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Physiological aspects of primary production in seagrasses
Aquatic Botany, 7 (1979) 139--150 © Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands
139
PHYSIOLOGICAL ASPECTS OF PRIMARY PRODUCTION IN SEAGRASSES
EDWARD A. DREW
Gatty Marine Laboratory, St. Andrews (Gt. Britain) (Accepted 23 April 1979)
ABSTRACT Drew, E.A., 1979. Physiological aspects of primary production in seagrasses. Aquat. Bot., 7: 139--150. P--I curves and dark-respiration data are presented for Cymodocea nodosa (Ucria) Aschers., Halophila stipulacea (Forsk.) Aschers, Phyllospadix torreyi S. Watson, Posidonia oceanica (L.) Delile, Zostera angustifolia (Hornem.) Reichb. and Z. marina L. (wide-leaved form), together with photosynthesis versus temperature data. Compensation irradiance, saturation irradiance (Ik) and photoinhibition are discussed with reference to habitat. Chloroplast movement in H. stipulacea is shown to be complex and to reduce photainhihition, whilst leaf senescence in iv. oceanica is suggested as a cause of marked seasonal variation in its photosynthetic rate, with high rates in spring and low rates in summer.
INTRODUCTION
Seagrasses grow in polar, temperate and tropical regions and as a group are thus subjected to a wide range of water temperatures (--2 to 30 ° C) and irradiance regimes. They also show very marked geographical limitations often closely correlated with these major environmental parameters, as postulated for instance by SetcheU (1920) with reference to temperature. Similarly, some seagrasses may grow at considerable depths with very dim illumination at the lower limit of perhaps 90 m for Halophila (den Hartog, 1970), whilst others grow intertidally in the tropics with very high irradiance. However, very little is known of the physiological adaptations of these superficially very similar plants. M c R o y and McMillan (1977) summarised the relevant data on the effects of temperature and light on seagrasses in only three pages. Other than observational work on tolerance of temperature extremes such as that by Zieman (1975), the only experimental data on temperature--photosynthesis relationships seem to be those of Biebl and M c R o y (1971) and Drew (1978). During the last few years I have carried o u t relevant experiments on a number of seagrasses in temperate and sub-tropical environments. This paper represents a summary of basic physiological data potentially useful to researchers attempting correlation of seagrass growth with environmental conditions or perhaps introducing species to new areas for coastal conservation or other purposes.
140 MATERIALS AND METHODS
The species used in experiments are listed in Table I. Plants, complete with leaves, rhizomes and roots, were collected by wading in shallow water or by SCUBA divers from the localities indicated. They were then transported in shaded, cool containers of seawater to the laboratory and maintained in flowing seawater aquaria under dim illumination (about 1 mW cm -2 PAR); plants were used in experiments within 12 h of collection. Photosynthesis and respiration were measured using the methods described by Drew (1978) in which sections of leaves 7 cm long, or whole leaves of Halophila stipulacea, were enclosed in 28-ml glass bottles and gently agitated for 1 h in a simple, portable incubation apparatus illustrated in Fig.1. Oxygen TABLE I Seagrasses used a n d sites o f c o l l e c t i o n for e x p e r i m e n t s
Cy modocea nodosa (Ucria) Aschers. Halophila stipulacea ( F o r s k . ) Aschers. Phyllospadix torreyi S. W a t s o n Posidonia oceanica (L.) Delile Zostera angustifolia ( H o r n e m . ) Reichb. Z. marina L (wide-leaved)
0.5 a n d 33 m d e p t h , Malta (35 ° 5 1 ' N ; 14 ° 3 0 ' E ) 0.5 m d e p t h , Malta; 2 a n d 18 m, Sinai (35° 5 1 ' N ; 14 ° 3 0 ' E ) I n t e r t i d a l , California (34 ° 1 6 ' N ; 119 ° 50' W) 5 a n d 33 m d e p t h , Malta ( 3 5 ° 5 1 ' N ; 14 ° 3 0 ' E ) Intertidal, Scotland (56 ° 27'N; 2° 53'W) 10 m d e p t h , California ( 3 4 ° 1 6 ' N ; 1 1 9 ° 5 0 ' W )
-
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---F=I .........oscillation I
linkage
Fig. 1. I n c u b a t i o n a p p a r a t u s used f o r d e t e r m i n a t i o n o f rates o f p h o t o s y n t h e s i s a n d r e s p i r a t i o n at various i r r a d i a n c e s a n d t e m p e r a t u r e s .
exchange was determined using the Winkler chemical method as modified by Drew and Robertson ( 1 9 7 4 ) and data converted to ttg carbon cm -2 h -~ assuming PQ and RQ of unity. Chlorophyll c o n t e n t was determined from the optical density at 6 4 5 and 665 nm of methanol extracts of the leaves, using the formulae of Vollenweider ( 1 9 7 4 ) with a correction factor of 1.15 for use of methanol rather than acetone.
141 RESULTS AND DISCUSSION
Photosynthesis versus irradiance Photosynthetic rates were measured at various irradiances between 20 mW cm -2 PAR (equivalent to 60% full sunlight) and 0.02 mW cm -2 PAR in all six species investigated. They all showed similar, classic photosynthesis versus itradiance (P--I) curves, were usually light-saturated (Ik of Talling, 1957) at 2--3 mW cm -2 PAR, and had correspondingly low compensation irradiances (Ic) since their dark-respiration rates were relatively low. The curves are shown in Fig. 2 and only in Halophila stipulacea was photosynthesis apparently photoinhibited at the highest irradiance used, a feature which will be discussed further. Although saturation and compensation irradiances can be estimated graphically from such P--I curves, more accurate values for Ik and Ic can be directly c o m p u t e d from net photosynthesis rates at the lower irradiances by linear regression analysis of these values, as illustrated in Fig. 3. This also allows estimation of respiratory 02 exchange at zero irradiance, providing an extrapolated measure of respiration rate. Appropriate values for the six seagrasses investigated are set out in Table II; r 2 values (goodness-of-fit coefficient) for the initial straight lines were all between 0.97 and 0.99, showing photosynthesis to be strictly proportional to irradiance. Despite a wide range of maximum photosynthetic rates (8.1--26.0 pg C cm -2 h-~), Ik values were all about 10% full sunlight and Ic a b o u t 1%. Direct comparison of results obtained under tungsten halide lamps in these experiments with natural sunlight is valid since calculations based on the photosynthetic action spectrum of Ulva indicate that green plants utilise light with the Phxllospadix ~
o_
10~/
,
_Cymodocea
~
o
Z. marina
Z. anqustifolia
c ~
¥
~
0
O ~
10 r
o
Posi donia
#_ 10
HaloRhila o
;
1'0
1'5
2'0
Fig. 2. P - - I c u r v e s f o r s i x s e a g r a s s e s a t a m b i e n t
I;
I;
2'0 mWcm-2 e n v i r o n m e n t a l temperatures.
142
~k /
m a x Dhs,
(J
-
resp. R~
-
linear regression where
y = z *mx y
= net
x
=
rn =
then
phs.
irrodionce slope
z
= y
I~
=
intercept
YN,
- Z
m ~.=
-z m
R, =
z
Fig. 3. M e t h o d o f calculating s a t u r a t i o n and c o m p e n s a t i o n irradiances f r o m P--I data. T A B L E II P h o t o s y n t h e t i c rates, r e s p i r a t i o n rates, s a t u r a t i o n a n d c o m p e n s a t i o n irradiances m e a s u r e d at a m b i e n t e n v i r o n m e n t a l t e m p e r a t u r e s in six seagrasses*
PhyUospadix torreyi Cymodocea nodosa Zostera angustifolia Zostera marina Halophila stipulacea Posidonia oceanica
Ymax
lk
lc
26.0 21.8 18.1 14.2 9.0 8.1
3.6 3.8 3.2 5.0 2.0 2.6
0.5 0.4 0.3 0.6 0.2 0.4
RI --3.8 - 2. 5 -- 1.8 - 1.7 -- 1.2 --1.3
Rd --5.3 -- 3.5 - 2.1 - 1.4 - 1.1 --1.5
Temp. (o C) 15 25 10 15 25 17
*Ymax, ~g C c m -2 h - l ; I k and I t , mW c m -2 P A R ; R 1 and Rd, #g C c m -2 h -1. E x p e r i m e n t a l details as in Fig. 2; calculations as in Fig. 3.
emission spectrum of tungsten halide lamps at least 94% as effectively as they use sunlight. Direct measurements of dark respiration (R d) are compared with extrapolated respiration rates (Rl) in Table II; the t w o estimates were generally very similar and in nearly all cases were equivalent to between 10 and 15% of the maximum photosynthetic rates. The data in Table II do not suggest that any of the six species investigated required particularly brightly illuminated environments, despite the fact that all were obtained using very shallow-growing material and t w o of the samples
143
--Phyllospadix torreyi and Zostera angustifolia -- were collected intertidally. It is perhaps surprising that the two species with particularly low maximum rates of photosynthesis -- Halophila stipulacea and Posidonia oceanica -- are also known to penetrate to at least 40 m in clear waters. The data reported are for shallow-growing plants in both cases, but Drew (1978) showed that Cymodocea nodosa and P. oceanica from 33 m both had lower rates of maximum photosynthesis than shallower material despite higher chlorophyll contents, suggesting deep-growing plants were no better adapted to use dim light than those from shallow water. A recent experiment with H. stipulacea in the Gulf of Eilat also showed that material from 18 m had slightly lower rates of photosynthesis than shallow-growing plants (see Fig.10). However, H. stipulacea and P. oceanica quoted in Table II did have the lowest Ik values measured, indicating that their maximal rates of photosynthesis could be achieved at low irradiances, whilst their relatively low respiration rates also ensured low Ic values. Photosynthesis versus temperature
Rates of photosynthesis and respiration discussed in the preceding section were measured at temperatures normally encountered in the habitats of the seagrasses concerned. The response of gross photosynthesis at saturating irradiance to a wide range of temperatures was investigated in four species, and the results are shown in Fig. 4 together with indication of their environmental temperature range and the temperature at the time of collection. All four 5040 Zostero EL
30"
0
20
,/
10-
,.a~, S
,i
Posidonio
Cymodocea
(J
,
20
"'=.... "....
10 | |
i
10
20
30
40
10
20
3o
Fig. 4. E f f e c t o f t e m p e r a t u r e s b o t h a b o v e a n d b e l o w a m b i e n t o n r a t e s o f gross p h o t o s y n thesis in f o u r seagrasses. O p e n h o r i z o n t a l b a r = r a n g e o f t e m p e r a t u r e s e x p e r i e n c e d ; vertical bar = temperature when collected.
144 showed exceptionally consistent linear increase of photosynthesis with increased temperature, with r 2 values of 0.99 or 1.00 for the linear regression lines pl ot t ed in that figure. In experiments with C y m o d o c e a n o d o s a and P o s i d o n i a oceanica, high enough temperatures were used to reach the point where thermal damage occurred, in the region 30--35 ° C. Biebl and McRoy (1971) found similar temperature optima for photosynthesis in Z o s t e r a m a r i n a in Alaska ( 3 0 - 3 5 ° C) in short-term experiments although their plants must have been accustomed to much lower water temperatures than the Mediterranean species discussed above, suggesting little adaptation of temperature maxima to environment. However, C. n o d o s a did have a slightly lower t e m per a t ure o p t i m u m in spring (30 ° C) than in summer (35 ° C), indicating slight seasonal adaptation, although its environmental temp e r a t ur e did n o t exceed 26 ° C even in summer. The Ql0 c o n c e p t is difficult to apply to such straight lines as it is primarily applicable to exponential curves; the slope factors of the linear regression analyses, shown in Table III, better describe the actual temperature--photosynthesis relationships with P h y l l o s p a d i x t o r r e y i responding most (2.44) and P o s i d o n i a o c e a n i c a least (0.55). Since prolonged p r e t r e a t m e n t of C. n o d o s a at temperatures up to the o p t i m u m had no deleterious effect on subsequent p h o t o s y n t h e t i c rates, it seems th at such short-term experiments (1 h duration) give a good indication of temperature tolerances. However, even an additional hour at temperatures above the o p t i m u m drastically reduced subsequent photosynthesis. The response o f dark-respiration rates to increased t em perat ure was considerably more erratic than that of photosynthesis, as shown by data in Fig. 5. TABLE III Regression analyses of the effect of increased temperature on rates of gross photosynthesis in four seagrasses Phyllospadix torreyi Zostera marina Cymodocea nodosa Posidonia oceanica
~. 10"
% u
p/p
/
5-
y = - 2.51 + 2.44 x y = -- 2.70 + 1.32 x y =--6.86 + 1.00 x y = + 0.65 + 0.55 x
Phy_llospadix
Z.angustifolia ~
_Cymodocea Posidonia 1'0 20 3'0 4'0 °C
Fig. 5. E f f e c t o f t e m p e r a t u r e s b o t h a b o v e a n d b e l o w a m b i e n t o n dark r e s p i r a t i o n in f o u r seagrasses.
145 Up to the m a x i m u m temperature experienced in their habitats, all four seagrasses investigated maintained a moderately low rate of respiration. The most dramatic response to higher temperatures was observed in Phyllospadix torreyi which actually had a much higher rate of respiration than the other three at all temperatures; its high rate of respiration was, however, associated with the highest rate of photosynthesis measured (Table II; Fig. 2).
Photosynthesis versus chlorophyll content Gabrielsen (1948) found that photosynthetic rates in foliage leaves were proportional to chlorophyll content up to levels of about 40 #g Chl cm -2. Seagrass leaves often vary considerably in green coloration along their lengths, from colourless sheaths to dark green mature blades, and data in Fig. 6 for various regions of Phyllospadix torreyi leaves show t h a t the rate of photosynthesis varied in direct proportion to total chlorophyll c o n t e n t up to at least 50 #g Chl cm -2. A similar gradation of coloration can also be seen in Posidonia oceanica leaves, and data in Fig. 7 indicate a linear relationship in that species up to the m a x i m u m of about 60 pg Chl cm -2. Rates of photosynthesis per unit chlorophyll are often a useful indication of the physiological state of a plant, and rates up to 1.04 #g C #g-~ Chl h -~ are shown in Table IV, although the average obtained in these experiments was about half that. Drew (1978) pointed out that in P. oceanica, summer rates of photosynthesis were considerably lower than those measured in spring, despite lower irradiance and water temperature then. This was in agreement with other workers' indications of m a x i m u m growth in this seagrass early in the year and cannot be correlated with seasonal fluctuation in nutrients, as spring maxima of phytoplankton growth in seas other than the Mediterranean often can be, since nutrient levels are always low there. Another cause of summer decline of photosynthesis, which did n o t occur in C. nodosa growing adjacent to the Posidonia meadows, had to be sought and it was found that leaf chlorophyll content also declined markedly in summer, hence the similar photosynthetic rates per unit chlorophyll recorded in Table IV for this species in spring and summer. Leaf senesPhyll0___%spadix ~n o
o
30
~'S 2o
IL
10 T2'O basal sheath
jO T iO lower leaf
50 pgChl cm-2 mid leaf
Fig. 6. Photosynthesis and chlorophyll content at various points along leaves of Phyllospadix
torreyi.
146
100 oE --
0
E ~ g E
I
N
sheath
tip
20-
15-
10-
P ,
10
,
,
,
i
20
30
40
50
,
60 pg Chl cm -2
Fig. 7. (a) Photosynthesis and chlorophyll content at various points along leaves of Posidonia oceanica. (b) Photosynthesis vs. chlorophyll content in all experiments on Posidonia oceanica. TABLE IV Gross photosynthetic rates per unit chlorophyll in six seagrasses Gross photosynthetic rate (~g C ug -1 Chl h -1)
Temp. (o C)
Season
Cymodocea nodosa
0.53 1.03
17 26
Spring Summer
Halophila stipulacea
0.75
26
Summer
PhyUospadix torreyi
0.58
16
Summer
Posidonia oceanica
0. 31 0.25
16 26
Spring Summer
Zostera angustifolia
1.04 0.60
10 12
Spring Autumn
Zostera marina
0.32
16
Summer
147 cence, probably triggered by daylength changes rather than by irradiance or temperature changes, was probably responsible for the summer reduction in chlorophyll and thence photosynthetic rates in Posidonia.
Leaf colour, chloroplast movement and photosynthesis in Halophila stipulacea In Malta, the leaves of Halophila stipulacea plants growing in unshaded positions at 0.5 m depth were almost white except in very oblique view when they appeared green. Only where the leaves overlapped at their bases, and also in the shade of Cymodocea meadows, did they appear bright green. Although this effect could be attributed to much lower chlorophyll content in the unshaded leaves, examination under the microscope showed that in the pale leaves the numerous chloroplasts were clumped tightly into a bright green mass in the centre of each cell. This condition may correspond to systrophy as described by Virgin (1964) and led to an appearance very similar to extreme plasmolysis; in bright green leaves the chloroplasts were uniformly distributed throughout the cells. Leaves which were initially green showed this clumping effect and overall pale coloration after less than 1 h exposure to bright sunlight (20 mW cm -2 PAR in mid-afternoon) and after about 2 h under the same irradiance from tungsten halide lamps. In total darkness, pale leaves with tightly clumped chloroplasts throughout returned to the bright green condition with uniformly dispersed chloroplasts only after 18--24 h, whilst cells with chloroplasts only partially clumped after short exposure to bright light showed a progressive increase in clumping during the first few hours of darkness before recovery began. Cells lower down the leaf were more susceptible to bright light, possibly because these cells were younger or because the leaves normally shade each other at the base, and cells there were not so well adapted to intense illumination as those of the upper leaf. Lipkin (1976)pointed out that in the Gulf of Eilat, Halophila stipulacea leaves growing in dim light were brighter green than those from bright sunlight, a situation reminiscent of that in Malta. A recent study of this population showed that in these plants similar chloroplast migrations occurred, leading to pale coloration of the leaves, and the effect of bright sunlight on chloroplast distribution therein over a prolonged period is illustrated in Fig. 8. These leaves were exposed to brighter light and for a longer period than those in the experiment in Malta and a further stage of clumping was observed in which the central clumps of chloroplasts moved t o the cell perimeter, taking up a position adjacent to a clump in a neighbouring cell. The initial migratory reaction of the chloroplasts in the leaves placed in total darkness may have resulted from 1--2 h moderately bright sunshine between dawn and the initial collection of plants at 08.00 h for this experiment, an effect similar to that of short periods of bright light observed in plants from Malta. Very pale leaves of Halophila can recover in total darkness to the bright green condition. Loss of colour in the leaves cannot, therefore, be due to rapid photodestruction of chlorophyll by the bright light, as its resynthesis could not occur
148
in the dark. Most of the chlorophyll must still be present in the pale leaves and the change in colour is presumably due to the clumping of the chloroplasts and resulting increase in the proportion of the leaf containing no chloroplasts. In this condition, much of the leaf transmits, scatters and reflects light nonselectively, thus appearing white. However, this reorientation of the chloroplasts does not adversely affect photosynthesis, as shown by data in Fig. 9; preexposure to bright light for a period sufficient to cause central clumping in all cells of the leaves resulted in increased photosynthesis at high irradiance, thereby reducing the photoinhibitory effect of bright light noted in Halophila in an earlier section. This may have been due to mutual shading of the chloroplasts, protecting most of them from the intense irradiance. Similarly, in an experiment to compare photosynthesis of shallow- and deep-growing Halophila, it was observed that the bright green leaves from 18 m depth became very pale after 1.25 h exposure to both 30 and 10 mW cm -2 PAR sunlight at the surface. There was, however, no reduction of photosynthesis at these irradiances in either shallow- or deep-growing leaves, as is shown by data set o u t in Fig. 10. It was shown in the preceding section that photosynthesis in seagrasses is proportional to chlorophyll content, and this is especially so at the relatively low chlorophyll content of Halophila leaves (13.7 ~g Chl cm -2 in Malta material). Thus, the increased rather than decreased rates of photosynthesis observed in pale Halophila leaves lend further support to the suggestion that chloroplast
L OH
--08.00h
NO
I
2
z.
27
hours
Fig. 8. Reorientation of chloroplasts during exposure of leaves of Halophila stipulacea to
bright sunlight. 15~f-.%
10Oh J pretreatment
o Q. (J
5
10
15
20 mWcm -2
-5
Fig. 9. Effect of pretreatment in bright light on photosynthesis in leaves of Halophila stipula-
cea.
149 20shallow
.~.o
deep ~n
~=
10-
o
5-
J
o_
5
10
15
20
25
30 mWcm-2
Fig. 10. Photosynthesis in shallow- (2 m) and deep- (18 m) growing leaves of Halophila stipulacea measured in bright sunlight.
clumping has a protective effect on the photosynthetic apparatus, and loss of leaf colour is n o t due to chlorophyll destruction b u t rather to chloroplast reorientation. CONCLUSIONS
(1) Seagrasses have classic P--I curves of plants intermediate between sun and shade adaptation with light-saturated photosynthetic rates between 26.0 and 8.1 pg C cm -2 h -1, depending on species. (2) The maximum photosynthetic rate a plant could achieve was not correlated with potential depth distribution, an intertidal species, Phyllospadix torreyi, having the highest rate and one from 33 m, Posidonia oceanica, having the lowest. (3} Light-saturated photosynthetic rates increased in direct proportion to temperature-increase up to a point between 30 and 35 ° C, above which thermal damage caused rapid reduction. (4) Respiration rates were n o t so dramatically affected by increased temperature. (5) Photosynthetic rate was closely correlated with chlorophyll content up to at least 50--60 gg Chl cm -2 and summer reduction of photosynthesis in Posidonia oceanica resulted from reduced chlorophyll content, probably due to general leaf senescence in this species in early summer. (6) Loss of colour in Halophila stipulacea leaves in bright light was due to chloroplast reorientation into tight clumps rather than chlorophyll photodestruction; it resulted in enhanced rather than reduced rates of photosynthesis and is presumably a protective mechanism.
150
REFERENCES Biebl, R. and McRoy, C.P., 1971. Plasmatic resistance and rate of respiration and photosynthesis of Zostera marina at different saiinities and temperatures. Mar. Biol., 8: 48--56. Den Hartog, C., 1970. The Seagrasses of the World. North-Holland, Amsterdam, 275 pp. Drew, KA., 1978. Factors affecting photosynthesis and its seasonal variation in the seagrasses Cymodocea nodosa (Ucria) Aschers., and Posidonia oceanica (L.) Delile in the Mediterranea~ J. Exp. Mar. Biol. Ecol., 31: 173--194. Drew, E..~ and Robertson, W.A.,~, 1974. A simple field version of the Winkler determination of dissolved oxygen. New Phytol., 73: 793--796. Gabrielsen, E.IC, 1948. Influence of light of different wavelengths on photosynthesis in foliage leaves. Physiol. Plant., 1: 113--123. Lipkin, Y., 1976. Seagrass vegetation of Sinai and Israel. In: C.P. McRoy and C. Helfferich (Eds.), Seagrass Ecosystems: A Scientific Perspective. Dekker, New York, pp. 263--293. McRoy, C.P. and McMillan, C., 1976. Production ecology and physiology of seagrasses. In: C.P. McRoy and C. Helfferich, (Ed~), Seagrass Ecosystems: A Scientific Perspective. Dekker, New York, pp. 53--87. Setchell, W.A., 1920. Geographic distribution of the marine spermatophytes. Bull. Torrey Bot. Club, 47: 563--579. Tailing, J.F., 1957. Photosynthetic characteristics of some freshwater plankton diatoms in relation to underwater radiation. New Phytol., 56: 29--50. Virgin, I-I.I., 1964. Some effects of light on chloroplasts and plant protoplasm. In: A.C. Giese (Ed.), Photophysiology, Vol. 1. Academic Press, New York and London, 282 pp. Vollenweider, R.A., 1974. A Manual of Methods of Measuring Primary Production in Aquatic Environment~ IBP Handbook No. 12, Blackwell Scientific, Oxford, 225 pp. Zieman, J.C., 1975. Seasonal variation of turtle grass, Thalassia testudinum K~nig, with reference to temperature and salinity effects. Aquat. Bot., 1: 107--123.
139
PHYSIOLOGICAL ASPECTS OF PRIMARY PRODUCTION IN SEAGRASSES
EDWARD A. DREW
Gatty Marine Laboratory, St. Andrews (Gt. Britain) (Accepted 23 April 1979)
ABSTRACT Drew, E.A., 1979. Physiological aspects of primary production in seagrasses. Aquat. Bot., 7: 139--150. P--I curves and dark-respiration data are presented for Cymodocea nodosa (Ucria) Aschers., Halophila stipulacea (Forsk.) Aschers, Phyllospadix torreyi S. Watson, Posidonia oceanica (L.) Delile, Zostera angustifolia (Hornem.) Reichb. and Z. marina L. (wide-leaved form), together with photosynthesis versus temperature data. Compensation irradiance, saturation irradiance (Ik) and photoinhibition are discussed with reference to habitat. Chloroplast movement in H. stipulacea is shown to be complex and to reduce photainhihition, whilst leaf senescence in iv. oceanica is suggested as a cause of marked seasonal variation in its photosynthetic rate, with high rates in spring and low rates in summer.
INTRODUCTION
Seagrasses grow in polar, temperate and tropical regions and as a group are thus subjected to a wide range of water temperatures (--2 to 30 ° C) and irradiance regimes. They also show very marked geographical limitations often closely correlated with these major environmental parameters, as postulated for instance by SetcheU (1920) with reference to temperature. Similarly, some seagrasses may grow at considerable depths with very dim illumination at the lower limit of perhaps 90 m for Halophila (den Hartog, 1970), whilst others grow intertidally in the tropics with very high irradiance. However, very little is known of the physiological adaptations of these superficially very similar plants. M c R o y and McMillan (1977) summarised the relevant data on the effects of temperature and light on seagrasses in only three pages. Other than observational work on tolerance of temperature extremes such as that by Zieman (1975), the only experimental data on temperature--photosynthesis relationships seem to be those of Biebl and M c R o y (1971) and Drew (1978). During the last few years I have carried o u t relevant experiments on a number of seagrasses in temperate and sub-tropical environments. This paper represents a summary of basic physiological data potentially useful to researchers attempting correlation of seagrass growth with environmental conditions or perhaps introducing species to new areas for coastal conservation or other purposes.
140 MATERIALS AND METHODS
The species used in experiments are listed in Table I. Plants, complete with leaves, rhizomes and roots, were collected by wading in shallow water or by SCUBA divers from the localities indicated. They were then transported in shaded, cool containers of seawater to the laboratory and maintained in flowing seawater aquaria under dim illumination (about 1 mW cm -2 PAR); plants were used in experiments within 12 h of collection. Photosynthesis and respiration were measured using the methods described by Drew (1978) in which sections of leaves 7 cm long, or whole leaves of Halophila stipulacea, were enclosed in 28-ml glass bottles and gently agitated for 1 h in a simple, portable incubation apparatus illustrated in Fig.1. Oxygen TABLE I Seagrasses used a n d sites o f c o l l e c t i o n for e x p e r i m e n t s
Cy modocea nodosa (Ucria) Aschers. Halophila stipulacea ( F o r s k . ) Aschers. Phyllospadix torreyi S. W a t s o n Posidonia oceanica (L.) Delile Zostera angustifolia ( H o r n e m . ) Reichb. Z. marina L (wide-leaved)
0.5 a n d 33 m d e p t h , Malta (35 ° 5 1 ' N ; 14 ° 3 0 ' E ) 0.5 m d e p t h , Malta; 2 a n d 18 m, Sinai (35° 5 1 ' N ; 14 ° 3 0 ' E ) I n t e r t i d a l , California (34 ° 1 6 ' N ; 119 ° 50' W) 5 a n d 33 m d e p t h , Malta ( 3 5 ° 5 1 ' N ; 14 ° 3 0 ' E ) Intertidal, Scotland (56 ° 27'N; 2° 53'W) 10 m d e p t h , California ( 3 4 ° 1 6 ' N ; 1 1 9 ° 5 0 ' W )
-
ware r
........... I
"'
I
wiper
motor
II
opal p e r s p ~ . . !1 f i l t e r s .............. ' . I ~ : ' tray of . . . . . . . . . . . . . . . . . . . . . bottles I water bath ........ [].....
'J-I
a
. . . . . . . . .
........... -k.I
if
' I| |~zll~ I I ~ 1
II I - - - ] ~ II •
i
a
......... overf,ow - -
| II~ii~]l I
---F=I .........oscillation I
linkage
Fig. 1. I n c u b a t i o n a p p a r a t u s used f o r d e t e r m i n a t i o n o f rates o f p h o t o s y n t h e s i s a n d r e s p i r a t i o n at various i r r a d i a n c e s a n d t e m p e r a t u r e s .
exchange was determined using the Winkler chemical method as modified by Drew and Robertson ( 1 9 7 4 ) and data converted to ttg carbon cm -2 h -~ assuming PQ and RQ of unity. Chlorophyll c o n t e n t was determined from the optical density at 6 4 5 and 665 nm of methanol extracts of the leaves, using the formulae of Vollenweider ( 1 9 7 4 ) with a correction factor of 1.15 for use of methanol rather than acetone.
141 RESULTS AND DISCUSSION
Photosynthesis versus irradiance Photosynthetic rates were measured at various irradiances between 20 mW cm -2 PAR (equivalent to 60% full sunlight) and 0.02 mW cm -2 PAR in all six species investigated. They all showed similar, classic photosynthesis versus itradiance (P--I) curves, were usually light-saturated (Ik of Talling, 1957) at 2--3 mW cm -2 PAR, and had correspondingly low compensation irradiances (Ic) since their dark-respiration rates were relatively low. The curves are shown in Fig. 2 and only in Halophila stipulacea was photosynthesis apparently photoinhibited at the highest irradiance used, a feature which will be discussed further. Although saturation and compensation irradiances can be estimated graphically from such P--I curves, more accurate values for Ik and Ic can be directly c o m p u t e d from net photosynthesis rates at the lower irradiances by linear regression analysis of these values, as illustrated in Fig. 3. This also allows estimation of respiratory 02 exchange at zero irradiance, providing an extrapolated measure of respiration rate. Appropriate values for the six seagrasses investigated are set out in Table II; r 2 values (goodness-of-fit coefficient) for the initial straight lines were all between 0.97 and 0.99, showing photosynthesis to be strictly proportional to irradiance. Despite a wide range of maximum photosynthetic rates (8.1--26.0 pg C cm -2 h-~), Ik values were all about 10% full sunlight and Ic a b o u t 1%. Direct comparison of results obtained under tungsten halide lamps in these experiments with natural sunlight is valid since calculations based on the photosynthetic action spectrum of Ulva indicate that green plants utilise light with the Phxllospadix ~
o_
10~/
,
_Cymodocea
~
o
Z. marina
Z. anqustifolia
c ~
¥
~
0
O ~
10 r
o
Posi donia
#_ 10
HaloRhila o
;
1'0
1'5
2'0
Fig. 2. P - - I c u r v e s f o r s i x s e a g r a s s e s a t a m b i e n t
I;
I;
2'0 mWcm-2 e n v i r o n m e n t a l temperatures.
142
~k /
m a x Dhs,
(J
-
resp. R~
-
linear regression where
y = z *mx y
= net
x
=
rn =
then
phs.
irrodionce slope
z
= y
I~
=
intercept
YN,
- Z
m ~.=
-z m
R, =
z
Fig. 3. M e t h o d o f calculating s a t u r a t i o n and c o m p e n s a t i o n irradiances f r o m P--I data. T A B L E II P h o t o s y n t h e t i c rates, r e s p i r a t i o n rates, s a t u r a t i o n a n d c o m p e n s a t i o n irradiances m e a s u r e d at a m b i e n t e n v i r o n m e n t a l t e m p e r a t u r e s in six seagrasses*
PhyUospadix torreyi Cymodocea nodosa Zostera angustifolia Zostera marina Halophila stipulacea Posidonia oceanica
Ymax
lk
lc
26.0 21.8 18.1 14.2 9.0 8.1
3.6 3.8 3.2 5.0 2.0 2.6
0.5 0.4 0.3 0.6 0.2 0.4
RI --3.8 - 2. 5 -- 1.8 - 1.7 -- 1.2 --1.3
Rd --5.3 -- 3.5 - 2.1 - 1.4 - 1.1 --1.5
Temp. (o C) 15 25 10 15 25 17
*Ymax, ~g C c m -2 h - l ; I k and I t , mW c m -2 P A R ; R 1 and Rd, #g C c m -2 h -1. E x p e r i m e n t a l details as in Fig. 2; calculations as in Fig. 3.
emission spectrum of tungsten halide lamps at least 94% as effectively as they use sunlight. Direct measurements of dark respiration (R d) are compared with extrapolated respiration rates (Rl) in Table II; the t w o estimates were generally very similar and in nearly all cases were equivalent to between 10 and 15% of the maximum photosynthetic rates. The data in Table II do not suggest that any of the six species investigated required particularly brightly illuminated environments, despite the fact that all were obtained using very shallow-growing material and t w o of the samples
143
--Phyllospadix torreyi and Zostera angustifolia -- were collected intertidally. It is perhaps surprising that the two species with particularly low maximum rates of photosynthesis -- Halophila stipulacea and Posidonia oceanica -- are also known to penetrate to at least 40 m in clear waters. The data reported are for shallow-growing plants in both cases, but Drew (1978) showed that Cymodocea nodosa and P. oceanica from 33 m both had lower rates of maximum photosynthesis than shallower material despite higher chlorophyll contents, suggesting deep-growing plants were no better adapted to use dim light than those from shallow water. A recent experiment with H. stipulacea in the Gulf of Eilat also showed that material from 18 m had slightly lower rates of photosynthesis than shallow-growing plants (see Fig.10). However, H. stipulacea and P. oceanica quoted in Table II did have the lowest Ik values measured, indicating that their maximal rates of photosynthesis could be achieved at low irradiances, whilst their relatively low respiration rates also ensured low Ic values. Photosynthesis versus temperature
Rates of photosynthesis and respiration discussed in the preceding section were measured at temperatures normally encountered in the habitats of the seagrasses concerned. The response of gross photosynthesis at saturating irradiance to a wide range of temperatures was investigated in four species, and the results are shown in Fig. 4 together with indication of their environmental temperature range and the temperature at the time of collection. All four 5040 Zostero EL
30"
0
20
,/
10-
,.a~, S
,i
Posidonio
Cymodocea
(J
,
20
"'=.... "....
10 | |
i
10
20
30
40
10
20
3o
Fig. 4. E f f e c t o f t e m p e r a t u r e s b o t h a b o v e a n d b e l o w a m b i e n t o n r a t e s o f gross p h o t o s y n thesis in f o u r seagrasses. O p e n h o r i z o n t a l b a r = r a n g e o f t e m p e r a t u r e s e x p e r i e n c e d ; vertical bar = temperature when collected.
144 showed exceptionally consistent linear increase of photosynthesis with increased temperature, with r 2 values of 0.99 or 1.00 for the linear regression lines pl ot t ed in that figure. In experiments with C y m o d o c e a n o d o s a and P o s i d o n i a oceanica, high enough temperatures were used to reach the point where thermal damage occurred, in the region 30--35 ° C. Biebl and McRoy (1971) found similar temperature optima for photosynthesis in Z o s t e r a m a r i n a in Alaska ( 3 0 - 3 5 ° C) in short-term experiments although their plants must have been accustomed to much lower water temperatures than the Mediterranean species discussed above, suggesting little adaptation of temperature maxima to environment. However, C. n o d o s a did have a slightly lower t e m per a t ure o p t i m u m in spring (30 ° C) than in summer (35 ° C), indicating slight seasonal adaptation, although its environmental temp e r a t ur e did n o t exceed 26 ° C even in summer. The Ql0 c o n c e p t is difficult to apply to such straight lines as it is primarily applicable to exponential curves; the slope factors of the linear regression analyses, shown in Table III, better describe the actual temperature--photosynthesis relationships with P h y l l o s p a d i x t o r r e y i responding most (2.44) and P o s i d o n i a o c e a n i c a least (0.55). Since prolonged p r e t r e a t m e n t of C. n o d o s a at temperatures up to the o p t i m u m had no deleterious effect on subsequent p h o t o s y n t h e t i c rates, it seems th at such short-term experiments (1 h duration) give a good indication of temperature tolerances. However, even an additional hour at temperatures above the o p t i m u m drastically reduced subsequent photosynthesis. The response o f dark-respiration rates to increased t em perat ure was considerably more erratic than that of photosynthesis, as shown by data in Fig. 5. TABLE III Regression analyses of the effect of increased temperature on rates of gross photosynthesis in four seagrasses Phyllospadix torreyi Zostera marina Cymodocea nodosa Posidonia oceanica
~. 10"
% u
p/p
/
5-
y = - 2.51 + 2.44 x y = -- 2.70 + 1.32 x y =--6.86 + 1.00 x y = + 0.65 + 0.55 x
Phy_llospadix
Z.angustifolia ~
_Cymodocea Posidonia 1'0 20 3'0 4'0 °C
Fig. 5. E f f e c t o f t e m p e r a t u r e s b o t h a b o v e a n d b e l o w a m b i e n t o n dark r e s p i r a t i o n in f o u r seagrasses.
145 Up to the m a x i m u m temperature experienced in their habitats, all four seagrasses investigated maintained a moderately low rate of respiration. The most dramatic response to higher temperatures was observed in Phyllospadix torreyi which actually had a much higher rate of respiration than the other three at all temperatures; its high rate of respiration was, however, associated with the highest rate of photosynthesis measured (Table II; Fig. 2).
Photosynthesis versus chlorophyll content Gabrielsen (1948) found that photosynthetic rates in foliage leaves were proportional to chlorophyll content up to levels of about 40 #g Chl cm -2. Seagrass leaves often vary considerably in green coloration along their lengths, from colourless sheaths to dark green mature blades, and data in Fig. 6 for various regions of Phyllospadix torreyi leaves show t h a t the rate of photosynthesis varied in direct proportion to total chlorophyll c o n t e n t up to at least 50 #g Chl cm -2. A similar gradation of coloration can also be seen in Posidonia oceanica leaves, and data in Fig. 7 indicate a linear relationship in that species up to the m a x i m u m of about 60 pg Chl cm -2. Rates of photosynthesis per unit chlorophyll are often a useful indication of the physiological state of a plant, and rates up to 1.04 #g C #g-~ Chl h -~ are shown in Table IV, although the average obtained in these experiments was about half that. Drew (1978) pointed out that in P. oceanica, summer rates of photosynthesis were considerably lower than those measured in spring, despite lower irradiance and water temperature then. This was in agreement with other workers' indications of m a x i m u m growth in this seagrass early in the year and cannot be correlated with seasonal fluctuation in nutrients, as spring maxima of phytoplankton growth in seas other than the Mediterranean often can be, since nutrient levels are always low there. Another cause of summer decline of photosynthesis, which did n o t occur in C. nodosa growing adjacent to the Posidonia meadows, had to be sought and it was found that leaf chlorophyll content also declined markedly in summer, hence the similar photosynthetic rates per unit chlorophyll recorded in Table IV for this species in spring and summer. Leaf senesPhyll0___%spadix ~n o
o
30
~'S 2o
IL
10 T2'O basal sheath
jO T iO lower leaf
50 pgChl cm-2 mid leaf
Fig. 6. Photosynthesis and chlorophyll content at various points along leaves of Phyllospadix
torreyi.
146
100 oE --
0
E ~ g E
I
N
sheath
tip
20-
15-
10-
P ,
10
,
,
,
i
20
30
40
50
,
60 pg Chl cm -2
Fig. 7. (a) Photosynthesis and chlorophyll content at various points along leaves of Posidonia oceanica. (b) Photosynthesis vs. chlorophyll content in all experiments on Posidonia oceanica. TABLE IV Gross photosynthetic rates per unit chlorophyll in six seagrasses Gross photosynthetic rate (~g C ug -1 Chl h -1)
Temp. (o C)
Season
Cymodocea nodosa
0.53 1.03
17 26
Spring Summer
Halophila stipulacea
0.75
26
Summer
PhyUospadix torreyi
0.58
16
Summer
Posidonia oceanica
0. 31 0.25
16 26
Spring Summer
Zostera angustifolia
1.04 0.60
10 12
Spring Autumn
Zostera marina
0.32
16
Summer
147 cence, probably triggered by daylength changes rather than by irradiance or temperature changes, was probably responsible for the summer reduction in chlorophyll and thence photosynthetic rates in Posidonia.
Leaf colour, chloroplast movement and photosynthesis in Halophila stipulacea In Malta, the leaves of Halophila stipulacea plants growing in unshaded positions at 0.5 m depth were almost white except in very oblique view when they appeared green. Only where the leaves overlapped at their bases, and also in the shade of Cymodocea meadows, did they appear bright green. Although this effect could be attributed to much lower chlorophyll content in the unshaded leaves, examination under the microscope showed that in the pale leaves the numerous chloroplasts were clumped tightly into a bright green mass in the centre of each cell. This condition may correspond to systrophy as described by Virgin (1964) and led to an appearance very similar to extreme plasmolysis; in bright green leaves the chloroplasts were uniformly distributed throughout the cells. Leaves which were initially green showed this clumping effect and overall pale coloration after less than 1 h exposure to bright sunlight (20 mW cm -2 PAR in mid-afternoon) and after about 2 h under the same irradiance from tungsten halide lamps. In total darkness, pale leaves with tightly clumped chloroplasts throughout returned to the bright green condition with uniformly dispersed chloroplasts only after 18--24 h, whilst cells with chloroplasts only partially clumped after short exposure to bright light showed a progressive increase in clumping during the first few hours of darkness before recovery began. Cells lower down the leaf were more susceptible to bright light, possibly because these cells were younger or because the leaves normally shade each other at the base, and cells there were not so well adapted to intense illumination as those of the upper leaf. Lipkin (1976)pointed out that in the Gulf of Eilat, Halophila stipulacea leaves growing in dim light were brighter green than those from bright sunlight, a situation reminiscent of that in Malta. A recent study of this population showed that in these plants similar chloroplast migrations occurred, leading to pale coloration of the leaves, and the effect of bright sunlight on chloroplast distribution therein over a prolonged period is illustrated in Fig. 8. These leaves were exposed to brighter light and for a longer period than those in the experiment in Malta and a further stage of clumping was observed in which the central clumps of chloroplasts moved t o the cell perimeter, taking up a position adjacent to a clump in a neighbouring cell. The initial migratory reaction of the chloroplasts in the leaves placed in total darkness may have resulted from 1--2 h moderately bright sunshine between dawn and the initial collection of plants at 08.00 h for this experiment, an effect similar to that of short periods of bright light observed in plants from Malta. Very pale leaves of Halophila can recover in total darkness to the bright green condition. Loss of colour in the leaves cannot, therefore, be due to rapid photodestruction of chlorophyll by the bright light, as its resynthesis could not occur
148
in the dark. Most of the chlorophyll must still be present in the pale leaves and the change in colour is presumably due to the clumping of the chloroplasts and resulting increase in the proportion of the leaf containing no chloroplasts. In this condition, much of the leaf transmits, scatters and reflects light nonselectively, thus appearing white. However, this reorientation of the chloroplasts does not adversely affect photosynthesis, as shown by data in Fig. 9; preexposure to bright light for a period sufficient to cause central clumping in all cells of the leaves resulted in increased photosynthesis at high irradiance, thereby reducing the photoinhibitory effect of bright light noted in Halophila in an earlier section. This may have been due to mutual shading of the chloroplasts, protecting most of them from the intense irradiance. Similarly, in an experiment to compare photosynthesis of shallow- and deep-growing Halophila, it was observed that the bright green leaves from 18 m depth became very pale after 1.25 h exposure to both 30 and 10 mW cm -2 PAR sunlight at the surface. There was, however, no reduction of photosynthesis at these irradiances in either shallow- or deep-growing leaves, as is shown by data set o u t in Fig. 10. It was shown in the preceding section that photosynthesis in seagrasses is proportional to chlorophyll content, and this is especially so at the relatively low chlorophyll content of Halophila leaves (13.7 ~g Chl cm -2 in Malta material). Thus, the increased rather than decreased rates of photosynthesis observed in pale Halophila leaves lend further support to the suggestion that chloroplast
L OH
--08.00h
NO
I
2
z.
27
hours
Fig. 8. Reorientation of chloroplasts during exposure of leaves of Halophila stipulacea to
bright sunlight. 15~f-.%
10Oh J pretreatment
o Q. (J
5
10
15
20 mWcm -2
-5
Fig. 9. Effect of pretreatment in bright light on photosynthesis in leaves of Halophila stipula-
cea.
149 20shallow
.~.o
deep ~n
~=
10-
o
5-
J
o_
5
10
15
20
25
30 mWcm-2
Fig. 10. Photosynthesis in shallow- (2 m) and deep- (18 m) growing leaves of Halophila stipulacea measured in bright sunlight.
clumping has a protective effect on the photosynthetic apparatus, and loss of leaf colour is n o t due to chlorophyll destruction b u t rather to chloroplast reorientation. CONCLUSIONS
(1) Seagrasses have classic P--I curves of plants intermediate between sun and shade adaptation with light-saturated photosynthetic rates between 26.0 and 8.1 pg C cm -2 h -1, depending on species. (2) The maximum photosynthetic rate a plant could achieve was not correlated with potential depth distribution, an intertidal species, Phyllospadix torreyi, having the highest rate and one from 33 m, Posidonia oceanica, having the lowest. (3} Light-saturated photosynthetic rates increased in direct proportion to temperature-increase up to a point between 30 and 35 ° C, above which thermal damage caused rapid reduction. (4) Respiration rates were n o t so dramatically affected by increased temperature. (5) Photosynthetic rate was closely correlated with chlorophyll content up to at least 50--60 gg Chl cm -2 and summer reduction of photosynthesis in Posidonia oceanica resulted from reduced chlorophyll content, probably due to general leaf senescence in this species in early summer. (6) Loss of colour in Halophila stipulacea leaves in bright light was due to chloroplast reorientation into tight clumps rather than chlorophyll photodestruction; it resulted in enhanced rather than reduced rates of photosynthesis and is presumably a protective mechanism.
150
REFERENCES Biebl, R. and McRoy, C.P., 1971. Plasmatic resistance and rate of respiration and photosynthesis of Zostera marina at different saiinities and temperatures. Mar. Biol., 8: 48--56. Den Hartog, C., 1970. The Seagrasses of the World. North-Holland, Amsterdam, 275 pp. Drew, KA., 1978. Factors affecting photosynthesis and its seasonal variation in the seagrasses Cymodocea nodosa (Ucria) Aschers., and Posidonia oceanica (L.) Delile in the Mediterranea~ J. Exp. Mar. Biol. Ecol., 31: 173--194. Drew, E..~ and Robertson, W.A.,~, 1974. A simple field version of the Winkler determination of dissolved oxygen. New Phytol., 73: 793--796. Gabrielsen, E.IC, 1948. Influence of light of different wavelengths on photosynthesis in foliage leaves. Physiol. Plant., 1: 113--123. Lipkin, Y., 1976. Seagrass vegetation of Sinai and Israel. In: C.P. McRoy and C. Helfferich (Eds.), Seagrass Ecosystems: A Scientific Perspective. Dekker, New York, pp. 263--293. McRoy, C.P. and McMillan, C., 1976. Production ecology and physiology of seagrasses. In: C.P. McRoy and C. Helfferich, (Ed~), Seagrass Ecosystems: A Scientific Perspective. Dekker, New York, pp. 53--87. Setchell, W.A., 1920. Geographic distribution of the marine spermatophytes. Bull. Torrey Bot. Club, 47: 563--579. Tailing, J.F., 1957. Photosynthetic characteristics of some freshwater plankton diatoms in relation to underwater radiation. New Phytol., 56: 29--50. Virgin, I-I.I., 1964. Some effects of light on chloroplasts and plant protoplasm. In: A.C. Giese (Ed.), Photophysiology, Vol. 1. Academic Press, New York and London, 282 pp. Vollenweider, R.A., 1974. A Manual of Methods of Measuring Primary Production in Aquatic Environment~ IBP Handbook No. 12, Blackwell Scientific, Oxford, 225 pp. Zieman, J.C., 1975. Seasonal variation of turtle grass, Thalassia testudinum K~nig, with reference to temperature and salinity effects. Aquat. Bot., 1: 107--123.