Screening of Freshwater Algae (Chlorophyta, Chromophyta) for Ultraviolet-B Sensitivity of the Photosynthetic Apparatus

j. Plant Phy5ioL. WlL. 148. pp. 42-48 (1996)

Screening of Freshwater Algae (Chlorophyta, Chromophyta) for Ultraviolet-B Sensitivity of the Photosynthetic Apparatus 1 FUSHENG XIONG ,2,3, FILIP LEDERER1,2, ]AROMfR LUKAvsral, 1

and

1 UDISLAV NEDBAL ,4

Institute of Microbiology, National Research Centre Global Climate Change and Photosynthesis, Opatovicky mlyn, CZ-37981 Hebon, Czech Republic .

2

University of South Bohemia, Faculty of Biological Sciences, Branisovska 31,

3

on leave from: Yangzhou University, People's Republic of China

4

to whom the correspondence should be sent

C. Budejovice, Czech Republic

Received June 26, 1995 . Accepted October 24, 1995

Summary

Sixty-seven species of algae (Chlorophyta, Chromophyta - Xanthophyceae) were used in a screening experiment in which the UV-B sensitivity of the photosynthetic apparatus was assessed. The species were selected to represent largely different natural environments ranging from mountain snow and high elevation lakes on one extreme and shaded thermal springs and soil on another. The study was aimed at the identification of algal species whose photosynthetic apparatus was either extremely sensitive or resistant to enhanced UVB radiation. Samples of algae were exposed to 2 W·m -2 of artificial UV-B radiation for 2 hours. The response of the algae to the UV-B exposure was monitored by following changes in photosynthetic oxygen evolution rates and changes in the PAM fluorescence emission. The most UV-B susceptible algal species lost 30-50% of their oxygen evolving capacity during the UV-B exposure. On the other extreme, UV-B exposure stimulated the oxygen evolving capacity of some algal species by as much as 20 %. The emission of chlorophyll fluorescence from algae was also modified by the UV-B exposure with Fm declining and Fo rising. The quantitative correlation between the UV-B-induced changes in oxygen evolving capacity and the fluorescence parameters was found to be weak for Fo and for photochemical quenching (qp), while, in most species, the correlation was strong for F)F m and e. The impact of the UV-B exposure on the capacity of the algae to develop non-photochemical quenching was largely species-dependent. Most UV-B-resistant algae have large cell cross-sections or are growing in large-sized colonies or cenobia that provide effective shading of the internal structures. Alternatively, UV-B-sensitive species tended to be small sized or filamentous. A high fraction of UV-B-resistant species was found among algae originally isolated from mountainous, sun-exposed locations. They have frequently a solid cell wall containing sporopollenin.

Key words: Algae, Chifluorescence, photosynthesis, WE radiation, sporopollenin. Abbreviations: Chi alb = chlorophyll alb; Fm = maximal fluorescence emitted with the primary quinone acceptor of Photosystem II reduced and quenching mechanisms ineffective; F0 = fluorescence emitted with the primary quinone acceptor of Photosystem II oxidised; Fv = variable fluorescence equals Fm-Fo; PSII = Photosystem II; OA = primary quinone acceptor of Photosystem II; qp = photochemical quenching of fluorescence; qNP = non-photochemical quenching of fluorescence; UV-B = ultraviolet-B radiation (280 - 320 nm); e = quantum efficiency oflinear electron transport estimated from fluorescence emission. © 1996 by Gustav Fischer Verlag, Stuttgart

Screening of algae for UV-B sensitivity

Introduction Anthropogenic depletion of stratospheric ozone layer results in increased levels of UV-B radiation reaching both terrestrial and aquatic ecosystems (Frederick and Lubin, 1988; Stolarski, 1988; Caldwell et aI., 1989, Hader and Worrest, 1991; Cullen and Neale, 1994). Assessment of the present impact of elevated UV-B radiation on autotrophic organisms as well as the prediction of future effects on ecosystems are the focus of numerous studies (for reviews see e.g. Tevini and Teramura, 1989; Hader, 1993; Karentz, 1994). Elevated UV-B radiation was found to affect various fundamental processes of plant cells e.g. photosynthesis (Bornman, 1989; Neale et al., 1993; Dohler and Haas et al., 1995), cell cycle (Behrenfeld et aI., 1992), growth and development (e.g. Bothwell et aI., 1993) and nitrogen assimilation (Dohler et aI., 1991). During evolution, plants have developed several mechanisms that provide protection against the relatively low ambient flux of UV-B photons occurring under the ozone layer (Bornman, 1989, Hader, 1993; Karentz, 1994). The damage caused by elevated UV-B exposure depends therefore on a complex interplay between the UV-B dose, the effectiveness of plant protection mechanisms and the sensitivity of the plant to UV-B. Tevini and Teramura (1989) reviewed screening of about 300 species of annual terrestrial plants for UV-B sensitivity and found a large amount of variability with about half of the species susceptible and half resistant. A wide range of interspecific UV-B sensitivity was also found in marine diatoms (Calkins and Thordardouir, 1980; Karentz et al., 1991; Lesser et aI., 1994; ) and sea macroalgae and seagrasses (Larkum and Wood, 1993). Microscopic algae play an important role in terrestrial as well as in freshwater and marine ecosystems. Parallel to their function in nature, algal biotechnology is of increasing importance. The impact of elevated UV-B radiation on the capacity of microscopic freshwater algae to maintain these functions is difficult to predict because of the scarcity of data on the UV-B susceptibility of these organisms. The available information concerning algae is mainly focused on marine organisms and ecosystems (Smith et al., 1992; Cullen and Neale, 1994; Karentz, 1994, Williamson and Zagarese, 1994). Taking into account the large variability in UV-B responses found in other organisms, screening of a large set of algal species is expected to provide information useful both in algal ecophysiology and biotechnology. Equally important, results of such a screening can enhance the potential of microscopic freshwater algae as relatively simple model organisms in the research ofUV-B damage and photoprotection. We have chosen a two-step approach: In the first phase of the experiment, which is reported here, we used a large number o~ al~al species su~ected.t~ a relatively high dose of UV-B radiation (2 W·m- admlllistered for 2 hours). In order to limit the complexity of the problem, the present study focused only on the response of the photosynthetic apparatus of the algae to a telatively high dose of UV-B radiation. The results of this study allow us to continue the project with a second phase of experiments, in which, the impact of realistic, long term UV-B exposure will be investigated using sets of resistant and susceptible species identified here.

43

Materials and Methods

Algae The 67 algal species used in the screening were obtained from the Culture Collection of Autotrophic Organisms of the Institute of Botany, Teebon (Lukavsky et al., 1992). The unialgal cultures listed in Table 1 were collected from ag'!rslants and transferred into 50 mL of growth medium after Zehnde'r (see Lukavsky et al., 1992 for composition of individual media). Cultures were then exposed to 10 W.m- 2 photosynthetically active radiation (PAR). After 10-15 days, the preinoculum cultures were transferred from Ehrlenmayer flasks to cylindrical tubes with 50 mL of fresh mc;dia. The algal suspension was bubbled with air enriched with 2 % COz, kept at 26 ·c and exposed to 1OW.m- 2 of PAR. After another 2-3 days, 100mL of fresh media were added and the irradiance was increased to 25 W.m- 2 • After 3-4 additional days, the cultures were in the exponential phase of growth.

UV-B exposure Exponentially growing cultures were diluted to 8-15 JlM Chi and placed into Petri dishes (50 mL, 7x 1.5 cm). The algal suspension was initially pretreated for 15 minutes at constant temperature (26 ± 2 'C) and irradiance (25 W.m- 2 PAR). After this pretreatment, UV-B radiation was added, which was produced by a single fluorescent tube (Philips TL20W/12) placed above algal suspension. The UV-C radiation was blocked by aged cellulose acetate filter (31 1000 inch thickness) that was regularly replaced after 20 hours of service. The ability of cellulose acetate filters to block the UV-C radiation was checked using a Shimadzu UV-3000 spectrophotometer. The unweighted UV-B irradiance incident at the surface of the algal suspension was measured by International Light SED 240 detector and adjusted to 2 W·m -2 by changing the distance between the UVB light and algal suspension. In order to avoid confusion of the UVB response and UV-A or visible light induced inhibition, control experiments were done with Mylar filters. The Mylar filter blocks both UV-C and UV-B and is transparent to visible and UV-A photons. No deteriorating effects were seen under Mylar protection when UV-B sensitive strains were exposed.

Photosynthetic oxygen evolution and Chi fluorescence Aliquots of suspension (6 mL) were taken during the UV-B exposure to measure oxygen evolution and Chi fluorescence. The sample was placed in a temperature-controlled cuvette (28 'C) and HC0 3 - was added to final concentration of 20 mM. After a dark adaptation of 10 minutes, Chi fluorescence parameters Fo and Fm were measured with a PAM fluorimeter (Walz, Germany, Schreiber et al., 1986) based on a protocol similar to that ofTing and Owens (1992) and Hofstraat et al. (1994). Using the same sample, the COzdependent. oxyge~ evolution was measured (YSI Clark-~e ~lec­ trode) dunng 5 mm of exposure of the algae to 150W·m- of hght from a Xenon lamp filtered by 2 % CUS04 solution. During this actinic light exposure, nearly steady-state fluorescence emission was established and the F" Fm ' and Fo' parameters were determined for calculation of the non-photochemical (qNP) and photochemical (qp) quenching (Schreiber et al., 1986) and of the e: the quantum efficiency of electron flow through PSII (Genty et al., 1989). The UV-B exposure as well as the accompanying oxygen and fluorescence measurements were repeated 3-5 times for each algal species and means as well as standard deviations were calculated for each of the measured parameters.

44

FUSHENG XIONG, FILIP LEDERER, JAROMiR LUKAVSKY, and UDISLAV NEDBAL

Table 1: The list of screened algal species*. No.

Strain identification

Note

Chlorophyta, Chlamydophyceae, Chlamydomorlluulles 1 Chlamydomonas chlorococcoitks, strain Schwarz 1975 2 C tkbaryana, strain Ettll960/4 3 C macropyrmoidosa, strain Hiibel1964/182 4 C peter.fii, strain Holubcova 1959/3

Yugoslavia, soil CS, mts. forest soil D, forest soil CS, mts., meadow

Chlorophyta, Chlamydophyceae, Chlorococcales 1 Characium sieboldii, strain Hindak 1963/70 2 C starrii. srrain Starr 1951/UTEX, 112 (strain-) 3 C terrestre. strain Trainor et Bold 1953/Gotr. 209-2 4 Chlorococcum echinozygotum, strain Starr 1965/118 5 C elbrose, srrain Hindak 1969/81 6 C ellipsoitkum. srrain Kovacik 1977/11 7 C hypnosporum. strain Pringsheim 1940/Camb. 237-1 8 C minutum. stain Bold/Camb. 213-7 9 C scabellum, strain Hindak 1969/125 10 C vacuolatum, srrain Starr 1952/UTEX 110 11 Chlorolunula sp. strain Kovacik 1988/5 12 Chlorosarcinopsis aggregata. srrain ARCE/UTEX 779 13 C minuta, strain Lukdova 1987/4

CS, mts. snow South Mrica, soil USA, Georgia, soil Phillipines, soil CS, lake CS, thermo spring U.K., soil India, soil CS, soil South Mrica, soil CS, mts., pool Cuba, soil CS, meadow soil

Chlorophyta. Chlorophyceae. Chlorellales 1 Ankistrotksmus spiralis, srrain Chrisrensen 1948/4879 2 Chlorella cf. minutissima, strain Cassie/Camb. 211-52 3 C sorokiniana, strain Prat et Basler, Praha AC.AI4 4 Choricystis sp. strain Lukdova 1988/8 5 Coelastrella multistriata, strain Kalina 1967/9 6 C multistriata. strain Trenkwalder 1975/Inns.T88 7 Dictyosphaerium pulchellum, strain Kovacik 1983/7 8 D. cf. tetrachotomum, strain Fott 1959/1 9 Diplosphaera cf. chodatii. strain Lukdova 1988/6 10 Enallax coelastroitks, strain Kalina 1966/1 11 E. sp., strain Kovacik 1984/ 12 Monoraphidium convolutum, strain Komarek 1964/28 13 M. convolutum. strain Komarek 1964/110 14 M. tortile, strain Hindak 1963/104 15 Pseudococcomyxa. sp., strain KovaCik 1977/4 16

17 18 19 20 21 22 23

24 25

26

P. sp., strain Wydrzycka 1981/GC-10 Raphidocelis inclinata, strain Pringsheim 1939/Gott. 243-1 R. valida. strain George/Camb. 243-2 Scmetksmus cf. corallinus, strain KovaCik 1988/4 S. sp., strain NEtAS 1965/N-508, GREI./I5, UV mutant Scotiella chlorelloitka, strain Komarek 1961/2 Scotiellopsis rubescens, strain Vinarzerllnns. V S. terrestris, strain Hindak 1963/59 Sphaerocystis bilobata, strain Lukdova 1987/3 Tetraedron minimum. strain Hindak 1964/24 Willea sp. strain Kovacik 1987/12

DK, freshwater N. Zealand, water CS, thermo spring CS, soil CS, mts., peat bog Italy, mts. soil CS, mts., lake CS, mts. CS, forest, soil CS, mts. meadow CS, mts., peat bog Cuba, pool Cuba, soil/dry pool CS, mts., soil CS, thermo spring Costa Rica, soil U.K., soil U. K. freshwater CS, mts. valley UV selected. U. K., subaerophyt CH, mts. soil CS, mts. snow CS, field soil CS, mts., wett stone wall CS, mts. lake

Chlorophyta. Chlorophyceae. Protosiphonales 1 Spongiochloris excmtrica, strain Bold/UTEX 108 2 S. excmtrica. strain Lukdova 1988/5 3 S. spongiosa, strain Vischer 1942/318

USA, soil CS, field soil CH,soil

Chlorophyta, Chlorophyceae. Chaetophonales 1 Fritschiella tuberosa, strain Andrews/112.80 2 Stigeocloniumsp., strain Gardavsky 1985/13

USA, field soil CS, fishpond

Chlorophyta, Pleurastrophyceae. Chlorosarchinales 1 Myrmecia bisecta, strain Lukdova 1987/10 2 Neochloris bilobata, strain Trenkwald. 1975/Inns.T58 3 Pleurastrum sarcinoitkum, strain Lukdova 1986/11

CS, meadow soil Italy, mts. soil CS, meadow soil

UV-B Response

+

+ +++

+

+

+++ +++ +

+

+++ +++ + + +++

+ +

45

Screening of algae for UV-B sensitivity

Table 1: Continued. Strain identification

No.

Note

Chlorophyta, Ulvophyceae, Ulotrichales 1 Interfilum paradoxum, strain Pringsheim/UTEX 177 2 Rhexinema errumpens, strain Lukdova 1987/8

U. K., soil CS, forest soil

Chlorophyta, Ulvophyceae, Neochloridales 1 Chlorotetraedron bitridem, strain starr 19521120 2 C polymorphum, strain DEAN/42

USA, soil Australia, soil

Chlorophyta, Charophyceae, Klebsormidiales 1 Chlorhormidium flacciolum, strain Hindik 1965/96 2 C sp., strain Kovacik 1983/6 3 Stichococcus exiguus, strain Komarek 1962/1

Cuba, garden soil CS, mts. lake, benthos CS, mts. snow

Chromophyta, Xanthophyceae (Heterokontae), Miscococcales 1 Botrydiopsis alpina, strain Vischer 1949/232 2 B. alpina, strain Vinatzer 1975/lnns.V181 3 Chloridella neglecta, strain Vischer 1940/216 4 C simplex, strain Hindik 1962116 5 Chlorobotrys regularis, strain Flint 1964/2 6 Heterococcus brevicellularis, strain Vischer 1945/351

CH,soil Italy, mts., soil CH, soil, meadow CS, mts., snow Antarctis, soil CH, soil, forest

Chromophyta, Xanthophyceae (Heterokontae), Tribonematales 1 Bumilleriopsis filiformis, strain Vischer 1943/360 2 Tribonema aequale, strain Pringsheim/Cambr. 880-1 3 Xanthonema bristolianum, strain Hindik 1966/38

CH,soil CS, soil CS, mts. snow

UV-B Response

+

+ +

+

* UV-B response is identified based on the relative change of photosynthetic oxygen evolution capacity caused by 90-min UV-B exposure. For Resistant: +++: stimulated more than 10%; +: change between -10 % and + 10%; For Sensitive: ---: reduced more than -40 %; -: reduced between -40 % and -10 %. CS = Czechoslovakia.

Results

Oxygen evolving capacity was affected by UV-B exposure in most of the algal species tested (Fig. 1). The response varied from reduction by about 50 % to stimulation of the oxygen evolution rate that was in some species 20 % higher than the pre-exposure value. There is no clear-cut correlation between taxonomic affiliation of the species and UV-B sensitivity. However, in general, relative sensitivity tends to be higher in Chromophyta as compared to Chlorophyta. Table 2 presents statistics of the UV-B response of the screened algae. Among the screened algal species 70 % were found to be sensitive to UV-B with suppression of oxygen evolving capacity by more than 10 % following the exposure. Out of the resistant species (30 % of total), 11 species were stimulated by UV-B exposure when the final oxygen evolving capacity was compared to the pre-UV-B exposure value. A relatively even proportion of sensitive to resistant species

Table 2: UV-B responses among the tested algal species. Location/number of species Response

Inhibition

Frequency

mts.lake or snow

mts. soil

soil

sensitive resistant

>10% < 10%

70% 30%

9 7

4

27 7

1

plankton

thermal spring

5

1 2

2

(9 : 7) was found among the algae originally collected from lakes or snow detritus in mountains. A high proportion of sensitive species was found in algae isolated from soil in mountains (4: 1), soil in lowlands (27: 7) and from lowlands plankton (5: 2). The statistics of the algae from thermal springs is not conclusive due to the low number of species in the screening. Among the most sensitive species are: Ankistrodesmus spiralis (no. 1, Chlorellales, Fig. 1), Heterococcus brevicellularis (no. 6, Mischococcales), Myrmecia bisecta (no. 1, Chlorosarcinales), Klebshormidium bisectum (no. 1, Klebsormidiales), Pseudococcomyxa sp. (no. 16, Chlorellales), and Xanthonema bristolianum (no. 3, lribonematales). Among the species exhibiting the greatest stimulation by UV-B are: Chloridella neglecta (no. 3, Mischococcales), Scenedesmus (no. 19, 20, Chlorellales) , Spongiochloris spongiosa (no. 3, Protosiphonales), Enallax coelastroides (no. 10, Chlorellales) and Chlorococcum ellipsoideum (no. 6, Chlorococ-

cales).

Fig. 2 presents a comparison of the relative changes in fluorescence parameters and in oxygen evolution capacity of all screened algae resulting from 90 minutes of UV-B exposure. No correlation was seen between the nearly universal UV-B induced rise in Fo and the change in the oxygen evolution capacity (Fig. 2A). Relatively small species to species variation in F0 response indicate that the underlying molecular mechanism is common for a vast majority of the screened algae. The capacity of the PSII reaction centers of the algae

46

FUSHENG XIONG, FILIP LEDERER, )AROM(R LUKAVSKY.

UV-B INDUCED CHANGE IN O2 ACTIVITY, % I

I

N

(J1

o

I

,

«

N

o

(J1

,

,

,

,

,

,

I

,

LAOISLAV NEDBAL

FJF m (Fig. 2 B) and for ~e' the quantum yield of PSII electron transfer (Fig. 2 C).

(J1

o

(J1

I

and

I

,

,

,

Discussion

Freshwater algae exhibit species-dependent sensltlvlty to UV-B exposure that ranges from reduction to stimulation of the oxygen evolving capacity. Similar variability was reported earlier in higher plants (Teramura et al., 1991, Sullivan et al., 1992) and with CO 2 fixation in algae (Smith et al., 1992). The mechanism underlying such a widely variable response Chlorococca/es of plants to elevated UV-B remains obscured by the com'""" plexity of damaging processes to several targets in the cell in'"'"..... cluding PSII damage and by the complexity of the protection co mechanisms (Hader and Worrest, 1991; Willimoson and Zealter, 1994). The results of this investigation lead to the identifica() -------------------------------~ tion of algal species that are extremely sensitive to UV-B and ChloreUa/es ., 0algal species that are resistant to or even stimulated by UV-B o '""" :3" radiation. Analysis of common features in the two groups re'< ; '"'"..... sulted in the hypothesis that Chromophyta tend to be more co sensitive than Chlorophyta. The minimal cross-cell or crosscolony/cenobium distance is larger in the resistant species (8, 3 X 4, 30, 2 X 7, 10 ~m) compared to the sensitive species (1, '" 6, 3, 6, 3 and 4 ~m). We propose that resistance to UV-B exposure is more likely to be observed in algae with a large '" minimal cross-cell distance or with cells organised in colonies or cenobia that can provide shading of the internal structures. '"o In addition, the resistant algae have frequently a solid cell '" wall containing sporopollenin. The spectroscopical analysis of '" '" the cell wall fraction isolated from resistant species is under'"~ -------------------------------way to test the hypothesis that the UV-B absorbing or scatProrosiphona/es tering structures are localized in the cell wall of these algae. '"~ - - - - - - - - - - - - - - c"h;;t;';';o-';/;'; The group of algae sensitive to UV-B is more heterogeneous. In addition to the small size of cell, characteristics that could lead to UV-B sensitivity include the occurrence of naked zoo""N -_ - - - - -U/;; trlct,;I;;_ spores as a part of the cell cycle. Neochloridales_ N Many of the algal species that were resistant to UV-B ra~ Klebsormidia/es diation were originally isolated from sunlight-exposed loca'""" tions of high elevation (cf. Larson et al., 1990). Also one of the resistant species, Scenedesmus sp., strain NECAS 1965/NFig. 1: The relative change in the photosynthetic oxygen evolution 508, GREI./15, UV (no. 20 Chlorellales, Fig. 1) was isolated capacity of the algae induced by· 90 minutes UV-B exposure. Each bar represents change in CO 2-dependent O 2 evolution capacity in as a UV resistant strain from Scenedesmus quadncautla culture an individual species of algae with error bar indicating standard 30 years ago during UV-assisted induction of random mutadeviation found in 3-5 independent experiments. The ordering of tions. These two examples demonstrate the persistence of the the algal species as well as numbering in individual families (bottom genetically encoded basis for UV-B resistance. On the other of the figure) is identical to that in Table 1. extreme, the incidence of sensitive species is found to be high among algae isolated from shaded soil (Table 2). The presence of several resistant species among the soil algae is more difficult to interpret because of lack of information about the depths for development of photochemical quenching (qp) was nearly and character of soil from which the organism was isolated. The comparison of fluorescence and oxygen evolution data unaffected by UV-B exposure (Fig. 2 D). Limited correlation can be recognised between the UV-B induced change in the obtained during the progressing UV-B exposure shows that capacity of the algae to develop non-photochemical quench- fluorescence can serve as a secondary indicator of the UV-B ing (qNP) and the change in the oxygen evolution capacity induced changes in photosynthetic activity. The highest de(Fig. 2 D). Large variations in the relationship indicate a spe- gree of reliability in the correlation with the oxygen evolution cies-specific character of the underlying molecular mecha- data was found with the ~e and F)F m (Fig. 2 B and Fig. nism. A relatively high correlation with the UV-B induced 2 C). The nearly uniform observation that UV-B induced an changes in the oxygen evolution capacity was found for the increase in F0 (Fig. 2 A) can be tentatively interpreted as a

. .

.

: -:~-~-~-~-~-~-:-:-:-~-~!-.!~-!-I-I---chi;r""a;a-r~;-";/;;

"

47

Screening of algae for UV-B sensitiviry

160 140

=> a: 0

u.

120

~ 0

o

& iO~

0 00

0

00'0 0 0

,&

0

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80

=> a: ell

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60

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0

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100 80

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80

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02 evolution capacity (R. U.) Fig. 2: Comparison of the relative changes in the oxygen evolution capaciry of the screened algae with the accompanying changes in the fluorescence parameters Fo (A), FJF m (B), <1>e (C), and qp and qNP (D) as response to enhanced UV-B exposure. For more detail, see Materials and Methods. r-coefficients of correlations for probabiliry p
process occurring in the proximal antenna ofPSII in almost all screened algae or as a process which changes the equilibrium of Q;./Q;.- in favour of the reduced form. To test these alternatives, future experiments will be done with added quinones. The largely species-dependent decline in the capacity of the algae to develop non-photochemical quenching which accompanies UV-B induced suppression of oxygen evolution capacity can be tentatively ascribed to UV-B-induced damage to the de-epoxidase of the carotenoid cycle (Pfundel et al., 1992). To test this hypothesis, the capacity of the algae to perform the light-induced carotenoid conversion will be compared with the oxygen evolution activity during UV-B exposure of sensitive algal species. In the next phase of the experiments, the hypothesis will be tested that algal species resistant to UV-B exposure on a shorr time scale will also be resistant to a more realistic longterm UV-B exposure. It is anticipated that some of the strains susceptible to the shorr time/high UV-B exposure will be severely affected during more realistic UV-B exposure. A comparative study on the impact of enhanced UV-B exposure on pigment composition of susceptible and resistant algal species will be presented elsewhere. This comparison of susceptible and resistant algae is expected to yield more insight into the molecular mechanisms of protection used by the algae against UV-B stress. Acknowledgements

The project was supported by contracts of Grant Agency of Czech Republic 206-93-0663 and 202-94-0457. Authors also appre-

ciate assistance of Mrs. Marie KaSparkova and Mrs. Helena Vondrkova from Culture Collection of Autotrophic Organisms at Tfebon and critical reading of the manuscript by Dr. Mary Poulson.

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48

FUSHENG XIONG, FILIP LEDERER, JAROMtR LUKAVSKY, and UDISLAV NEDBAL

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