INTERACTIONS BETWEEN LIGHT,NH4+, AND CO2 IN BUOYANCY REGULATION OF ANABAENA FLOS-AQUAE (CYANOPHYCEAE)1

Buoyancy of the gas-vacuolate alga Anabaena flosaquae Brébisson was measured under various levels of light, NH₄⁺, and CO₂. At high irradiance (50 μE · m^?2·^?1) the alga was non-buoyant regardless of the availability of CO₂ and NH₄⁺. At low irradiance (≤10 μE · m ^?2· s^?1) buoyancy was controlled by the availability of NH₄⁺ and CO₂. When NH₄⁺ was abundant, algal buoyancy was high over a wide range of CO₂ concentrations. In the absence of NH₄⁺, algal buoyancy was reduced at high CO₂ concentrations, however as the CO₂ concentration declined below about 5 μmol · L^?1, algal buoyancy increased. These results help explain why gas vacuolate, nitrogen-fixing blue-green algae often form surface blooms in eutrophic lakes.     

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.     

INTERACTIVE EFFECTS OF IRRADIANCE AND CO2 ON CO2 FIXATION AND N2 FIXATION IN THE DIAZOTROPH TRICHODESMIUM ERYTHRAEUM (CYANOBACTERIA)1

The diazotrophic cyanobacteria Trichodesmium spp. contribute approximately half of the known marine dinitrogen (N₂) fixation. Rapidly changing environmental factors such as the rising atmospheric partial pressure of carbon dioxide (pCO₂) and shallower mixed layers (higher light intensities) are likely to affect N₂‐fixation rates in the future ocean. Several studies have documented that N₂ fixation in laboratory cultures of T. erythraeum increased when pCO₂ was doubled from present‐day atmospheric concentrations (～380 ppm) to projected future levels (～750 ppm). We examined the interactive effects of light and pCO₂ on two strains of T. erythraeum Ehrenb. (GBRTRLI101 and IMS101) in laboratory semicontinuous cultures. Elevated pCO₂ stimulated gross N₂‐fixation rates in cultures growing at 38 μmol quanta · m^?2 · s^?1 (GBRTRLI101 and IMS101) and 100 μmol quanta · m^?2 · s^?1 (IMS101), but this effect was reduced in both strains growing at 220 μmol quanta · m^?2 · s^?1. Conversely, CO₂‐fixation rates increased significantly (P < 0.05) in response to high pCO₂ under mid‐ and high irradiances only. These data imply that the stimulatory effect of elevated pCO₂ on CO₂ fixation and N₂ fixation by T. erythraeum is correlated with light. The ratio of gross:net N₂ fixation was also correlated with light and trichome length in IMS101. Our study suggests that elevated pCO₂ may have a strong positive effect on Trichodesmium gross N₂ fixation in intermediate and bottom layers of the euphotic zone, but perhaps not in light‐saturated surface layers. Climate change models must consider the interactive effects of multiple environmental variables on phytoplankton and the biogeochemical cycles they mediate.     

The role of extracellular carbonic anhydrase for accumulation of inorganic carbon in the green alga Chlamydomonas reinhardtii. A comparison between wild-type and cell-wall-less mutant cells

The role of extracellular carbonic anhydrase (CA_ex) for dissolved inorganic carbon (DIC) accumulation in the green alga Chlamydomonas reinhardtii was investigated. It was found that when algal cells were bubbled with ambient air, cell-wall-less mutant cells exhibited the same high photosynthetic affinity for CO₂ as wild-type cells despite a 10 times lower activity of CA^ex. It was also found that the affinity for CO₂ was further increased when the total DIC concentration of the algal medium was reduced from that in equilibrium with ambient air to even lower levels. This increased affinity was not correlated with any further increase in the CA_ex activity. Dextran-bound sulfonamide (DBS. 100 μM bound ligand) completely inhibited the activity of CA_ex in intact, low-DIC grown, wild-type cells, while photosynthesis at <2 μM CO₂(aq) proceeded at a far greater rate than could be maintained by CO₂ supplied from the spontaneous dehydration of HCO^?₃. DBS-inhibition of CA_ex, during the induction of the DIC-accumulating mechanism in previously high-DIC grown cells, only caused a 50% inhibition of photosynthesis at 10 μM CO₂(aq) after 1 h of low-DIC acclimation. It was also shown that 50 μM acetazolamide (AZ) inhibited photosynthesis at low DIC concentrations to a relatively higher degree than DBS, suggesting that AZ inhibited intracellular CA as well. Taken together, these results suggest that low-DIC grown cells of C. reinhardtii have the ability to transport HCO^?₃ across the plasma membrane in addition to the CA_ex-mediated, facilitated diffusion and/or transport of CO₂. It is also suggested that the relative importance of these two fluxes (CO₂ or HCO^?₃) is dependent on the growth and experimental conditions. Facilitated CO₂ uptake seems to be most prevalent, supported by HCO^?₃-transport under more or less extreme situations, such as a reduction of CO₂ to extremely low concentrations, leakage of CA_ex to the medium as in cultures of cell-wall-less mutant cells or when the activity of CA_ex has been artificially inhibited.     

DIEL PATTERNS IN LIGHT ABSORPTION AND ABSORPTION EFFICIENCY FACTORS OF ISOCHRYSIS GALBANA (PRYMNESIOPHYCEAE)1

The chl a specific absorption coefficients [a* (λ), m²·mg chl a ^{? 1}] were examined in chemostat culture of the Prymnesiophyceae Isochrysis galbana (Parke) under a 12:12‐h light:dark cycle at low light (75 μmol photons·m ^{? 2}·s ^{? 1}) and high light (500 μmol photons· m ^{? 2}·s ^{? 1}) conditions. Other associated measurements such as pigment composition, cell density, and diameter as the measure of cell size were also made at the two light regimes every 2 h for 2 days to confirm the periodicity. A distinct diel variability was observed for the a* (λ) with maxima near dawn and minima near dusk. The magnitude of diel variation in a* (440) was 15% at low light and 22% at high light. Pronounced diel patterns were observed for cell size with minima near dawn and maxima near dusk. The magnitude of diel variation in cell size was 9.3% at low light and 21% at high light. The absorption efficiency factors [Q _a (440)] were determined by reconstruction using intracellular concentrations of pigments and cell size. The Q _a (440) also showed a distinct diel variability, with minima near dawn and maxima near dusk. The diel variation in a* (λ) and Q _a (λ) was primarily caused by changes in cell size due to growth, although there was some influence from diel variations in the intracellular pigment concentrations. The results presented here indicated that diel variation in a* (λ) was an important component of the optical characterization of phytoplankton.     

ACCLIMATION OF SYNECHOCOCCUS STRAIN WH7803 TO AMBIENT CO2 CONCENTRATION AND TO ELEVATED LIGHT INTENSITY1

A CO₂ concentrating mechanism has been identified in the phycoerythrin-possessing Synechococcus sp. WH7803 and has been observed to be severely inhibited by short exposure to elevated light intensities. A light treatment of 300–2000 μmol quanta·m^?2·s^?1 resulted in a considerable decay in the variable fluorescence of PSII with time, suggesting decreased efficiency of energy transfer from the phycobilisomes, direct damage to the reaction center II, or both. Measurements of the activity of PSII and changes in fluorescence emission spectra during a light treatment of 1000 μmol quanta·m^?2·s^?1 indicated considerable reduction in the energy flow from the phycocyanin to the phycobilisome terminal acceptor and chlorophyll a. Consequently, whereas the maximal photosynthetic rate, at saturating light and Co₂ concentration, was hardly affected by a light treatment of 1000 μmol quanta·m^?2·s^?1 for 2 h, the light intensity required to reach that maximum increased with the duration of the light treatment.     

Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO₂, ambient + 165 μmol mol^?1 CO₂ or ambient + 330 μmol mol^?1 CO₂ concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO₂ treatments in all seasons. On the basis of A/C_i, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO₂ treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO₂ treatment, rates of net assimilation were ～1·6 times greater in the ambient + 165 μmol mol^?1 CO₂ treatment and 2·2 times greater in the ambient + 330 μmol mol^?1 CO₂ treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO₂ concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO₂ (+ 165 or + 330 μmol mol^?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.     

Carbon dioxide biofixation by Chlorella vulgaris at different CO2 concentrations and light intensities

In order to develop an effective CO₂ mitigation process using microalgae for potential industrial application, the growth and physiological activity of Chlorella vulgaris in photobioreactor cultures were studied. C. vulgaris was grown at two CO₂ concentrations (2 and 13% of CO₂ v/v) and at three incident light intensities (50, 120 and 180 μmol m^?2 s^?1) for 9 days. The measured specific growth rate was similar under all conditions tested but an increase in light intensity and CO₂ concentration affected the biomass and cell concentrations. Although carbon limitation was observed at 2% CO₂, similar cellular composition was measured in both conditions. Light limitation induced a net change in the growth behavior of C. vulgaris. Nitrogen limitation seemed to decrease the nitrogen quota of the cells and rise the intracellular carbon:nitrogen ratio. Exopolysaccharide production per cell appeared to be affected by light intensity. In order to avoid underestimation of the CO₂ biofixation rate of the microalgae, exopolysaccharide production was taken into account. The maximum CO₂ removal rate (0.98 g CO₂ L^?1 d^?1) and the highest biomass concentration (4.14 g DW L^?1) were determined at 13% (v/v) CO₂ and 180 μmol m^?2 s^?1. Our results show that C. vulgaris has a real potential for industrial CO₂ remediation.     

RELATIONSHIP BETWEEN PHOTOSYNTHESIS AND CELL SIZE OF MARINE DIATOMS12

Three photosynthetic parameters of 7 species of marine diatoms were studied using Na₂¹⁴CO₃ at 5–8 C using log phase axenic cultures. The cell volumes of the different species varied from 70 μm³ to 40 × 10⁵μm³. The present experiment is consistent with the interpretation that the initial slope α (mg C · [mg chl a]^?1· h^?1· w^?1· m²) of photosynthesis vs. light curves is controlled by self-shading of chlorophyll a in the cell. Pm, the rate of photosynthesis at light saturation (mg C · [mg cell, C]^?1· h^?1) and R, the intercept at zero light intensity (mg C · [mg cell C]^?1· H^?1) are both dependent on the ratio of surface area to volume of cell.     

SHORT‐ AND LONG‐TERM EFFECTS OF ELEVATED CO2 ON PHOTOSYNTHESIS AND RESPIRATION IN THE MARINE MACROALGA HIZIKIA FUSIFORMIS (SARGASSACEAE,PHAEOPHYTA) GROWN AT LOW AND HIGH N SUPPLIES1

The short‐term and long‐term effects of elevated CO₂ on photosynthesis and respiration were examined in cultures of the marine brown macroalga Hizikia fusiformis (Harv.) Okamura grown under ambient (375 μL · L^?1) and elevated (700 μL · L^?1) CO₂ concentrations and at low and high N availability. Short‐term exposure to CO₂ enrichment stimulated photosynthesis, and this stimulation was maintained with prolonged growth at elevated CO₂, regardless of the N levels in culture, indicating no down‐regulation of photosynthesis with prolonged growth at elevated CO₂. However, the photosynthetic rate of low‐N‐grown H. fusiformis was more responsive to CO₂ enrichment than that of high‐N‐grown algae. Elevation of CO₂ concentration increased the value of K_1/2(Ci) (the half‐saturation constant) for photosynthesis, whereas high N supply lowered it. Neither short‐term nor long‐term CO₂ enrichment had inhibitory effects on respiration rate, irrespective of the N supply, under which the algae were grown. Under high‐N growth, the Q₁₀ value of respiration was higher in the elevated‐CO₂‐grown algae than the ambient‐CO₂‐grown algae. Either short‐ or long‐term exposure to CO₂ enrichment decreased respiration as a proportion of gross photosynthesis (Pg) in low‐N‐grown H. fusiformis. It was proposed that in a future world of higher atmospheric CO₂ concentration and simultaneous coastal eutrophication, the respiratory carbon flux would be more sensitive to changing temperature.     

EXTRACTION,PARTIAL PURIFICATION AND CHARACTERIZATION OF PHOSPHOENOLPYRUVATE CARBOXYKINASE FROM ASCOPHYLLIUM NODOSUM (PHAEOPHYCEAE)1

Phosphoenolpyruvate carboxykinase (PEPCK) from Ascophyllum nodosum (L.) Le Jolis was partially purified and characterized to investigate its role in inorganic carbon assimilation in macroalgae. Inorganic carbon isotopic disequilibrium studies showed that the carboxylation of phosphoenolpyruvate utilized CO₂ rather than HCO₃^?as its source of inorganic carbon. This is consistent with the enzyme being a phosphoenolpyruvate carboxykinase rather than a phosphoenolpyruvate carboxylase. Pre-incubation with Mn²⁺alone activated PEPCK more effectively than when combinations of Mn²⁺, ADP and HCO₃^?were used as activators. Activation of PEPCK during catalysis was found not to occur. Although the activation of PEPCK reduced the K_m for CO₂ by a factor of 2.25, the value reported here of 1.084 mM CO₂ for the activated enzyme at pH 7.0 is at the top of the range of previously reported values for brown algal PEPCK. The specific activity of PEPCK was increased from 0.268 μmol·min^?1·mg^?1in the crude extract to 33.03 μmol·min^?1·mg^?1in the partially purified preparations. Whether PEPCK can act as an initial carboxylating enzyme is discussed. Triton X-100 at 0.57% (v/v) was found to be the optimum detergent and concentration for the extraction of enzymes from A. nodosum. When high concentrations of detergents -were used, a low (NH₄)₂SO₄ cut was required to remove the free detergent from solution, which was extracted by centrifugation. Q Sepharose was used to partially purify PEPCK and separate it from pyruvate kinase. Good protein separations were consistently obtained.     

EFFECT OF TEMPERATURE,LIGHT AND pH ON GROWTH,PHOTOSYNTHESIS AND RESPIRATION OF THE DINOFLAGELLATE PERIDINIUM CINCTUM FA. WESTII IN LABORATORY CULTURES1

Optimum light, temperature, and pH conditions for growth, photosynthetic, and respiratory activities of Peridinium cinctum fa. westii (Lemm.) Lef were investigated by using axenic clones in batch cultures. The results are discussed and compared with data from Lake Kinneret (Israel) where it produces heavy blooms in spring. Highest biomass development and growth rates occurred at ca. 23° C and ≥50 μE· m^?2·s¹ of fluorescent light with energy peaks at 440–575 and 665 nm. Photosynthetic oxygen release was more efficient in filtered light of blue (BG 12) and red (RG 2) than in green (VG 9) qualities. Photosynthetic oxygen production occurred at temperatures ranging from 5° to 32° C in white fluorescent light from 10 to 105 μE·m^?2·s^?1 with a gross maximum value of 1500 × 10^?12 g·cell^?1·h^?1 at the highest irradiance. The average respiration amounted to ca. 12% of the gross production and reached a maximum value of ca. 270·10^?12 g·cell^?1·h^?1 at 31° C. A comparison of photosynthetic and respiratory Q₁₀-values showed that in the upper temperature range the increase in gross production was only a third of the corresponding increase in respiration, although the gross production was at maximum. Short intermittent periods of dark (>7 min) before high light exposures from a halogen lamp greatly increased oxygen production. Depending on the physiological status of the alga, light saturation values were reached at 500–1000 μE·m^?2·s^?1 of halogen light with compensation points at 20–40 μE·m^?2·s^?1 and I_k-values at 100–200 μE·m^?2·s^?1. The corresponding values in fluorescent light in which it was cultured and adapted, were 25 to 75% lower indicating the ability of the alga to efficiently utilize varying light conditions, if the adaptation time is sufficient. Carbon fixation was most efficient at ca. pH 7, but the growth rates and biomass development were highest at pH 8.3.     

EFFECTS OF CO2 ENRICHMENT ON THE BLOOM‐FORMING CYANOBACTERIUM MICROCYSTIS AERUGINOSA (CYANOPHYCEAE): PHYSIOLOGICAL RESPONSES AND RELATIONSHIPS WITH THE AVAILABILITY OF DISSOLVED INORGANIC CARBON1

Microcystis aeruginosa Kütz. 7820 was cultured at 350 and 700 μL·L ^{? 1} CO₂ to assess the impacts of doubled atmospheric CO₂ concentration on this bloom‐forming cyanobacterium. Doubling of CO₂ concentration in the airflow enhanced its growth by 52%–77%, with pH values decreased and dissolved inorganic carbon (DIC) increased in the medium. Photosynthetic efficiencies and dark respiratory rates expressed per unit chl a tended to increase with the doubling of CO₂. However, saturating irradiances for photosynthesis and light‐saturated photosynthetic rates normalized to cell number tended to decrease with the increase of DIC in the medium. Doubling of CO₂ concentration in the airflow had less effect on DIC‐saturated photosynthetic rates and apparent photosynthetic affinities for DIC. In the exponential phase, CO₂ and HCO₃ ^? levels in the medium were higher than those required to saturate photosynthesis. Cultures with surface aeration were DIC limited in the stationary phase. The rate of CO₂ dissolution into the liquid increased proportionally when CO₂ in air was raised from 350 to 700 μL·L ^{? 1}, thus increasing the availability of DIC in the medium and enhancing the rate of photosynthesis. Doubled CO₂ could enhance CO₂ dissolution, lower pH values, and influence the ionization fractions of various DIC species even when the photosynthesis was not DIC limited. Consequently, HCO₃ ^? concentrations in cultures were significantly higher than in controls, and the photosynthetic energy cost for the operation of CO₂ concentrating mechanism might decrease.     

INORGANIC CARBON ACQUISITION BY CHRYSOPHYTES1

Twelve species, representing 12 families of the chrysophytes sensu lato, were tested for their ability to take up inorganic carbon. Using the pH‐drift technique, CO₂ compensation points generally varied between 1 and 20 μmol · L^?1 with a mean concentration of 5 μmol · L^?1. Neither pH nor alkalinity affected the CO₂ compensation point. The concentration of oxygen had a relatively minor effect on CO₂‐uptake kinetics, and the mean CO₂ compensation point calculated from the kinetic curves was 3.6 μmol · L^?1 at 10–15 kPa starting oxygen partial pressure and 3.8 μmol · L^?1 at atmospheric starting oxygen partial pressure (21 kPa). Similarly, uptake kinetics were not affected by alkalinity, and hence concentration of bicarbonate. Membrane inlet mass spectrometry (MIMS) in the presence and absence of acetazolamide suggested that external carbonic anhydrase in Dinobryon sertularia Ehrenb. and Synura petersenii Korschikov was either very low or absent. Rates of net HCO₃^? uptake were very low (～5% of oxygen evolution) using MIMS and decreased rather than increased with increasing HCO₃^? concentration, suggesting that it was not a real uptake. The CO₂ compensation points determined by MIMS for CO₂ uptake and oxygen evolution were similar to those determined in pH‐drift and were >1 μmol · L^?1. Overall, the results suggest that chrysophytes as a group lack a carbon‐concentrating mechanism (CCM), or an ability to make use of bicarbonate as an alternative source of inorganic carbon. The possible evolutionary and ecological consequences of this are briefly discussed.     

PHYSIOLOGICAL RESPONSES OF ANABAENA VARIABILIS (CYANOPHYCEAE) TO INSTANTANEOUS EXPOSURE TO VARIOUS COMBINATIONS OF LIGHT INTENSITY AND TEMPERATURE1

Light intensity and temperature interactions have a complex effect on the physiological process rates of the filamentous bluegreen alga Anabaena variabilis Kütz. The optimum temperature for photosynthesis increased with increasing light intensity from 10°C at 42 μE·m^?2·s^?1 to 35°C at 562 μE·m^?2·s^?1. The light saturation parameter, I_K, increased with increasing temperatures. The maximum photosynthetic rate (2.0 g C·g dry wt.^?1·d^?1) occurred at 35°C and 564 μE·m^?2·s^?1. At 15°C, the maximum rate was 1.25 g C·g dry wt.^?1·d^?1 at 332 μE·m^?2·s^?1. The dark respiration rate increased exponentially with temperature. Under favorable conditions of light intensity and temperature the percent of extracellular release of dissolved organic carbon was less than 5% of the total C fixed. This release increased to nearly 40% under combinations of low light intensity and high temperature. A mathematical model was developed to simulate the interaction of light intensity and temperature on photosynthetic rate. The interactive effects were represented by making the light-saturation parameters a function of temperature.     

LIGHT AND TEMPERATURE AS FACTORS REGULATING SEASONAL GROWTH AND DISTRIBUTION OF ULOTHRIX ZONATA (ULVOPHYCEAE)1

Ulothrix zonata (Weber and Mohr) Kütz. is an unbranched filamentous green alga found in rocky littoral areas of many northern lakes. Field observations of its seasonal and spatial distribution indicated that it should have a low temperature and a high irradiance optimum for net photosynthesis, and at temperatures above 10°C it should show an increasingly unfavorable energy balance. Measurements of net photosynthesis and respiration were made at 56 combinations of light and temperature. Optimum conditions were 5°C and 1100 μE·m^?2·s^?1 at which net photosynthesis was 16.8 mg O₂·g^?1·h^?1. As temperature increased above 5° C optimum irradiance decreased to 125 μE·m^?2·s^?1 at 30°C. Respiration rates increased with both temperature and prior irradiance. Light-enhanced respiration rates were significantly greater than dark respiration rates following irradiance exposures of 125 μE·m^?2·s^?1 or greater. Polynomials were fitted to the data to generate response surfaces. Polynomial equations represent statistical models which can accurately predict photosynthesis and respiration for inclusion in ecosystem models.     

THE RELATIONSHIP OF LIPIDS WITH LIGHT AND CHLOROPHYLL MEASUREMENTS IN FRESHWATER ALGAE AND PERIPHYTON1

Lipid content and lipid class composition were determined in stream periphyton and the filamentous green algae Cladophora sp. and Spirogyra sp, Sterols and phospholipids were compared to chlorophyll a (chl a) as predictors of biomass for stream periphyton and algae. Chlorophyll a, phospholipids, and sterols were each highly correlated with ash-free dry mass (AFDM) (r² > 0.98). Stream periphyton exposed naturally to high light (HL) and low light (LL) had chl a concentrations (μg chl a-mg^?1AFDM) of 7.9± 0.7 and 12.4 ± 2.9, respectively, while the sterol concentrations of these HL and LL stream periphyton (1.6 ± 0.4) were not significantly different (P > 0.05). Periphyton exposed to an irradiance of 300 μmol photons·m^?2s^?1 in the laboratory for 60 h had 5.6 ± 0.55 μg chl a·mg^?1 AFDM, but the same periphyton exposed to 2% incident light for the same amount of time had 11.0 ± 0.56 μg chl a·mg^?1 AFDM. Sterol concentrations in these periphyton communities remained unchanged (1.5 ± 0.3 μg·mg^?1AFDM), Similar results (i.e. changes in chl a but stability of sterol concentrations in response to irradiance changes) were also found for Cladophora and Spirogyra in laboratory experiments. Sterols can be quantified rapidly from a few milligrams of algae and appear to be a useful predictor of eukaryote biomass, whereas cellular levels of chl a vary substantially with light conditions. Phospholipids (or phospholipid fatty acids) are considered to be a reliable measure of viable microbial biomass. Nevertheless, phospholipid content varied substantially and unpredictably among algae and periphyton under different light regimes. Irradiance also had a significant effect on storage lipids: HL Cladophora and HL periphyton had 2 × and 5 × greater concentrations of triacylglycerols, respectively, compared to their LL forms. HL and LL algae also differed in the concentration of several major fatty acids. These light-induced changes in algal lipids and fatty acids have important implications for grazers.     

THE SHORT‐TERM EFFECT OF IRRADIANCE ON THE PHOTOSYNTHETIC PROPERTIES OF ANTARCTIC FAST‐ICE MICROALGAL COMMUNITIES1

Although sea‐ice represents a harsh physicochemical environment with steep gradients in temperature, light, and salinity, diverse microbial communities are present within the ice matrix. We describe here the photosynthetic responses of sea‐ice microalgae to varying irradiances. Rapid light curves (RLCs) were generated using pulse amplitude fluorometry and used to derive photosynthetic yield (Φ_PSII), photosynthetic efficiency (α), and the irradiance (E_k) at which relative electron transport rate (rETR) saturates. Surface brine algae from near the surface and bottom‐ice algae were exposed to a range of irradiances from 7 to 262 μmol photons · m^?2 · s^?1. In surface brine algae, Φ_PSII and α remained constant at all irradiances, and rETR_max peaked at 151 μmol photons · m^?2 · s^?1, indicating these algae are well acclimated to the irradiances to which they are normally exposed. In contrast, Φ_PSII, α, and rETR_max in bottom‐ice algae reduced when exposed to irradiances >26 μmol photons · m^?2 · s^?1, indicating a high degree of shade acclimation. In addition, the previous light history had no significant effect on the photosynthetic capacity of bottom‐ice algae whether cells were gradually exposed to target irradiances over a 12 h period or were exposed immediately (light shocked). These findings indicate that bottom‐ice algae are photoinhibited in a dose‐dependent manner, while surface brine algae tolerate higher irradiances. Our study shows that sea‐ice algae are able to adjust to changes in irradiance rapidly, and this ability to acclimate may facilitate survival and subsequent long‐term acclimation to the postmelt light regime of the Southern Ocean.     

CARBON SKELETON SOURCES FOR AMMONIUM ASSIMILATION IN N-SUFFICIENT AND N-LIMITED CELLS OF CYANIDIUM CALDARIUM (RHODOPHYTA)1

The possible origin of carbon skeletons for ammonium assimilation in Cyanidium caldarium (Tilden) Geitler was investigated. N-sufficient cells assimilated ammonium at a rate of 182 ± 18 μmol·mL packed cell volume (pcv)^-1· h^-1. Removal of CO₂ or darkening almost immediately prevented ammonium assimilation. N-limited cells in light assimilated ammonium at a rate of 493 ± 45 μmol · mL pcv^-1· h^-1 in the presence of CO₂ and at a lower rate of 168 ± 17 μmol · mL pcv^-1· h^-1 in the absence of CO₂. In darkness they assimilated ammonium at a rate of 293 ± 29 μmol · mL pcv^-1 h^-1 in the presence of CO₂, only 60% of the assimilation rate in light. In the absence of CO₂, ammonium was assimilated at a similar rate of 325 ± 14 μmol · mL pcv^-1· h^-1. Under the latter conditions, however, assimilation was inhibited after 40 min and ceased after 70 min; it resumed upon resupply of CO₂. We suggest that N-sufficient cells of C. caldarium obtain carbon skeletons for ammonium assimilation exclusively by photosynthetic reactions. Upon N-limitation they develop the ability, apparently through derepression or activation of regulatory enzyme system(s), to obtain a consistent quantity of additional carbon skeletons and ATP from mobilization of carbon reserves. This enables the N-limited cell to assimilate ammonium not only in light but also in darkness, and at a higher rate than N-sufficient cells. The fact that ammonium assimilation in light occurs at a higher rate than in darkness suggests that ammonium assimilation in light is the sum of both light and dark ammonium assimilation, which implies separate metabolic reactions for the two processes. These results suggest the existence of two distinct and differently controlled pathways in N-limited cells, but not in N-sufficient cells, through which carbon skeletons for ammonium assimilation originate. An important role for dark CO₂ fixation in dark or light ammonium assimilation is also indicated.