ACCLIMATION OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) TO PHOTON FLUX DENSITY1

Growth rate, pigment composition, and noninvasive chl a fluorescence parameters were assessed for a noncalcifying strain of the prymnesiophyte Emiliania huxleyi Lohman grown at 50, 100, 200, and 800 μmol photons·m^?2·s^?1. Emiliania huxleyi grown at high photon flux density (PFD) was characterized by increased specific growth rates, 0.82 d^?1 for high PFD grown cells compared with 0.38 d^?1 for low PFD grown cells, and higher in vivo chl a specific attenuation coefficients that were most likely due to a decreased pigment package, consistent with the observed decrease in cellular photosynthetic pigment content. High PFD growth conditions also induced a 2.5‐fold increase in the pool of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin responsible for dissipation of excess energy. Dark‐adapted maximal photochemical efficiency (F_v/F_m) remained constant at around 0.58 for cells grown over the range of PFDs, and therefore the observed decline, from 0.57 to 0.33, in the PSII maximum efficiency in the light‐adapted state, (F_v′/F_m′), with increasing growth PFD was due to increased dissipation of excess energy, most likely via the xanthophyll cycle and not due to photoinhibition. The PSII operating efficiency (F_q′/F_m′) decreased from 0.48 to 0.21 with increasing growth PFD due to both saturation of photochemistry and an increase in nonphotochemical quenching. The changes in the physiological parameters with growth PFD enable E. huxleyi to maximize rates of photosynthesis under subsaturating conditions and protect the photosynthetic apparatus from excess energy while supporting higher saturating rates of photosynthesis under saturating PFDs.     

Microphytobenthic species composition,pigment concentration,and primary production in sublittoral sediments of the Trondheimsfjord (Norway)1

In spring 2005, monthly sampling was carried out at a sublittoral site near Tautra Island. Microphytobenthic identification, abundance (ABU), and biomass (BIOM), were performed by microscopic analyses. Bacillariophyceae accounted for 67% of the total ABU, and phytoflagellates constituted 30%. The diatom floristic list consisted of 38 genera and 94 species. Intact light‐harvesting pigments chl a, chl c, and fucoxanthin and their derivatives were identified and quantified by HPLC. Photoprotective carotenoids were also observed (only as diadinoxanthin; no diatoxanthin was detected). Average fucoxanthin content was 4.57 ± 0.45 μg fucoxanthin · g sediment dry mass^?1, while the mean chl a concentration was 2.48 ± 0.15 μg · g^?1 dry mass. Both the high fucoxanthin:chl a ratio (considering nondegraded forms) and low amounts of photoprotective carotenoids indicated that the benthic microalgal community was adapted to low light. Microphytobenthic primary production was estimated in situ (MPPs, from 0.15 to 1.28 mg C · m^?2 · h^?1) and in the laboratory (MPPp, from 6.79 to 34.70 mg C · m^?2 · h^?1 under light saturation) as ¹⁴C assimilation; in April it was additionally estimated from O₂‐microelectrode studies (MPP_O2) along with the community respiration. MPP_O2 and the community respiration equaled 22.9 ± 7.0 and 7.4 ± 1.8 mg C · m^?2 · h^?1, respectively. A doubling of BIOM from April to June in parallel with a decreasing photosynthetic activity per unit chl a led us to suggest that the microphytobenthic community was sustained by heterotrophic metabolism during this period.     

A COMPARISON OF PHOTORESPONSE AMONG TEN DIFFERENT KARENIA BREVIS (DINOPHYCEAE) ISOLATES1

Many laboratories have solely used the Wilson isolate to physiologically characterize the harmful algal bloom (HAB) dinoflagellate Karenia brevis (C. C. Davis) G. Hansen et Moestrup. However, analysis of one isolate may lead to misinterpretations when extrapolating measurements to field populations. In this study, pulse‐amplitude‐modulated chlorophyll fluorometer (PAM‐FL) relative electron transport rate (ETR), F_v/F_m, and chl were compared with traditional techniques, such as ¹⁴C photosynthesis versus irradiance (P–E) curves, DCMU [3‐(3′,4′‐dichlorophenyl)‐1,1‐dimethyl urea] F_v/F_m, and extracted chl. The DCMU and PAM‐FL values of F_v/F_m (r² = 0.51) and chl (r² = 0.58) were in good agreement. There was no correlation between ¹⁴C and PAM‐FL α, P_max, and β parameters because PAM‐FL ETR was only a relative measurement. The PAM‐FL techniques were then used to investigate P–E curves, quantum yield of PSII (F_v/F_m), and chl from 10 K. brevis isolates to determine whether one or all isolates would better represent the species. Comparisons were made with a radial photosynthetron, which allowed for controlled conditions of light and temperature. Isolate α, P_max, and β varied between 0.097 and 0.204 μmol e^? · m^?2 · s^?1 · (μmol quanta · m^?2 · s^?1)^?1, 80.41 and 241 μmol e^? · m^?2 · s^?1, and 0.005 and 0.160 μmol e^? · m^?2 · s^?1 · (μmol quanta · m^?2 · s^?1)^?1, respectively. Either carbon limitation and/or bacterial negative feedback were implicated as the cause of the P–E parameter variability. Furthermore, these results directly contradicted some literature suggestions that K. brevis is a low‐light‐adapted dinoflagellate. Results showed that K. brevis was more than capable of utilizing and surviving in light conditions that may be present on cloudless days off Florida.     

LIPID CLASS,CAROTENOID, AND TOXIN DYNAMICS OF KARENIA BREVIS (DINOPHYCEAE) DURING DIEL VERTICAL MIGRATION1

The internal lipid, carotenoid, and toxin concentrations of Karenia brevis (C. C. Davis) Gert Hansen and Moestrup are influenced by its ability to use ambient light and nutrients for growth and reproduction. This study investigated changes in K. brevis toxicity, lipid class, and carotenoid concentrations in low‐light, nitrate‐replete (250 μmol quanta · m^?2 · s^?1, 80 μM NO₃); high‐light, nitrate‐replete (960 μmol quanta · m^?2 · s^?1, 80 μM NO₃); and high‐light, nitrate‐reduced (960 μmol quanta · m^?2 · s^?1, <5 μM NO₃) mesocosms. Reverse‐phase HPLC quantified the epoxidation state (EPS) of the xanthophyll‐cycle pigments diadinoxanthin and diatoxanthin, and a Chromarod Iatroscan thin layer chromatography/flame ionization detection (TLC/FID) system quantified changes in lipid class concentrations. EPS did not exceed 0.20 in the low‐light mesocosm, but increased to 0.65 in the high‐light mesocosms. Triacylglycerol and monogalactosyldiacylglycerol (MGDG) were the largest lipid classes consisting of 9.3% to 48.7% and 37.3% to 69.7% of total lipid, respectively. Both lipid classes also experienced the greatest concentration changes in high‐light experiments. K. brevis increased EPS and toxin concentrations while decreasing its lipid concentrations under high light. K. brevis may mobilize its toxins into the surrounding environment by reducing lipid concentrations, such as sterols, limiting competition, or toxins are released because lipids are decreased in high light, reducing any protective mechanism against their own toxins.     

EFFECT OF SEASONAL SEA ICE BREAKOUT ON THE PHOTOSYNTHESIS OF BENTHIC DIATOM MATS AT CASEY,ANTARCTICA1

Photosynthesis of marine benthic diatom mats was examined before and after sea ice breakout at a coastal site in eastern Antarctica (Casey). Before ice breakout the maximum under‐ice irradiance was between 2.5 and 8.2 μmol photons·m^?2·s^?1 and the benthic microalgal community was characterized by low E_k (12.1–32.3 μmol photons·m^?2·s^?1), low relETR_max (9.2–32.9), and high alpha (0.69–1.1). After breakout, 20 days later, the maximum irradiance had increased to between 293 and 840 μmol photons·m^?2·s^?1, E_k had increased by more than an order of magnitude (to 301–395 μmol photons·m^?2·s^?1), relETR_max had increased by more than five times (to 104–251), and alpha decreased by approximately 50% (to 0.42–0.68). During the same time interval the species composition of the mats changed, with a decline in the abundance of Trachyneis aspera (Karsten) Hustedt, Gyrosigma subsalsum Van Heurck, and Thalassiosira gracilis (Karsten) Hustedt and an increase in the abundance of Navicula glaciei Van Heurck. The benthic microalgal mats at Casey showed that species composition and photophysiology changed in response to the sudden natural increase in irradiance. This occurred through both succession shifts in the species composition of the mats and also an ability of individual cells to photoacclimate to the higher irradiances.     

OXYGEN MICROSENSOR STUDIES ON ZOOXANTHELLATE CLIONAID SPONGES FROM THE COSTA BRAVA,MEDITERRANEAN SEA1

Photosynthetic properties of two symbiotic demosponges were compared using Clark‐type oxygen microsensors. The putatively distinct sponge species, Cliona viridis (Schmidt, 1862) and Cliona nigricans (Schmidt, 1862) were discriminated by their mean megasclere lengths of 296 and 387 μm, respectively. Photosynthetic behavior was used to generate additional taxonomic information. Sponge–dinoflagellate symbioses were well adapted to low light due to the hosts' endolithic lifestyle. Both sponges reached light compensation and saturation at similar light levels with means close to 10 and 30 μmol photons·m^?2·s^?1, respectively. The gross photosynthetic activity was closely related to symbiont cell density in the sponge surface tissue. Mean symbiont densities, chl a content, and gross photosynthesis were about six times higher in C. viridis than in C. nigricans, with respective values of 3000 and 440 symbiont·mm^?2, 1.3 and 0.2 μg chl a·g^?1, and 5.4 and 1.0 μmol O₂·cm^?3·s^?1 gross photosynthesis. Net photosynthesis and respiration could not be calculated accurately from the oxygen gradients, because significant gas exchange occurs through the pumping activity. Thus, assumptions of diffusional oxygen exchange via the surface do not hold for sponges. Combined data of this study indicate that the metabolic activity of C. viridis depends on photosynthetic activity of its symbionts, whereas C. nigricans appears to have a higher pumping intensity and is more actively filter feeding. The difference in photosynthetic activities is not caused by different light adaptations but provides new evidence against the conspecifity of C. viridis and C. nigricans.     

SHORT‐TERM EFFECT OF TEMPERATURE ON THE PHOTOKINETICS OF MICROALGAE FROM THE SURFACE LAYERS OF ANTARCTIC PACK ICE1

Microalgae growing within brine channels (85 psu salinity) of the surface ice layers of Antarctic pack ice showed considerable photosynthetic tolerance to the extreme environmental condition. Brine microalgae exposed to temperatures above ?5°C and at irradiances up to 350 μmol photons·m^?2·s^?1 showed no photosynthetic damage or limitations. Photosynthesis was limited (but not photoinhibited) when brine microalgae were exposed to ?10°C, provided the irradiance remained under 50 μmol photons·m^?2·s^?1. The highest level of photosynthetic activity (maximum relative electron transport rate [rETR_max]) in brine microalgae growing within the surface layer of sea ice was at approximately 18 μmol electrons·m^?2·s^?1, which occurred at ?1.8°C. Effective quantum yield of PSII and rETR_max of the halotolerant brine microalgae exhibited a temperature‐dependent pattern, where both parameters were higher at ?1.8°C and lower at ?10°C. Relative ETR_max at temperatures above ?5°C were stable across a wide range of irradiance.     

CULTURE OF THE TERRESTRIAL CYANOBACTERIUM,NOSTOC FLAGELLIFORME (CYANOPHYCEAE), UNDER AQUATIC CONDITIONS1

Both colonies and free‐living cells of the terrestrial cyanobacterium, Nostoc flagelliforme (Berk. & Curtis) Bornet & Flahault, were cultured under aquatic conditions to develop the techniques for the cultivation and restoration of this endangered resource. The colonial filaments disintegrated with their sheaths ruptured in about 2 days without any desiccating treatments. Periodic desiccation played an important role in preventing the alga from decomposing, with greater delays to sheath rupture with a higher frequency of exposure to air. The bacterial numbers in the culture treated with seven periods of desiccation per day were about 50% less compared with the cultures without the desiccation treatment. When bacteria in the culture were controlled, the colonial filaments did not disintegrate and maintained the integrity of their sheath for about 20 days even without the desiccation treatments, indicating the importance of desiccation for N. flagelliforme to prevent them from being disintegrated by bacteria. On the other hand, when free‐living cells obtained from crushed colonial filaments were cultured in liquid medium, they developed into single filaments with sheaths, within which multiple filaments were formed later on as a colony. Such colonial filaments were developed at 15, 25, and 30° C at either 20 or 60 μmol photons·m^?2·s^?1; colonies did not develop at 180 μmol photons·m^?2·s^?1, though this light level resulted in the most rapid growth of the cells. Conditions of 60 μmol photons·m^?2·s^?1 and 25° C appeared to result in the best colonial development and faster growth of the sheath‐held colonies of N. flagelliforme when cultured indoor under aquatic conditions.     

AMMONIUM AND UV RADIATION STIMULATE THE ACCUMULATION OF MYCOSPORINE‐LIKE AMINO ACIDS IN PORPHYRA COLUMBINA (RHODOPHYTA) FROM PATAGONIA,ARGENTINA1

The combined effects of ammonium concentration and UV radiation on the red alga Porphyra columbina (Montagne) from the Patagonian coast (Chubut, Argentina) was determined using short‐term (less than a week) experimentation. Discs of P. columbina were incubated with three ammonium concentrations (0, 50, and 300 μM NH₄Cl) in anilluminated chamber (PAR=300 μmol photons·m^?2·s^?1, UVA=15 W·m^?2, UVB=0.7 W·m^?2) at 15°C. Algae incubated at 300 μM ammonium showed a significant increase (P<0.05) in the concentration of mycosporine‐like amino acids (MAAs) compared with the initial value or with the other ammonium treatments. The increase of MAAs was, however, a function of the quality of irradiance received, with a higher increase in samples exposed to UVA compared with UVB (29% and 5% increase, respectively). However, UVB radiation was more effective in inducing MAA synthesis per unit energy received by the algae. Samples exposed to PAR only had an intermediate increase in MAA concentration of 16%. Chl a concentration decreased through the incubation with the greatest decrease at high ammonium concentration. Phycobiliprotein (BP) decreased through time with the smallest decrease occurring at high ammonium concentration. Photoinhibition (as a decrease of optimal quantum yield) was significantly greater under nitrogen‐deprived conditions than that under replete ammonium levels. Maximal gross photosynthesis (GP_max), as oxygen evolution, and maximal electron transport rate (ETR_max), as chl fluorescence, increased with the ammonium concentration. Positive relationships between maximal GP or ETR and pigment ratios (BP/chl a and MAAs/chl a) and negative relationships with chl a concentration were found.     

PHYSIOLOGICAL ECOTYPES IN THE MARINE ALGA BOSTRYCHIA RADICANS (CERAMIALES,RHODOPHYTA) FROM THE EAST COAST OF THE U.S.A1

The comparative ecophysiology of nine culture isolates of the eulittoral red alga Bostrychia radicans (Montagne) Montague collected at sites from seven states along the east coast of the U.S.A. was investigated. The growth response in relation to different salinity and light conditions as well as photosynthesis-irradiance curves were studied. In addition, the effect of salt treatment on the content of the isomeric polyols d -sorbitol and d -dulcitol was also studied. All isolates grew between salinities of 5.3 and 70 ppt but with quite different optima and maxima. The isolates were all adapted to low light levels, i.e. growth was already recorded at 2.5 μmol photons·m^?2·s^?1, and growth rates peaked between 40 and 60 μmol photons·m^?2·s_-1. These low-light requirements were also reflected by the photosynthesis-irradiance curves: all plants had low light compensation points (2.5–9.7 μmol photons ·m^?2·^?1) and low photon fluence rates for initial saturation of photosynthesis (38.1–84.7 μmol photons·m^?2·s^?1, indicating that these isolates are “shade-adapted.” Isolates from Florida and Georgia synthesized and accumulated both the osmolytes d -sorbitol and d -dulcitol in increasing salinities, whereas only d -sorbitol was present in plants from North Carolina north to Connecticut. d -sorbitol was always strongly involved in osmotic acclimation. In various isolates from the same location in South Carolina, both polyol patterns were found, i.e. d -sorbitol plus d -dulcitol and d -sorbitol only. All data indicate that B. radicans exhibits a broad salinity tolerance and a low-light preference, which explain the successful colonization of this alga on various intertidal and shaded substrates. The data also clearly indicate intraspecific differences among the nine isolates, which is interpreted as development of different physiological ecotypes.     

PHOTOINHIBITION OF MECHANICALLY STIMULABLE BIOLUMINESCENCE IN THE AUTOTROPHIC DINOFLAGELLATE CERATIUM FUSUS (PYRROPHYTA)1

Ceratium fusus (Ehrenb.) Dujardin was exposed to light of different wavelengths and photon flux densities (PFDs) to examine their effects on mechanically stimulable bioluminescence (MSL). Photoinhibition of MSL was proportional to the logarithm of PFD. Exposure to I μmol photons·m^?2s^?1 of broadband blue light (ca. 400–500 nm) produced near-complete photoinhibition (≥90% reduction in MSL) with a threshold at ca. 0.01 μmol photons·m^?2·s^?1. The threshold of photoinhibition was ca. an order of magnitude greater for both broadband green (ca. 500–580 nm) and red light (ca. 660–700 nm). Exposure to narrow spectral bands (ca. 10 nm half bandwidth) from 400 and 700 nm at a PFD of 0.1 μmol photons·m^?2·s^?1 produced a maximal response of photoinhibition in the blue wavelengths (peak ca. 490 nm). A photoinhibition response (≥ 10%) in the green (ca. 500–540 nm) and red wavelengths (ca. 680 nm) occurred only at higher PFDs (1 and 10 μmol photons·m^?2·s^?1). The spectral response is similar to that reported for Gonyaulax polyedra Stein and Pyrocystis lunula Schütt and unlike that of Alexandrium tamarense (Lebour) Balech et Tangen. The dinoflagellate's own bioluminescence is two orders of magnitude too low to result in self-photoinhibition. The quantitative relationships developed in the laboratory predict photoinhibition of bioluminescence in populations of C. fusus in the North Atlantic Ocean.     

TOXIN PROFILE,PIGMENT COMPOSITION,AND LARGE SUBUNIT RDNA PHYLOGENETIC ANALYSIS OF AN ALEXANDRIUM MINUTUM (DINOPHYCEAE) STRAIN ISOLATED FROM THE FLEET LAGOON,UNITED KINGDOM1

Paralytic shellfish toxins, pigment composition, and large subunit (LSU) rDNA sequence were analyzed for a clonal culture of Alexandrium minutum Halim isolated in 2000 from the coastal Fleet Lagoon, Dorset, United Kingdom. The HPLC pigment analysis revealed the presence of chl a, peridinin, and diadinoxanthin as major pigments and chl c₁+c₂ and c₃, diatoxanthin, and β‐carotene as minor components. The toxins responsible for paralytic shellfish poisoning were analyzed by HPLC with postcolumn derivatization and fluorescence detection. The paralytic shellfish poisoning toxin profile of the Fleet Lagoon strain of A. minutum in exponential growth phase was dominated by gonyautoxin‐3 up to 54%, whereas gonyautoxin‐2 made up 10% and saxitoxin (STX) 36%. The average toxicity of the culture was 3.8 pg STX Eq·cell^?1, and total toxin content varied from 5.6 fmol·cell^?1 on day 1 to a maximum of 16.8 fmol·cell^?1 during the early stationary phase. Sequence analysis of the LSU rDNA revealed the strain to be closely related to several European strains of A. minutum and one isolated from Australian waters, although most of these do not produce STX. The shallow Fleet Lagoon may provide a favorable environment for A. minutum to bloom, and the presence of highly potent saxitoxins in this strain indicates potential for future shellfish contamination.     

EFFECTS OF LIGHT AND TEMPERATURE ON GROWTH,NITRATE UPTAKE,AND TOXIN PRODUCTION OF TWO TROPICAL DINOFLAGELLATES: ALEXANDRIUM TAMIYAVANICHII AND ALEXANDRIUM MINUTUM (DINOPHYCEAE)1

The two tropical estuarine dinoflagellates, Alexandrium tamiyavanichii Balech and A. minutum Halim, were used to determine the ecophysiological adaptations in relation to their temperate counterparts. These species are the two main causative organisms responsible for the incidence of paralytic shellfish poisoning (PSP) in Southeast Asia. The effects of light (10, 40, 60, and 100 μmol photons·m^?2·s^?1) and temperature (15, 20, and 25°C) on the growth, nitrate assimilation, and PST production of these species were investigated in clonal batch cultures over the growth cycle. The growth rates of A. tamiyavanichii and A. minutum increased with increasing temperature and irradiance. The growth of A. tamiyavanichii was depressed at lower temperature (20°C) and irradiance (40 μmol photons·m^?2·s^?1). Both species showed no net growth at 10 μmol photons·m^?2·s^?1 and a temperature of 15°C, although cells remained alive. Cellular toxin quotas (Q_t) of A. tamiyavanichii and A. minutum varied in the range of 60–180 and 10–42 fmol PST·cell^?1, respectively. Toxin production rate, R_tox, increased with elevated light at both 20 and 25°C, with a pronounced effect observed at exponential phase in both species (A. tamiyavanichii, r²=0.95; A. minutum, r²=0.96). Toxin production rate also increased significantly with elevated temperature (P<0.05) for both species examined. We suggest that the ecotypic variations in growth adaptations and toxin production of these Malaysian strains may reveal a unique physiological adaptation of tropical Alexandrium species.     

PRIMARY PRODUCTION OF CRUSTOSE CORALLINE RED ALGAE IN A HIGH ARCTIC FJORD1

Crustose coralline algae occupied ～1%–2% (occasionally up to 7%) of the sea floor within their depth range of 15–50 m, and they were the dominant encrusting organisms and macroalgae beyond 20 m depth in Young Sound, NE Greenland. In the laboratory, oxygen microelectrodes were used to measure net photosynthesis (P) versus downwelling irradiance (E_d) and season for the two dominant corallines [Phymatolithon foecundum (Kjellman) Düwel et Wegeberg 1996 and Phymatolithon tenue (Rosenvinge) Düwel et Wegeberg 1996] representing> 90% of coralline cover. Differences in P‐E_d curves between the two species, the ice‐covered and open‐water seasons, or between specimens from 17 and 36 m depth were insignificant. The corallines were low light adapted, with compensation irradiances (E_c) averaging 0.7–1.8 μmol photons·m ^{? 2}·s ^{? 1} and light adaptation (E_k) indices averaging 7–17 μmol photons·m ^{? 2}·s ^{? 1}. Slight photoinhibition was evident in most plants at irradiances up to 160 μmol photons·m ^{? 2}·s ^{? 1}. Photosynthetic capacity (P_m) was low, averaging 43–67 mmol O₂·m ^{? 2} thallus·d ^{? 1} (～250–400 g C·m ^{? 2} thallus·yr ^{? 1}). Dark respiration rates averaged ～5 mmol O₂·m ^{? 2} thallus·d ^{? 1}. In ice covered periods, E_d at 20 m depth averaged ～1 μmol photons·m ^{? 2}·s ^{? 1}, with daily maxima of 2–3 μmol photons·m ^{? 2}·s ^{? 1}. During the open water season, E_d at 20 m depth averaged ～7 μmol photons·m ^{? 2}·s ^{? 1} with daily maxima of ～30 μmol photons·m ^{? 2}·s ^{? 1}. Significant net primary production of corallines was apparently limited to the 2–3 months with open water, and the small contribution of corallines to primary production seems due to low P_m values, low in situ irradiance, and their relatively low abundance in Young Sound.     

Dynamic responses of photosystem II and phycobilisomes to changing light in the cyanobacterium Synechococcus sp. PCC 7942

We have examined the molecular and photosynthetic responses of a planktonic cyanobacterium to shifts in light intensity over periods up to one generation (7 h). Synechococcus sp. PCC 7942 possesses two functionally distinct forms of the D1 protein, D1∶1 and D1∶2. Photosystem II (PSII) centers containing D1∶1 are less efficient and more susceptible to photoinhibition than are centers containing D 1∶2. Under 50 μmol photons· m^?2·s^?1, PSII centers contain D1∶1, but upon shifts to higher light (200 to 1000 μmol photons·m^?2·s^?1), D1∶1 is rapidly replaced by D 1∶2, with the rate of interchange dependent on the magnitude of the light shift. This interchange is readily reversed when cells are returned to 50 μmol photons·m^?2·s^?1. If, however, incubation under 200 μmol photons·m^?2·s^?1 is extended, D1∶1 content recovers and by 3 h after the light shift D1∶1 once again predominates. Oxygen evolution and chlorophyll (Chl) fluorescence measurements spanning the light shift and D1 interchanges showed an initial inhibition of photosynthesis at 200 μmol photons·m^?2·s^?1, which correlates with a proportional loss of total D1 protein and a cessation of growth. This was followed by recovery in photosynthesis and growth as the maximum level of D 1∶2 is reached after 2 h at 200 μmol photons·m^?2·s^?1. Thereafter, photosynthesis steadily declines with the loss of D1∶2 and the return of the less-efficient D1∶1. During the D1∶1/D1∶2 interchanges, no significant change occurs in the level of phycocyanin (PC) and Chl a, nor of the phycobilisome rod linkers. Nevertheless, the initial PC/Chl a ratio strongly influences the magnitude of photo inhibition and recovery during the light shifts. In Synechococcus sp. PCC 7942, the PC/Chl a ratio responds only slowly to light intensity or quality, while the rapid but transient interchange between D1∶1 and D 1∶2 modulates PSII activity to limit damage upon exposure to excess light.     

HETEROGENEITY OF OXYGEN PRODUCTION AND CONSUMPTION IN A PHOTOSYNTHETIC MICROBIAL MAT AS STUDIED BY PLANAR OPTODES

By applying planar optodes and imaging techniques to a benthic photosynthetic mat, we demonstrated an extensive vertical and horizontal variation in O₂ concentrations, O₂ consumption, and O₂ production. In light, the oxic zone could be divided into three horizons: 1) an upper zone dominated by diatoms that had a moderate net O₂ production, 2) another zone dominated by Microcoleus-like cyanobacteria with a high net O₂ production, and 3) a lower zone with disintegrating microalgae and cyanobacteria with a high O₂ consumption rate. From the O₂ images, the net O₂ production/consumption was calculated at a spatial resolution of 130 μM. This allowed us to identify microsites with high rates of O₂ turnover within the photic zone. Sites with high net O₂ consumption (>1.5 nmol·cm^?3·s^?1) were typically situated next to sites with a relatively high net production (>2 nmol·cm^?3·s^?1), revealing a mosaic in which the highest O₂ consumption sites were surrounded by the highest O₂ production sites. This suggested a tight spatial coupling between production and consumption of O₂ within the photic zone. Light stimulated the O₂ consumption within the photic zone. At irradiances above 400 μmol photons·m^?2·s^?1, the stimulated O₂ production was almost completely balanced by enhanced O₂ consumption at microsites exhibiting net consumption of O₂ even at maximum irradiance (578 μmol photons·m^?2·s^?1). Our observations strongly supported the idea that light-stimulated respiration was caused by stimulated heterotrophic activity fueled by organic carbon leakage from the phototrophs. Despite microsites with high net O₂ consumption, anoxic microniches were not encountered in the investigated mat. Images of gross photosynthetic rates also revealed an extensive horizontal variation in gross rates, with microsites of low or no photosynthesis within the otherwise photic zone. Calculations based on the obtained images revealed that at maximum light (578 μmol photons·m^?2·s^?1), 90% of the O₂ produced was consumed within the photic zone. The presented data demonstrate the great potential offered by planar optode for studies of benthic photosynthetic communities.     

PHOTOSYNTHETIC FUNCTION IN DUNALIELLA TERTIOLECTA (CHLOROPHYTA) DURING A NITROGEN STARVATION AND RECOVERY CYCLE

Phytoplankton can be exposed to periods of N starvation with episodic N resupply. N starvation in Dunaliella tertiolecta (Butcher) measured over 4 days was characterized by slow reduction in cell chl and protein content and chl/carotenoid ratio and a decline in photosynthetic capacity and maximum quantum yield of photosynthesis (F_v/F_m). In the early stages of N starvation, cell division was maintained despite reduction in cellular chl. Chl content was more sensitive than carotenoids to N deprivation, and cellular chl a was maintained preferentially over chl b under N starvation. NO₃^? resupply stimulated rapid and complete recovery of F_v/F_m (from 0.4 to 0.7) within 24 h and commencement of cell division after 10 h, although N‐replete levels of cell chl and protein were not reestablished within 24 h. Recovery of F_v/F_m was correlated with increases in cell chl and protein and was more related to increases in F_m than to changes in F₀. Recovery of F_v/F_m was biphasic with a second phase of recovery commencing 4–6 h after resupply of NO₃^?. Uptake of NO₃^? from the external medium and the recovery of F_v/F_m, cell chl, and protein were inhibited when either cytosolic or chloroplastic protein synthesis was inhibited by cycloheximide or lincomycin, respectively; a time lag observed before maximum NO₃^? uptake was consistent with synthesis of NO₃^? transporters and assimilation enzymes. When both chloroplastic and cytosolic translation was inhibited, F_v/F_m declined dramatically. Dunaliella tertiolecta demonstrated a capacity to rapidly reestablish photosynthetic function and initiate cell division after N resupply, an important strategy in competing for limiting inorganic N resources.     

PHOTOSYNTHESIS-IRRADIANCE RELATIONSHIPS IN SEA ICE MICROALGAE FROM McMURDO SOUND,ANTARCTICA1

Sea ice microalgae in McMurdo Sound, Antarctica were examined for photosynthesis-irradiance relationships and for the extent and time course of their photoadaptation to a reduction in in situ irradiance. Algae were collected from the bottom centimeter of coarse-grained congelation ice in an area free of natural snow cover. Photosynthetic rate was determined in short term (1 h) incubations at ?2° C over a range of irradiance from 0 to 286 μE·m^?2·s^?1. Assimilation numbers were consistently below 0.1 mg C·mg chl a^?1·h^?1. The I_k's³ averaged only 7 μE·m^?2·s^?1, and photosynthesis was inhibited at irradiances above 25 μE·m^?2·s^?1. Photosynthetic parameters of the ice algal community were examined over a nine day period following the addition of 4 cm of surface snow while a control area remained snow-free. A reduction of 40% in P_m^B relative to the control occurred after two days of snow cover; α, β, I_k, and I_m were not significantly altered. Low assimilation numbers and constant standing crop size, however, suggested that the algal bloom may have already reached stationary growth phase, possibly minimizing their photoadaptive response.     

ACCLIMATION OF PALMARIA PALMATA (RHODOPHYTA) TO LIGHT INTENSITY: COMPARISON BETWEEN ARTIFICIAL AND NATURAL LIGHT FIELDS

The acclimation of the photosynthetic apparatus of Palmaria palmata (L.) to light intensity was examined in the field and under laboratory conditions. Algae from 3 different shore levels and from laboratory cultures adapted to 6 different photon flux densities were compared. This was done on the basis of light doses, which were delivered by different light regimes in the field and in the laboratory. Laboratory samples were adjusted to constant photon flux densities between 7 and 569 μmol photons·m ^{? 2}·s ^{? 1} in a 16:8 light:dark photoperiod. Under field conditions the daily amplitudes reached up to approximately 2000 μmol photons·m ^{? 2}·s ^{? 1} within a natural daily light course. Over the course of 14 days the light doses resulting from those different regimes are similar for both treatments. An increasing growth rate per day with increasing light doses was observed in the laboratory. Growth was saturated at 113 mol photons·m ^{? 2}·14 d ^{? 1}. Light saturation points (E_k) of photosynthesis increased with increasing light doses for both field and laboratory samples, and all E_k values were significantly related to the growth light dose. A correlation between fresh weight‐related lutein content and growth light dose was found for laboratory samples only, whereas the lutein:chlorophyll a (chl a) ratio was strongly correlated with E_k for laboratory and field samples. The content of chl a and phycoerythrin (PE) per fresh weight decreased significantly with increasing light doses under field conditions. Simultaneously, the PE:chl a ratio increased, whereas this ratio was not influenced by laboratory treatments. The correspondence of E_k values for field and laboratory treatments indicated that they were affected mainly by light dose. However, the variability in pigmentation was mainly dependent on temporal variability in light intensity (the amplitude of variations in incident light).