short timescales2 but in marine systems the changes are more likely to occur over relatively long timescales (days to seasons). Although nutrient availability is critical for determining population dynamics3 and the ultimate determinant of the geographic abundance of phytoplankton (Fig. 1), nutrient availability is less likely to drive short-term changes in photosynthetic rates. Because of water’s capacity for high levels of latent heat, temperature is more likely to vary on daily to seasonal scales, and in a manner predictable from the balance of radiative transfer and evaporative cooling. By contrast, phytoplankton are subject to relatively rapid changes in both the intensity and the spectral quality of light as they move vertically in a water column (Box 1). In addition to the changes imposed by mixing, there are variations in the light field with timescales of hours, minutes and milliseconds4 as a result of changes in solar elevation, cloud cover and subsurface focusing by waves. Photosynthesis and growth rates appear to be insensitive to variability in the millisecond domain5, therefore we will focus on variations that occur on a timescale of minutes and hours. Photoacclimation of pigment content
conditions, plastoquinone is largely oxidized, and transcription of the mRNAs that code for the proteins that bind chlorophyll in the antenna occurs at maximal rates. As irradiance increases, photosynthesis control passes from light harvesting to the maximum capacity for carbon dioxide fixation, the plastoquinone becomes increasingly reduced, and transcription of the mRNAs for pigment–protein complexes declines. The end results of this mode of regulation can be mimicked by ‘energy balance’ models developed recently10,11. These are based on the concept of a physiological light sensor that regulates the pigment quota to maintain a balance between the harvesting of excitation energy by means of light absorption and photochemistry on the one hand and the energetic demands of growth on the other. Conceptually, this is comparable to the redox regulation of the antenna protein LHCII (Refs 9,12). These models can account for steady-state responses and for the differences in the rates of acclimation that are observed following shifts from low-to-high versus high-to-low irradiance, without the potential errors imposed by specifying a mathematical function to describe the kinetics of photoacclimation13. Photosynthetic Induction
The time-course of light intensity changes in estuarine waters is much faster than in coastal or open-ocean waters because both the rate of light attenuation and the rate of mixing are much higher (Box 1). Two mechanisms, the activation and deactivation of Rubisco and state transitions, have time constants that are comparable to the timescale of mixing in estuarine waters and probably dominate short-term rates of photosynthesis (Fig. 2). Photosynthetic regulation operating by means of the catalytic activity of Rubisco can be controlled by variations in the enzyme concentration or, in the short-term, by its activation state. Although the enzyme cellular concentration might not be regulated during photoacclimation under nutrient-replete conditions, depending on the taxon8, it is regulated in response to chronic phosphorus or nitrogen limitation14. Because there is a correlation between the maximum quantum efficiency (fm) and the ratio of Rubisco to the PSII reaction center protein D1, changes in the pool size of Rubisco might play a role in regulating acclimated photosynthetic rates during nutrient-limited growth. A potential role for activation and deactivation of Rubisco in the short-term limitation of photosynthetic rates hinges on the assumption that at light saturation, the rate of photosynthesis depends on the enzyme’s activity. Broad conclusions are complicated by the diversity in microalgal Rubisco structure and
For a given temperature and nutrient status, photoacclimation in phytoplankton can be described in terms of regulation of the cell concentrations of the catalysts that determine light-limited and light-saturated photosynthetic rates. The rate of light absorption, which co-varies with a cell’s chlorophyll concentration, is often the primary determinant of light-limited photosynthesis, whereas the maximum rate of carbon dioxide fixation, which co-varies with Rubisco concentration, is the primary determinant of lightsaturated photosynthesis. The cell chlorophyll content is usually higher in cells that have Box 1. Variations in the submarine light field grown under low light6. Variations in other constituents of The light attenuation within any body of water follows (approximately) the Lambert–Beer law, decreasing expothe photosynthetic apparatus nentially with depth. The rate and spectral dependence of attenuation depends on the abundance of phytoplankton, appear to be taxon-specific: of detritus (which includes organic material of biogenic origin and suspended sediment) and chromophoric two studies of cells grown dissolved organic matter (CDOM, mainly humic and fulvic acids of terrestrial origin). under nutrient-replete condiIn estuarine waters, terrestrial runoff that is high in humics and plant nutrients, and close coupling between the benthos and water column results in high absorption by CDOM, phytoplankton and detritus in blue light. The domtions, the concentration of inant penetrating wavelengths are therefore red and the intermediate green (Fig. 3). Attenuation is rapid: in an Rubisco decreased at low extreme case, the euphotic zone (the depth to which 1% of sunlight penetrates) is ,0.5 m. growth irradiance in a diatom7 In oceanic waters, the abundance of CDOM, detritus and phytoplankton is low and the dominant attenuator is whereas it remained unchanged water itself, which is highly absorbent in red light. The dominant penetrating wavelengths are therefore blue light 8 in a chlorophyte . Synthesis (Fig. 3). The euphotic zone might be up to 140 m deep. Coastal waters are intermediate with respect to the magof pigment–protein complexes nitude and spectral dependence of attenuation. appears to be transcriptionally The light regime experienced by phytoplankton also depends on the rate of mixing, which is much more rapid in regulated by a mechanism estuarine waters than coastal or open ocean waters. The effect of this, in combination with the more rapid attenthat is under the control of the uation of light, means that estuarine phytoplankton can pass through a light gradient ranging from darkness to full redox state of the plastosunlight in minutes. The same transient would take hours to days in the more stable and clearer ocean (Fig. 3). quinone pool9. Under low-light .
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trends in plant science Reviews in illumination as it is mixed back up are insufficient to allow much deactivation18. There appears to be little role for Rubisco oxygenase activity in phytoplankton. Although the abundance of free CO2 in seawater (~10–15 mM) is well below the half-saturation concentration of Rubisco (30–170 mM)15,16, carbon concentrating mechanisms that involve either active transport of HCO32 or coupled dehydration of HCO32 by a cell-surface carbonic anhydrase and CO2 transport, have been documented for cyanobacteria, chlorophytes19, diatoms20 and dinoflagellates21,22. In addition, microalgal Rubiscos have high CO2 specificities compared with those from terrestrial plants. This might be because of differences in the structure of the loop-6 region15 or in the length of the large subunit C-terminus16, both of which appear to play a role in the conformation of the active site during catalysis. The dinoflagellates are distinct from all other eukaryotes in having a prokaryotic-type form II Rubisco23 (i.e. one lacking the small subunit characteristic of the eukaryotic form I). Although the specificity of dinoflagellate form II Rubisco is higher than bacterial form II Rubisco24, it is unlikely that it could support measured rates Fig. 2. (a) The first-order reaction time constants (t, in minutes) of photosynthetic responses to increases and decreases in light intensity vary from 104 min for changes in the chlorophyll of photosynthesis without the presence of a quota to 1021 min for inter-conversion of the xanthophylls diadinoxanthin and diatoxanthin. carbon-concentrating mechanism. Note that time constants for increases in irradiance (denoted ‘up’) are shorter than for A second phenomenon that might drive decreases (denoted ‘down’), except in the case of the chlorophyll-specific light-saturated rate the short-term photosynthetic response in of photosynthesis, Pm/Chl, where the kinetics are driven by changes in the chlorophyll quota. estuarine waters is the occurrence of state The time constant defines the rate of change of species X over a time-step of duration Dt as: transitions25, which are rapid adjustments Xt 1 Dt 5 Xt exp(Dt/2tdown) in the relative magnitudes of the photosysfor a decrease in irradiance and as: tem I (PS I) and photosystem II (PS II) Xt 1 Dt 5 Xt [1 2 exp(Dt/2tup)] antennae. State 2 (preferential excitation of PS I) is favored under conditions of high for an increase. absolute irradiance and under darkness, The time taken for the light level to either double or halve for cells in simulated oceanic and estuarine water (Fig. 3) is indicated by arrows. Sources for the time constants are given in whereas State 1 (preferential excitation of parentheses. (b) The essence of mixing-based productivity models is defining a step-change in PS II) is favored under conditions in which irradiance because of a cell’s motion through a light gradient (compare with Fig. 3) and implethe spectrum is dominated by red light. menting the time-dependence of photosynthetic response via time constants (tup and tdown) that Consequently cells in estuarine waters are describe the change in response to increases and decreases in light intensity (a). These contrast driven to State 2 at aphotic depths (depths with steady-state models (black unbroken line), in which no time-dependence is allowed (i.e. to which light does not penetrate because the effect of light level fluctuations is ignored). In acclimation or induction models (left), of complete attenuation by the overlying instantaneous estimates of productivity are almost always lower than in the steady-state rate, water), to State 1 at intermediate depths because of the time-lag inherent in the response as the photosynthetic mechanism adjusts to a where the spectrum is weighted towards change in light level. In inhibition models (right), instantaneous estimates of productivity can long wavelengths (Fig. 3), and back to be higher or lower than in the steady-state condition, because the steady-state rate is an empirically-derived average over a period that might be long compared with the accumulation of phoState 2 at the surface4,25. It is not clear what effect, if any, state transitions have on photosynthetic rates, but the interchange time constants are close enough to the rate of catalytic characteristics15,16. However, activity changes in those mixing that the transitions can be observed in cyanobacteria and few microalgae that have been studied are consistent with models chlorophytes from turbid waters25. State transitions are unlikely to of regulation by means of carbamylation–decarbamylation and, in limit photosynthetic rates in the ocean because the timescale of addition, possibly, by a tight-binding inhibitor, such as carboxy- mixing is slower, and because red light is more rapidly attenuated arabinitol-1-phosphate17. The kinetics of activation and deacti- than blue light (Fig. 3). State transitions have yet to be docuvation could affect photosynthetic rates in response to rapid mented from chromophytic microalgae. increases in light intensity during mixing. Because activation is much faster than deactivation, the effect is less pronounced during Photoprotection rapid mixing because the interval between decreases in illumi- The most complex group of photosynthetic responses includes the nation as a cell is mixed away from the well-lit surface and increases transients associated with photoprotection and photoinhibition. 14
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trends in plant science Reviews Photoprotective mechanisms can broadly be described as those that either decrease the absorption of light energy (by reducing the absorption cross section or effective photosynthetic cross-section of the reaction centers), or provide alternative energy sinks when photosynthetic capacity is exceeded. Change in the excitation delivery includes rapid responses, such as induction of energy-dissipating pigments in xanthophyll cycles26,27, and slower responses, such as photoacclimative changes in the size and pigment composition of the antennae (Fig. 2). Two different xanthophyll cycles are found in chlorophytes and chromophytes: in chlorophytes, violaxanthin is de-epoxidated to zeaxanthin in a two-step pathway, with antheraxanthin as an intermediate; in chromophytes diadinoxanthin is de-epoxidated to diatoxanthin in a single step. Although the chlorophytes have a pathway that is structurally similar to vascular plants’, the relationship between non-photochemical energy quenching and the level of de-epoxidation is different28. Conversely, although chromophytes have a different pathway, the relationship between non-photochemical quenching and the level of de-epoxidation is comparable to that in vascular plants. Other electron sinks
In addition to xanthophyll cycling, there are other mechanisms that might act as sinks for electrons when PS II activity exceeds photosynthetic capacity. For example, cyanobacteria lack a xanthophyll cycle but exhibit strong Mehler activity at light saturation29, and diatoms appear to be unique in using non-assimilatory nitrate reduction as a sink30. Both pathways bleed off excess energy from the electron transport chain when NADPH turnover is operating at the maximum rate, preventing over-excitation of the photosynthetic antennae. Photoinhibition
When the photoprotective mechanisms already described are exceeded, damaged PS II reaction centers, lacking a functional D1 protein, can accumulate. The reconstitution of functional PS II reaction centers can be described by first-order reaction kinetics when cells are moved to non-inhibitory irradiance31. Modeling the accumulation of damaged D1 might be more difficult because of the diversity of photoprotective responses, the protective capacity of which depends on both the light history and nutrient status of the cell26. However, photo-inactivation of D1 can be modeled by target theory32, opening the possibility that photosynthesis could then be described using the relationship between the proportion of inactive PS II reaction centers and the quantum efficiency. Kinetic models and the time-dependence of photosynthetic physiology
Short-term variability in photosynthetic rates can be accounted for in productivity models using time-dependent relationships. Generally, two assumptions are made, namely that the ocean can be considered as a one-dimensional system in the vertical and (in many models) that the time-dependence of photosynthetic processes follow firstorder reaction kinetics. With an adequate understanding of the vertical variation in mixing rate, the trajectories of groups of cells can be described by a random walk simulation (the vertical steps of which are defined in time by the local turbulent diffusivity) using a Lagrangian ensemble model. Given the rate of light attenuation, changes in light intensity can be described as a function of depth and time (Fig. 3). The sensitivity of photosynthesis to mixing can then be described by specifying the time-dependence of the response to changing light intensity with one or more time constants (Fig. 2). Separate time constants for upward and downward movements are usually employed to account for different physiological rate constants associated with responses to increases and decreases in irradiance.
Fig. 3. Light absorption in oceanic waters (a) is almost wholly dependent on phytoplankton (black line) and water itself (gray line), in contrast with estuarine waters (b), where detritus (broken black line) and chromophoric dissolved organic material (bold, black line) also make a substantial contribution (note different scales). As a consequence, both the magnitude and spectral dependence of underwater light are different. (c) The attenuation of incident sunlight (0 m) is more pronounced in red light (depicted by light gray) than blue light (depicted by dark gray) in oceanic water, in contrast with estuarine water (d), were blue light is attenuated more rapidly than red light. Light intensity is expressed as PAR (photosynthetically active radiation), which is the integral of irradiance between 400 and 700 nm. The blue and red absorbance peaks of plant pigments are shown. The spectral composition of light is shown at different depths that correspond to either a doubling or a halving in the intensity of PAR. Higher turbulence and more rapid attenuation in the estuarine water results in entrained cells experiencing more rapid transients in light intensity (f) than in the oceanic water (e). Note the difference in scales. Exposure was calculated from the spectrally dependent attenuation coefficient for a cell that was released into the middle of a mixed layer and whose trajectory was based on a random walk model.
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trends in plant science Reviews The approach has been used to model the sensitivity of photosynthesis to changes that are characteristic of photoacclimation33, photoinhibition34 and photosynthetic induction18. The relative importance of different photosynthetic mechanisms depends on the timescale and the magnitude of the changes in the light field (i.e. if the time constant for the metabolic response is longer than the characteristic timescale of change, the mechanism has the potential to limit the reaction rate13; Fig. 2). Constructing a general model of the effect of mixing on photoinhibition that is analogous to the acclimative models depends on a somewhat arbitrary level of calibration because both the degree and rate of accumulation of photoinhibitory damage are less constrained than acclimative changes (Fig. 2). One approach, which is based on first principles, involves defining an action spectrum of the doseresponse of photoinhibition and using a biological weighting function to drive the onset of photoinhibition and the subsequent recovery as cells are mixed through a water column35. A description based on energy-balance models is an alternative to the descriptions of photoacclimation based on explicit timeconstants of chlorophyll-specific photosynthetic responses. In energy-balance models, a physiological sensor regulates the pigment quota as a means of balancing excitation-energy harvesting with the energetic demands of growth. By specifying both as rates and using units of inverse time (which necessitates converting the currency of electrons to carbon equivalents), the imbalance can be used to drive changes in energy harvesting, either by inducing chlorophyll synthesis or D1 turnover10,11 until equilibrium is reestablished. Embedding such an energy-balance model into a physical model of mixing produces results that are consistent with field measurements over wide temporal and spatial scales (annual cycles from 0 to 608 N)36. Because the regulated term is the ratio of phytoplankton chlorophyll to carbon (an index of the pigment quota), energy-balance models can be reconciled with large-scale estimates of phytoplankton biomass (Fig. 1), and the response has an explicit dependence on temperature and nutrient (nitrogen) availability. These dependencies have not been quantified for first-order kinetic models, in which both the time constants and the end-points of acclimation might depend on temperature and nutrient status. The future
It is now possible to evaluate how the photosynthetic apparatus is regulated under complex environmental forcing. The complexity of the environment–photosynthesis relationship is tractable using the notion of energy-balance regulation: the redox state of the light reactions is a universal signal in microalgae for regulating cellular light harvesting efficiency. The success of recent pigment-photoacclimation models that incorporate an energy-balance regulatory ‘signal’ based on the redox state, holds promise for more sophisticated models that incorporate short-term energy modulation mechanisms. These models can be made species-specific by setting parameters according to the specific energy modulation processes that are present. Future work on environmental effects will distinguish short-term kinetically constrained responses from longer-term energybalance-constrained responses. This will clarify the relative importance of kinetic versus acclimative responses to particular environments. Acknowledgements
We thank two anonymous reviewers for their comments on the manuscript. This work was supported by Grants OCE-9730098 and OCE-9633633 from the National Science Foundation (USA) and is contribution No. 3239 from Horn Point Laboratory. 16
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References 1 Falkowski, P.G. and Raven, J.A. (1997) Aquatic Photosynthesis, Blackwell Science 2 Holmes, J.J. et al. (1989) Chlorophyll a fluorescence predicts total photosynthetic electron flow to CO2 or NO32/NO22 under transient conditions. Plant Physiol. 91, 331–337 3 Broekhuisen, N. et al. (1998) Seasonal photoadaptation and diatom dynamics in temperate waters. Mar. Ecol. Prog. Ser. 175, 227–239 4 Schubert, H. and Forester, R.M. (1997) Sources of variability in the factors used for modelling primary productivity in eutrophic waters. Hydrobiolgia 349, 75–85 5 Mouget, J-L. et al. (1995) Long-term acclimatization of Scenedesmus bicellularis to high-frequency intermittent lighting (100 Hz). I. Growth, photosynthesis and photosystem II activity. J. Plankt. Res. 17, 859–874 6 Geider, R.J. et al. (1997) Dynamic model of phytoplankton growth and acclimation: responses of the balanced growth rate and the chlorophyll a:carbon ratio to light, nutrient-limitation and temperature. Mar. Ecol. Prog. Ser. 148, 187–200 7 Orellana, M.V. and Perry, M.J. (1992) An immunoprobe to measure Rubisco concentrations and maximal photosynthetic rates of individual phytoplankton cells. Limnol. Oceanogr. 37, 478–490 8 Sukenik, A. et al. (1987) Light-saturated photosynthesis – limitation by electron transport or carbon fixation? Biochim. Biophys. Acta 891, 205–215 9 Escoubas, J-M. et al. (1995) Light-intensity regulation of CAB gene transcription is signaled by the redox state of the plastiquinone pool. Proc. Natl. Acad. Sci. U. S. A. 92, 10237–10241 10 Geider, R.J. et al. (1998) A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients and temperature. Limnol. Oceanogr. 43, 679–694 11 Kana, T.M. et al. (1997) Photosynthetic pigment regulation in microalgae by multiple environmental sensors: a dynamic balance hypothesis. New Phytol. 137, 628–638 12 Maxwell, D.P. et al. (1995) Redox regulation of light-harvesting complex II and cab mRNA abundance in Dunaliella salina. Plant Phys. 109, 787–795 13 Cullen, J.J. and Lewis, M.R. (1988) The kinetics of algal photoadaptation in the context of vertical mixing. J. Plankt. Res. 10, 1039–1063 14 Geider, R.J. et al. (1998) Responses of the photosynthetic apparatus of Dunaliella teriolecta (Chlorophyceae) to nitrogen and phosphorus limitation. Eur. J. Phycol. 33, 315–332 15 Read, B.A. and Tabita, F.R. (1994) High substrate specificity factor ribulose bisphosphate carboxylase/oxygenase from eukaryotic marine algae and properties of recombinant cyanobacterial Rubisco containing ‘algal’ residue modifications. Arch. Biochem. Biophys. 312, 210–218 16 Zhu, G. et al. (1998) Dependence of catalysis and CO2/O2 specificity of Rubisco on the carboxy-terminus of the large subunit at different temperatures. Photosynth. Res. 57, 71–79 17 MacIntyre, H.L. et al. (1997) Activation and deactivation of ribulose-1,5bisphosphate carboxylase/oxygenase (Rubisco) in three marine microalgae. Photosynth. Res. 51, 93–106 18 MacIntyre, H.L. and Geider, R.J. (1996) Regulation of Rubisco activity and its potential effect on photosynthesis during mixing in a turbid estuary. Mar. Ecol. Prog. Ser. 144, 247–264 19 Badger, M.R. and Price, G.D. (1992) The CO2 concentrating mechanism in cyanobacteria and microalgae. Physiol. Plant. 84, 606–615 20 Tortell, P.D. et al. (1997) Active uptake of bicarbonate by diatoms. Nature 390, 243–244 21 Berman-Frank, I. et al. (1998) Changes in inorganic carbon uptake during the progression of a dinoflagellate bloom in a lake ecosystem. Can. J. Bot. 76, 1043–1051 22 Nimer, N. et al. (1999) Extracellular carbonic anhydrase facilitates carbon dioxide availability for photosynthesis in the marine dinoflagellate Prorocentrum micans. Plant. Physiol. 120, 105–111 23 Morse, D. et al. (1995) A nuclear-encoded form II Rubisco in dinoflagellates. Science 268, 1622–1624 24 Whitney, S.M. and Andrews, T.J. (1998) The CO2/O2 specificity of singlesubunit ribulose-bisphosphate carboxylase from the dinoflagellate, Amphidinium carterae. Aust. J. Plant Physiol. 25, 131–138
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33 Barkmann, W. and Woods, J.D. (1996) On using a Lagrangian model to calibrate primary production determined from in vitro incubation measurements. J. Plankt. Res. 18, 767–788 34 Franks, P.J.S. and Marra, J. (1994) A simple new formulation for phytoplankton photoresponse and an application in a wind-driven mixed-layer model. Mar. Ecol. Prog. Ser. 111, 143–153 35 Neale, J.P. et al. (1998) Inhibition of marine photosynthesis by ultraviolet radiation: variable sensitivity of phytoplankton in the Weddell–Scotia confluence during the austral spring. Limnol. Oceanogr. 43, 433–448 36 Taylor, A.H. et al. (1997) Seasonal and latitudinal dependencies of phytoplankton carbon-to-chlorophyll a ratios: results of a modelling study. Mar. Ecol. Prog. Ser. 152, 51–66
Hugh L. MacIntyre* and Todd M. Kana are at the University of Maryland Center for Environmental Research, Horn Point Laboratory, PO Box 775, Cambridge, MD 21613, USA; Richard J. Geider is at the Dept of Biological Sciences, University of Essex, Colchester, UK CO3 4SQ. *Author for correspondence (tel 11 410 221 8430; fax 11 410 221 8490; e-mail [email protected]).
Floral induction and determination: where is flowering controlled? Frederick D. Hempel, David R. Welch and Lewis J. Feldman Flowering is controlled by a variety of interrelated mechanisms. In many plants, the environment controls the production of a floral stimulus, which moves from the leaves to the shoot apex. Apices can become committed to the continuous production of flowers after the receipt of sufficient amounts of floral stimulus. However, in some plants, the commitment to continued flower production is evidently caused by a plant’s commitment to perpetually produce floral stimulus in the leaves. Ultimately, the induction of flowering leads to the specification of flowers at the shoot apex. In Arabidopsis, floral specification and inflorescence patterning are regulated largely by the interactions between the genes TERMINAL FLOWER, LEAFY and APETALA1/CAULIFLOWER.
F
loral induction is the process by which stimuli originating outside the shoot apex induce the formation of flower primordia (Fig. 1). The photoperiodic induction of flowering was discovered 86 years ago by Julien Tornois in hops1. Shortly afterwards, additional experiments suggested that the photoperiodic control of flowering was a general phenomenon, which controlled flowering in most plants2. Later, focused-light experiments showed that leaves perceive photoperiodic signals3. These studies, and numerous grafting experiments, indicate that the production of the photoperiod-induced floral stimulus4 occurs in the leaves of a wide variety of flowering plants5–7. In contrast with floral induction, floral determination can be defined as the assignment of flower(ing) fate, which is persistent even when the flower-inducing conditions no longer exist8,9. Assays for floral determination include: • Changing environmental conditions (from inductive to noninductive). • Microsurgical removal of shoot apices, and the placement of those apices into neutral environments8,10.
However, both types of determination assay have limitations, and it is important to note that different determination assays might yield alternative conclusions for the same primordia (the caveats associated with determination experiments are discussed in Ref. 11). A third type of assay has been used to test leaf commitment to the continued production of floral stimulus: in this assay, photoinduced leaves are removed from the plant following an inductive treatment12. In this review we discuss firstly a variety of experiments that indicate the site(s) that control flowering. Secondly, we review recent studies that indicate how a few key molecular players regulate the specification of flower primordia in Arabidopsis. Floral determination assays Photoperiodic assays for floral determination
The simplest type of determination assay is one in which plants are moved to non-inductive conditions after various lengths of time under inductive conditions. Using this method, the duration of photoinduction treatment required to produce flowers can be
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The effect of water motion on short-term rates of photosynthesis by marine phytoplankton Hugh L. MacIntyre, Todd M. Kana and Richard J. Geider Phytoplankton respond to variations in light intensity as they are mixed through the water column. Changes in pigment content are characteristic of the relatively slow response of ‘sun–shade’ photoacclimation that occurs on timescales typical of mixing in the open ocean. In estuaries, the variations are much faster and induce correspondingly rapid changes in the activity (rather than abundance) of different components of the photosynthetic apparatus. These components modulate light harvesting and Calvin cycle activity, or protect the pigment bed from excess energy absorption. When the protective capacity is exceeded, photoinhibition occurs. All these mechanisms modulate the rate of photosynthesis in situ.
P
lanktonic microalgae (phytoplankton) live in diverse and highly variable environments. Their photosynthetic apparatus is subject to significant stresses because of rapid changes or imbalances in irradiance and nutrient supply imposed by the physics and chemistry of natural water bodies. The ways in which the photosynthetic apparatus adjusts to these temporally variable and complex environments are of interest for both practical and fundamental reasons, because phytoplankton photosynthesis is responsible for ~50% of global productivity. Modern techniques for assessing productivity on global scales rely on remotely sensing plankton’s optical properties, using the signature of chlorophyll as an index of abundance (Fig. 1). The emphasis on chlorophyll is inevitable given chlorophyll’s distinctive optical characteristics, which enables plant material to be distinguished from other suspended matter. The relationship between chlorophyll and organic carbon (the desired currency for productivity models) is highly plastic, varying with growth, irradiance, nutrient availability and temperature. However, the variability is ordered, not random, and knowledge about the effects of environmental variables on the efficiency of light absorption and its conversion to biomass during photosynthesis allows estimates of chlorophyll abundance to be translated into carbon equivalents.
Understanding the effect of varying environmental conditions on photosynthetic rates can be considered in terms of the regulation of the amounts and the specific activities of components of the photosynthetic apparatus. Regulation can be accomplished by variations in the relative abundance of the constituents [e.g. chlorophyll and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)] or, on a shorter timescale, by varying the efficiency of their coupling and activity (e.g. the activation state of Rubisco). The distinction is blurred at some levels but serves to distinguish between those means of photosynthetic regulation that depend on synthesis or turnover (usually of chlorophyll) in photoacclimation, and those that depend on more rapid changes in activation states or efficiencies, independent of turnover. Work in this area is complicated by the fact that phytoplankton include representatives of four kingdoms1 and are highly variable in their molecular, structural and optical properties. Whereas physiological studies have focused largely on taxa that are easy to maintain in culture, particularly chlorophytes and diatoms. It has been challenging to bring qualitative information on photosynthetic mechanisms together with quantitative kinetic information on rates of response to understand cellular photosynthesis in the natural environment. Here we review recent developments in our understanding of how phytoplankton photosynthesis adjusts to naturally variable environments. The principal environmental factors that affect phytoplankton photosynthesis are light, nutrient availability and temperature. In nature, these factors operate independently on timescales that match photosynthetic physiology, presenting a complex and unpredictable environment to the cells. Phytoplankton respond to such stochastic environments using a variety of physiological processes that affect light Fig. 1. Composite false-color image of mean annual ocean chlorophyll concentrations, as detected by satellite remote-sensing. Chlorophyll is used as an index of biomass in seawater because, unlike other organic compounds, harvesting efficiency and it is unique to phytoplankton and common to all autotrophs. Note the higher concentrations near the land photosynthetic capacity. Two key issues are at the forefront of ecophysiological research:
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trends in plant science Reviews • What are the kinetic constraints on processes that modulate photosynthesis in rapidly fluctuating environments? • What are the mechanisms by which cells integrate multiple environmental factors in regulating their physiological responses? These questions are related: the photosynthetic mechanisms that modulate energy and material flow through the cell are common to phytoplankton in spite of their diversity. Significant conceptual advances have been made recently that tie whole-cell physiology with photosynthetic regulatory processes operating at the biochemical and biophysical levels. Environmental control of photosynthetic processes
By definition, phytoplankton are incapable of sustained directional movement and therefore are subject to the environmental conditions in their parent body of water. Photosynthesis is responsive to changes in nutrient availability on short timescales2 but in marine systems the changes are more likely to occur over relatively long timescales (days to seasons). Although nutrient availability is critical for determining population dynamics3 and the ultimate determinant of the geographic abundance of phytoplankton (Fig. 1), nutrient availability is less likely to drive short-term changes in photosynthetic rates. Because of water’s capacity for high levels of latent heat, temperature is more likely to vary on daily to seasonal scales, and in a manner predictable from the balance of radiative transfer and evaporative cooling. By contrast, phytoplankton are subject to relatively rapid changes in both the intensity and the spectral quality of light as they move vertically in a water column (Box 1). In addition to the changes imposed by mixing, there are variations in the light field with timescales of hours, minutes and milliseconds4 as a result of changes in solar elevation, cloud cover and subsurface focusing by waves. Photosynthesis and growth rates appear to be insensitive to variability in the millisecond domain5, therefore we will focus on variations that occur on a timescale of minutes and hours. Photoacclimation of pigment content
conditions, plastoquinone is largely oxidized, and transcription of the mRNAs that code for the proteins that bind chlorophyll in the antenna occurs at maximal rates. As irradiance increases, photosynthesis control passes from light harvesting to the maximum capacity for carbon dioxide fixation, the plastoquinone becomes increasingly reduced, and transcription of the mRNAs for pigment–protein complexes declines. The end results of this mode of regulation can be mimicked by ‘energy balance’ models developed recently10,11. These are based on the concept of a physiological light sensor that regulates the pigment quota to maintain a balance between the harvesting of excitation energy by means of light absorption and photochemistry on the one hand and the energetic demands of growth on the other. Conceptually, this is comparable to the redox regulation of the antenna protein LHCII (Refs 9,12). These models can account for steady-state responses and for the differences in the rates of acclimation that are observed following shifts from low-to-high versus high-to-low irradiance, without the potential errors imposed by specifying a mathematical function to describe the kinetics of photoacclimation13. Photosynthetic Induction
The time-course of light intensity changes in estuarine waters is much faster than in coastal or open-ocean waters because both the rate of light attenuation and the rate of mixing are much higher (Box 1). Two mechanisms, the activation and deactivation of Rubisco and state transitions, have time constants that are comparable to the timescale of mixing in estuarine waters and probably dominate short-term rates of photosynthesis (Fig. 2). Photosynthetic regulation operating by means of the catalytic activity of Rubisco can be controlled by variations in the enzyme concentration or, in the short-term, by its activation state. Although the enzyme cellular concentration might not be regulated during photoacclimation under nutrient-replete conditions, depending on the taxon8, it is regulated in response to chronic phosphorus or nitrogen limitation14. Because there is a correlation between the maximum quantum efficiency (fm) and the ratio of Rubisco to the PSII reaction center protein D1, changes in the pool size of Rubisco might play a role in regulating acclimated photosynthetic rates during nutrient-limited growth. A potential role for activation and deactivation of Rubisco in the short-term limitation of photosynthetic rates hinges on the assumption that at light saturation, the rate of photosynthesis depends on the enzyme’s activity. Broad conclusions are complicated by the diversity in microalgal Rubisco structure and
For a given temperature and nutrient status, photoacclimation in phytoplankton can be described in terms of regulation of the cell concentrations of the catalysts that determine light-limited and light-saturated photosynthetic rates. The rate of light absorption, which co-varies with a cell’s chlorophyll concentration, is often the primary determinant of light-limited photosynthesis, whereas the maximum rate of carbon dioxide fixation, which co-varies with Rubisco concentration, is the primary determinant of lightsaturated photosynthesis. The cell chlorophyll content is usually higher in cells that have Box 1. Variations in the submarine light field grown under low light6. Variations in other constituents of The light attenuation within any body of water follows (approximately) the Lambert–Beer law, decreasing expothe photosynthetic apparatus nentially with depth. The rate and spectral dependence of attenuation depends on the abundance of phytoplankton, appear to be taxon-specific: of detritus (which includes organic material of biogenic origin and suspended sediment) and chromophoric two studies of cells grown dissolved organic matter (CDOM, mainly humic and fulvic acids of terrestrial origin). under nutrient-replete condiIn estuarine waters, terrestrial runoff that is high in humics and plant nutrients, and close coupling between the benthos and water column results in high absorption by CDOM, phytoplankton and detritus in blue light. The domtions, the concentration of inant penetrating wavelengths are therefore red and the intermediate green (Fig. 3). Attenuation is rapid: in an Rubisco decreased at low extreme case, the euphotic zone (the depth to which 1% of sunlight penetrates) is ,0.5 m. growth irradiance in a diatom7 In oceanic waters, the abundance of CDOM, detritus and phytoplankton is low and the dominant attenuator is whereas it remained unchanged water itself, which is highly absorbent in red light. The dominant penetrating wavelengths are therefore blue light 8 in a chlorophyte . Synthesis (Fig. 3). The euphotic zone might be up to 140 m deep. Coastal waters are intermediate with respect to the magof pigment–protein complexes nitude and spectral dependence of attenuation. appears to be transcriptionally The light regime experienced by phytoplankton also depends on the rate of mixing, which is much more rapid in regulated by a mechanism estuarine waters than coastal or open ocean waters. The effect of this, in combination with the more rapid attenthat is under the control of the uation of light, means that estuarine phytoplankton can pass through a light gradient ranging from darkness to full redox state of the plastosunlight in minutes. The same transient would take hours to days in the more stable and clearer ocean (Fig. 3). quinone pool9. Under low-light .
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trends in plant science Reviews in illumination as it is mixed back up are insufficient to allow much deactivation18. There appears to be little role for Rubisco oxygenase activity in phytoplankton. Although the abundance of free CO2 in seawater (~10–15 mM) is well below the half-saturation concentration of Rubisco (30–170 mM)15,16, carbon concentrating mechanisms that involve either active transport of HCO32 or coupled dehydration of HCO32 by a cell-surface carbonic anhydrase and CO2 transport, have been documented for cyanobacteria, chlorophytes19, diatoms20 and dinoflagellates21,22. In addition, microalgal Rubiscos have high CO2 specificities compared with those from terrestrial plants. This might be because of differences in the structure of the loop-6 region15 or in the length of the large subunit C-terminus16, both of which appear to play a role in the conformation of the active site during catalysis. The dinoflagellates are distinct from all other eukaryotes in having a prokaryotic-type form II Rubisco23 (i.e. one lacking the small subunit characteristic of the eukaryotic form I). Although the specificity of dinoflagellate form II Rubisco is higher than bacterial form II Rubisco24, it is unlikely that it could support measured rates Fig. 2. (a) The first-order reaction time constants (t, in minutes) of photosynthetic responses to increases and decreases in light intensity vary from 104 min for changes in the chlorophyll of photosynthesis without the presence of a quota to 1021 min for inter-conversion of the xanthophylls diadinoxanthin and diatoxanthin. carbon-concentrating mechanism. Note that time constants for increases in irradiance (denoted ‘up’) are shorter than for A second phenomenon that might drive decreases (denoted ‘down’), except in the case of the chlorophyll-specific light-saturated rate the short-term photosynthetic response in of photosynthesis, Pm/Chl, where the kinetics are driven by changes in the chlorophyll quota. estuarine waters is the occurrence of state The time constant defines the rate of change of species X over a time-step of duration Dt as: transitions25, which are rapid adjustments Xt 1 Dt 5 Xt exp(Dt/2tdown) in the relative magnitudes of the photosysfor a decrease in irradiance and as: tem I (PS I) and photosystem II (PS II) Xt 1 Dt 5 Xt [1 2 exp(Dt/2tup)] antennae. State 2 (preferential excitation of PS I) is favored under conditions of high for an increase. absolute irradiance and under darkness, The time taken for the light level to either double or halve for cells in simulated oceanic and estuarine water (Fig. 3) is indicated by arrows. Sources for the time constants are given in whereas State 1 (preferential excitation of parentheses. (b) The essence of mixing-based productivity models is defining a step-change in PS II) is favored under conditions in which irradiance because of a cell’s motion through a light gradient (compare with Fig. 3) and implethe spectrum is dominated by red light. menting the time-dependence of photosynthetic response via time constants (tup and tdown) that Consequently cells in estuarine waters are describe the change in response to increases and decreases in light intensity (a). These contrast driven to State 2 at aphotic depths (depths with steady-state models (black unbroken line), in which no time-dependence is allowed (i.e. to which light does not penetrate because the effect of light level fluctuations is ignored). In acclimation or induction models (left), of complete attenuation by the overlying instantaneous estimates of productivity are almost always lower than in the steady-state rate, water), to State 1 at intermediate depths because of the time-lag inherent in the response as the photosynthetic mechanism adjusts to a where the spectrum is weighted towards change in light level. In inhibition models (right), instantaneous estimates of productivity can long wavelengths (Fig. 3), and back to be higher or lower than in the steady-state condition, because the steady-state rate is an empirically-derived average over a period that might be long compared with the accumulation of phoState 2 at the surface4,25. It is not clear what effect, if any, state transitions have on photosynthetic rates, but the interchange time constants are close enough to the rate of catalytic characteristics15,16. However, activity changes in those mixing that the transitions can be observed in cyanobacteria and few microalgae that have been studied are consistent with models chlorophytes from turbid waters25. State transitions are unlikely to of regulation by means of carbamylation–decarbamylation and, in limit photosynthetic rates in the ocean because the timescale of addition, possibly, by a tight-binding inhibitor, such as carboxy- mixing is slower, and because red light is more rapidly attenuated arabinitol-1-phosphate17. The kinetics of activation and deacti- than blue light (Fig. 3). State transitions have yet to be docuvation could affect photosynthetic rates in response to rapid mented from chromophytic microalgae. increases in light intensity during mixing. Because activation is much faster than deactivation, the effect is less pronounced during Photoprotection rapid mixing because the interval between decreases in illumi- The most complex group of photosynthetic responses includes the nation as a cell is mixed away from the well-lit surface and increases transients associated with photoprotection and photoinhibition. 14
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trends in plant science Reviews Photoprotective mechanisms can broadly be described as those that either decrease the absorption of light energy (by reducing the absorption cross section or effective photosynthetic cross-section of the reaction centers), or provide alternative energy sinks when photosynthetic capacity is exceeded. Change in the excitation delivery includes rapid responses, such as induction of energy-dissipating pigments in xanthophyll cycles26,27, and slower responses, such as photoacclimative changes in the size and pigment composition of the antennae (Fig. 2). Two different xanthophyll cycles are found in chlorophytes and chromophytes: in chlorophytes, violaxanthin is de-epoxidated to zeaxanthin in a two-step pathway, with antheraxanthin as an intermediate; in chromophytes diadinoxanthin is de-epoxidated to diatoxanthin in a single step. Although the chlorophytes have a pathway that is structurally similar to vascular plants’, the relationship between non-photochemical energy quenching and the level of de-epoxidation is different28. Conversely, although chromophytes have a different pathway, the relationship between non-photochemical quenching and the level of de-epoxidation is comparable to that in vascular plants. Other electron sinks
In addition to xanthophyll cycling, there are other mechanisms that might act as sinks for electrons when PS II activity exceeds photosynthetic capacity. For example, cyanobacteria lack a xanthophyll cycle but exhibit strong Mehler activity at light saturation29, and diatoms appear to be unique in using non-assimilatory nitrate reduction as a sink30. Both pathways bleed off excess energy from the electron transport chain when NADPH turnover is operating at the maximum rate, preventing over-excitation of the photosynthetic antennae. Photoinhibition
When the photoprotective mechanisms already described are exceeded, damaged PS II reaction centers, lacking a functional D1 protein, can accumulate. The reconstitution of functional PS II reaction centers can be described by first-order reaction kinetics when cells are moved to non-inhibitory irradiance31. Modeling the accumulation of damaged D1 might be more difficult because of the diversity of photoprotective responses, the protective capacity of which depends on both the light history and nutrient status of the cell26. However, photo-inactivation of D1 can be modeled by target theory32, opening the possibility that photosynthesis could then be described using the relationship between the proportion of inactive PS II reaction centers and the quantum efficiency. Kinetic models and the time-dependence of photosynthetic physiology
Short-term variability in photosynthetic rates can be accounted for in productivity models using time-dependent relationships. Generally, two assumptions are made, namely that the ocean can be considered as a one-dimensional system in the vertical and (in many models) that the time-dependence of photosynthetic processes follow firstorder reaction kinetics. With an adequate understanding of the vertical variation in mixing rate, the trajectories of groups of cells can be described by a random walk simulation (the vertical steps of which are defined in time by the local turbulent diffusivity) using a Lagrangian ensemble model. Given the rate of light attenuation, changes in light intensity can be described as a function of depth and time (Fig. 3). The sensitivity of photosynthesis to mixing can then be described by specifying the time-dependence of the response to changing light intensity with one or more time constants (Fig. 2). Separate time constants for upward and downward movements are usually employed to account for different physiological rate constants associated with responses to increases and decreases in irradiance.
Fig. 3. Light absorption in oceanic waters (a) is almost wholly dependent on phytoplankton (black line) and water itself (gray line), in contrast with estuarine waters (b), where detritus (broken black line) and chromophoric dissolved organic material (bold, black line) also make a substantial contribution (note different scales). As a consequence, both the magnitude and spectral dependence of underwater light are different. (c) The attenuation of incident sunlight (0 m) is more pronounced in red light (depicted by light gray) than blue light (depicted by dark gray) in oceanic water, in contrast with estuarine water (d), were blue light is attenuated more rapidly than red light. Light intensity is expressed as PAR (photosynthetically active radiation), which is the integral of irradiance between 400 and 700 nm. The blue and red absorbance peaks of plant pigments are shown. The spectral composition of light is shown at different depths that correspond to either a doubling or a halving in the intensity of PAR. Higher turbulence and more rapid attenuation in the estuarine water results in entrained cells experiencing more rapid transients in light intensity (f) than in the oceanic water (e). Note the difference in scales. Exposure was calculated from the spectrally dependent attenuation coefficient for a cell that was released into the middle of a mixed layer and whose trajectory was based on a random walk model.
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trends in plant science Reviews The approach has been used to model the sensitivity of photosynthesis to changes that are characteristic of photoacclimation33, photoinhibition34 and photosynthetic induction18. The relative importance of different photosynthetic mechanisms depends on the timescale and the magnitude of the changes in the light field (i.e. if the time constant for the metabolic response is longer than the characteristic timescale of change, the mechanism has the potential to limit the reaction rate13; Fig. 2). Constructing a general model of the effect of mixing on photoinhibition that is analogous to the acclimative models depends on a somewhat arbitrary level of calibration because both the degree and rate of accumulation of photoinhibitory damage are less constrained than acclimative changes (Fig. 2). One approach, which is based on first principles, involves defining an action spectrum of the doseresponse of photoinhibition and using a biological weighting function to drive the onset of photoinhibition and the subsequent recovery as cells are mixed through a water column35. A description based on energy-balance models is an alternative to the descriptions of photoacclimation based on explicit timeconstants of chlorophyll-specific photosynthetic responses. In energy-balance models, a physiological sensor regulates the pigment quota as a means of balancing excitation-energy harvesting with the energetic demands of growth. By specifying both as rates and using units of inverse time (which necessitates converting the currency of electrons to carbon equivalents), the imbalance can be used to drive changes in energy harvesting, either by inducing chlorophyll synthesis or D1 turnover10,11 until equilibrium is reestablished. Embedding such an energy-balance model into a physical model of mixing produces results that are consistent with field measurements over wide temporal and spatial scales (annual cycles from 0 to 608 N)36. Because the regulated term is the ratio of phytoplankton chlorophyll to carbon (an index of the pigment quota), energy-balance models can be reconciled with large-scale estimates of phytoplankton biomass (Fig. 1), and the response has an explicit dependence on temperature and nutrient (nitrogen) availability. These dependencies have not been quantified for first-order kinetic models, in which both the time constants and the end-points of acclimation might depend on temperature and nutrient status. The future
It is now possible to evaluate how the photosynthetic apparatus is regulated under complex environmental forcing. The complexity of the environment–photosynthesis relationship is tractable using the notion of energy-balance regulation: the redox state of the light reactions is a universal signal in microalgae for regulating cellular light harvesting efficiency. The success of recent pigment-photoacclimation models that incorporate an energy-balance regulatory ‘signal’ based on the redox state, holds promise for more sophisticated models that incorporate short-term energy modulation mechanisms. These models can be made species-specific by setting parameters according to the specific energy modulation processes that are present. Future work on environmental effects will distinguish short-term kinetically constrained responses from longer-term energybalance-constrained responses. This will clarify the relative importance of kinetic versus acclimative responses to particular environments. Acknowledgements
We thank two anonymous reviewers for their comments on the manuscript. This work was supported by Grants OCE-9730098 and OCE-9633633 from the National Science Foundation (USA) and is contribution No. 3239 from Horn Point Laboratory. 16
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References 1 Falkowski, P.G. and Raven, J.A. (1997) Aquatic Photosynthesis, Blackwell Science 2 Holmes, J.J. et al. (1989) Chlorophyll a fluorescence predicts total photosynthetic electron flow to CO2 or NO32/NO22 under transient conditions. Plant Physiol. 91, 331–337 3 Broekhuisen, N. et al. (1998) Seasonal photoadaptation and diatom dynamics in temperate waters. Mar. Ecol. Prog. Ser. 175, 227–239 4 Schubert, H. and Forester, R.M. (1997) Sources of variability in the factors used for modelling primary productivity in eutrophic waters. Hydrobiolgia 349, 75–85 5 Mouget, J-L. et al. (1995) Long-term acclimatization of Scenedesmus bicellularis to high-frequency intermittent lighting (100 Hz). I. Growth, photosynthesis and photosystem II activity. J. Plankt. Res. 17, 859–874 6 Geider, R.J. et al. (1997) Dynamic model of phytoplankton growth and acclimation: responses of the balanced growth rate and the chlorophyll a:carbon ratio to light, nutrient-limitation and temperature. Mar. Ecol. Prog. Ser. 148, 187–200 7 Orellana, M.V. and Perry, M.J. (1992) An immunoprobe to measure Rubisco concentrations and maximal photosynthetic rates of individual phytoplankton cells. Limnol. Oceanogr. 37, 478–490 8 Sukenik, A. et al. (1987) Light-saturated photosynthesis – limitation by electron transport or carbon fixation? Biochim. Biophys. Acta 891, 205–215 9 Escoubas, J-M. et al. (1995) Light-intensity regulation of CAB gene transcription is signaled by the redox state of the plastiquinone pool. Proc. Natl. Acad. Sci. U. S. A. 92, 10237–10241 10 Geider, R.J. et al. (1998) A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients and temperature. Limnol. Oceanogr. 43, 679–694 11 Kana, T.M. et al. (1997) Photosynthetic pigment regulation in microalgae by multiple environmental sensors: a dynamic balance hypothesis. New Phytol. 137, 628–638 12 Maxwell, D.P. et al. (1995) Redox regulation of light-harvesting complex II and cab mRNA abundance in Dunaliella salina. Plant Phys. 109, 787–795 13 Cullen, J.J. and Lewis, M.R. (1988) The kinetics of algal photoadaptation in the context of vertical mixing. J. Plankt. Res. 10, 1039–1063 14 Geider, R.J. et al. (1998) Responses of the photosynthetic apparatus of Dunaliella teriolecta (Chlorophyceae) to nitrogen and phosphorus limitation. Eur. J. Phycol. 33, 315–332 15 Read, B.A. and Tabita, F.R. (1994) High substrate specificity factor ribulose bisphosphate carboxylase/oxygenase from eukaryotic marine algae and properties of recombinant cyanobacterial Rubisco containing ‘algal’ residue modifications. Arch. Biochem. Biophys. 312, 210–218 16 Zhu, G. et al. (1998) Dependence of catalysis and CO2/O2 specificity of Rubisco on the carboxy-terminus of the large subunit at different temperatures. Photosynth. Res. 57, 71–79 17 MacIntyre, H.L. et al. (1997) Activation and deactivation of ribulose-1,5bisphosphate carboxylase/oxygenase (Rubisco) in three marine microalgae. Photosynth. Res. 51, 93–106 18 MacIntyre, H.L. and Geider, R.J. (1996) Regulation of Rubisco activity and its potential effect on photosynthesis during mixing in a turbid estuary. Mar. Ecol. Prog. Ser. 144, 247–264 19 Badger, M.R. and Price, G.D. (1992) The CO2 concentrating mechanism in cyanobacteria and microalgae. Physiol. Plant. 84, 606–615 20 Tortell, P.D. et al. (1997) Active uptake of bicarbonate by diatoms. Nature 390, 243–244 21 Berman-Frank, I. et al. (1998) Changes in inorganic carbon uptake during the progression of a dinoflagellate bloom in a lake ecosystem. Can. J. Bot. 76, 1043–1051 22 Nimer, N. et al. (1999) Extracellular carbonic anhydrase facilitates carbon dioxide availability for photosynthesis in the marine dinoflagellate Prorocentrum micans. Plant. Physiol. 120, 105–111 23 Morse, D. et al. (1995) A nuclear-encoded form II Rubisco in dinoflagellates. Science 268, 1622–1624 24 Whitney, S.M. and Andrews, T.J. (1998) The CO2/O2 specificity of singlesubunit ribulose-bisphosphate carboxylase from the dinoflagellate, Amphidinium carterae. Aust. J. Plant Physiol. 25, 131–138
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33 Barkmann, W. and Woods, J.D. (1996) On using a Lagrangian model to calibrate primary production determined from in vitro incubation measurements. J. Plankt. Res. 18, 767–788 34 Franks, P.J.S. and Marra, J. (1994) A simple new formulation for phytoplankton photoresponse and an application in a wind-driven mixed-layer model. Mar. Ecol. Prog. Ser. 111, 143–153 35 Neale, J.P. et al. (1998) Inhibition of marine photosynthesis by ultraviolet radiation: variable sensitivity of phytoplankton in the Weddell–Scotia confluence during the austral spring. Limnol. Oceanogr. 43, 433–448 36 Taylor, A.H. et al. (1997) Seasonal and latitudinal dependencies of phytoplankton carbon-to-chlorophyll a ratios: results of a modelling study. Mar. Ecol. Prog. Ser. 152, 51–66
Hugh L. MacIntyre* and Todd M. Kana are at the University of Maryland Center for Environmental Research, Horn Point Laboratory, PO Box 775, Cambridge, MD 21613, USA; Richard J. Geider is at the Dept of Biological Sciences, University of Essex, Colchester, UK CO3 4SQ. *Author for correspondence (tel 11 410 221 8430; fax 11 410 221 8490; e-mail [email protected]).
Floral induction and determination: where is flowering controlled? Frederick D. Hempel, David R. Welch and Lewis J. Feldman Flowering is controlled by a variety of interrelated mechanisms. In many plants, the environment controls the production of a floral stimulus, which moves from the leaves to the shoot apex. Apices can become committed to the continuous production of flowers after the receipt of sufficient amounts of floral stimulus. However, in some plants, the commitment to continued flower production is evidently caused by a plant’s commitment to perpetually produce floral stimulus in the leaves. Ultimately, the induction of flowering leads to the specification of flowers at the shoot apex. In Arabidopsis, floral specification and inflorescence patterning are regulated largely by the interactions between the genes TERMINAL FLOWER, LEAFY and APETALA1/CAULIFLOWER.
F
loral induction is the process by which stimuli originating outside the shoot apex induce the formation of flower primordia (Fig. 1). The photoperiodic induction of flowering was discovered 86 years ago by Julien Tornois in hops1. Shortly afterwards, additional experiments suggested that the photoperiodic control of flowering was a general phenomenon, which controlled flowering in most plants2. Later, focused-light experiments showed that leaves perceive photoperiodic signals3. These studies, and numerous grafting experiments, indicate that the production of the photoperiod-induced floral stimulus4 occurs in the leaves of a wide variety of flowering plants5–7. In contrast with floral induction, floral determination can be defined as the assignment of flower(ing) fate, which is persistent even when the flower-inducing conditions no longer exist8,9. Assays for floral determination include: • Changing environmental conditions (from inductive to noninductive). • Microsurgical removal of shoot apices, and the placement of those apices into neutral environments8,10.
However, both types of determination assay have limitations, and it is important to note that different determination assays might yield alternative conclusions for the same primordia (the caveats associated with determination experiments are discussed in Ref. 11). A third type of assay has been used to test leaf commitment to the continued production of floral stimulus: in this assay, photoinduced leaves are removed from the plant following an inductive treatment12. In this review we discuss firstly a variety of experiments that indicate the site(s) that control flowering. Secondly, we review recent studies that indicate how a few key molecular players regulate the specification of flower primordia in Arabidopsis. Floral determination assays Photoperiodic assays for floral determination
The simplest type of determination assay is one in which plants are moved to non-inductive conditions after various lengths of time under inductive conditions. Using this method, the duration of photoinduction treatment required to produce flowers can be
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