The effects of iron limitation on photoacclimation to dynamic irradiance were studied in Phaeocystis antarctica G. Karst. and Fragilariopsis cylindrus (Grunow) W. Krieg. in terms of growth rate, photosynthetic parameters, pigment composition, and fluorescence characteristics. Under dynamic light conditions mimicking vertical mixing below the euphotic zone, P. antarctica displayed higher growth rates than F. cylindrus both under iron (Fe)-replete and Fe-limiting conditions. Both species showed xanthophyll de-epoxidation that was accompanied by low levels of nonphotochemical quenching (NPQ) during the irradiance maximum of the light cycle. The potential for NPQ at light levels corresponding to full sunlight was substantial in both species and increased under Fe limitation in F. cylindrus. Although the decline in Fv /Fm under Fe limitation was similar in both species, the accompanying decrease in the maximum rate of photosynthesis and growth rate was much stronger in F. cylindrus. Analysis of the electron transport rates through PSII and on to carbon (C) fixation revealed a large potential for photoprotective cyclic electron transport (CET) in F. cylindrus, particularly under Fe limitation. Probably, CET aided the photoprotection in F. cylindrus, but it also reduced photosynthetic efficiency at higher light intensities. P. antarctica, on the other hand, was able to efficiently use electrons flowing through PSII for C fixation at all light levels, particularly under Fe limitation. Thus, Fe limitation enhanced the photophysiological differences between P. antarctica and diatoms, supporting field observations where P. antarctica is found to dominate deeply mixed water columns, whereas diatoms dominate shallower mixed layers.

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Abstract. The hydrographic properties of the KongsfjordenKrossfjorden system (79 • N, Spitsbergen) are affected by Atlantic water incursions as well as glacier meltwater runoff. This results in strong physical gradients (temperature, salinity and irradiance) within the fjords. Here, we tested the hypothesis that glaciers affect phytoplankton dynamics as early as the productive spring bloom period. During two campaigns in 2007 (late spring) and 2008 (early spring) we studied hydrographic characteristics and phytoplankton variability along two transects in both fjords, using highperformance liquid chromatography (HPLC)-CHEMTAX pigment fingerprinting, molecular fingerprinting (denaturing gradient gel electrophoresis, or DGGE) and sequencing of 18S rRNA genes. The sheltered inner fjord locations remained colder during spring as opposed to the outer locations. Vertical light attenuation coefficients increased from early spring onwards, at all locations, but in particular at the inner locations. In late spring meltwater input caused stratification of surface waters in both fjords. The inner fjord locations were characterized by overall lower phytoplankton biomass. Furthermore HPLC-CHEMTAX data revealed that diatoms and Phaeocystis sp. were replaced by small nano-and picophytoplankton during late spring, coinciding with low nutrient availability. The innermost stations showed higher relative abundances of nano-and picophytoplankton throughout, notably of cyanophytes and cryptophytes. Molecular fingerprinting revealed a high similarity between inner fjord samples from early spring and late spring samples from all locations, while outer samples from early spring clustered separately. We conclude that glacier influence, mediated by early meltwater input, modifies phytoplankton biomass and composition already during the spring bloom period, in favor of low biomass and small cell size communities. This may affect higher trophic levels especially when regional warming further increases the period and volume of meltwater.

To study how natural Southern Ocean phytoplankton communities acclimate to rapid fluctuations in irradiance levels that result from deep wind-driven mixing of the upper water column, we measured their fluorescence properties (Fv : Fm, maximum quantum yield of photosystem II; and qN, non-photochemical quenching) and pigment composition. Values of Fv : Fm were low (, 0.46) and qN was high (. 0.67) throughout the upper mixed layer (UML). Short-term (20-min) exposure to incident surface irradiance strongly reduced Fv : Fm and recovery was slow under subsequent incubation at low irradiance. This suggests that phytoplankton cells are frequently photodamaged when mixed up to the surface from depth. Recovery of Fv : Fm was suppressed when lincomycin was added, inhibiting synthesis of the photosystem II reaction center D1 protein. This indicates that D1 protein repair is crucial in maintaining photosynthetic performance under fluctuating irradiance levels.

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Effects of fluctuating irradiance regimes on excessive photosynthetically active radiation (PAR) and ultraviolet (UV) radiation sensitivity were assessed for Emiliania huxleyi (Lohman) and Thalassiosira weissflogii (Grunow) Fryxell and Hasle. Cultures acclimated to low irradiance were subjected to two irradiance regimes of equal daily dose: dynamic irradiance simulating vertical mixing within the water column and constant irradiance. For each regime two irradiance levels were studied. Growth was monitored for 3 d, after which pigment composition was determined. Next, excessive PAR and UV sensitivity was measured by studying viability loss during 4-h exposure to simulated surface irradiance (SSI). Furthermore, the effects of inhibition of D1 reaction center protein turnover were investigated by incubating samples with lincomycin prior to exposure. Dynamic irradiance reduced growth rates of both species as compared to constant irradiance. Pools of light-harvesting pigments increased in dynamic irradiance, whereas the protective pigment pools decreased compared to constant irradiance. Excessive irradiance sensitivity was enhanced in cells grown in fluctuating irradiance. Furthermore, viability loss was most pronounced in UV treatments combined with lincomycin. E. huxleyi was more sensitive to excessive irradiance than T. weissflogii, which coincided with a lower ratio between protective and light-harvesting pigments in the former species. Irradiance modulation by deep vertical mixing influences growth, pigment composition, and excessive PAR and UV sensitivity within days.

The enzyme superoxide dismutase (SOD) holds a key position in the microalgal antioxidant network. The present research focused on oxidative stress responses in the Antarctic diatom Chaetoceros brevis F. Schütt during transition to excess (including ultraviolet radiation [UVR]) and limiting irradiance conditions. Over a 4 d period, cellular responses of thiobarbituric acid reactive substances (TBARS, a general oxidative stress indicator), SOD activity, photosynthetic and xanthophyll cycle pigments, PSII efficiency, and growth were determined. In addition, oxidative responses were measured during a daily cycle. Changing irradiance conditions significantly affected growth rates of C. brevis. PSII efficiency decreased significantly during periodic excess irradiance and increased under low irradiance conditions. Transition to excess irradiance increased the ratio of xanthophyll to light-harvesting pigments, whereas the opposite was observed for cultures transferred to low irradiance. This acclimation process was completed after 2 d in the new irradiance environment. SOD activity increased significantly after the first day regardless of the new irradiance environment but returned to preexposure values on the fourth day. We hypothesize that SOD activity may be temporarily elevated in C. brevis after irradiance shifts, thereby reducing oxidative stress when photoacclimation is in progress.