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Waters of the Southern Ocean are characterized by high macronutrient concentrations but limited availability of trace metals and light, often making it difficult for phytoplankton to achieve maximum growth rates. One strategy employed by Southern Ocean phytoplankton in culture to cope with low light and low dissolved iron (DFe) is to enhance light absorption by increasing their antenna size rather than the number of reaction centers, thereby reducing their Fe demand. Here we provide physiological evidence that natural populations of Southern Ocean phytoplankton employ a similar photoacclimation strategy to cope with low ambient DFe concentrations. During a research cruise to the Ross Sea in 2013-2014, we conducted 4 bioassay experiments in which we manipulated light and DFe concentrations and measured changes in phytoplankton biomass, growth rate, photosynthetic parameters, fluorescence parameters, and pigment composition. Phytoplankton responded strongly to DFe additions, exhibiting significantly higher biomass, growth rates, and photosynthetic competency. At low light, the maximum photosynthetic rate (P*max) was significantly reduced and the photosynthetic efficiency (α*) was unchanged compared to the high light treatment, regardless of phytoplankton species composition or DFe concentration. Our data suggest that Southern Ocean phytoplankton have evolved an Fe-saving strategy whereby they photoacclimate to low light by increasing their photosynthetic unit size, rather than photosynthetic unit number, even when DFe is available. It appears this Fe-saving strategy is characteristic of both Phaeocystis Antarctica and diatoms, suggesting that it is a common adaptation among phytoplankton taxa that grow under Fe limitation in the Southern Ocean.

The western Antarctic Peninsula has experienced extreme changes in the timing of sea ice melt and freeze up, shortening the duration of the seasonal sea ice cycle. While previous research demonstrated connections between multiple pelagic trophic levels and the physics of the sea ice, few studies have assessed the sea ice ecosystem or its linkage to the ocean ecosystem in this region. Through a field survey and shipboard experiments, our study focused on characterizing the spring ice algal bloom and elucidating its role in seeding phytoplankton communities post-ice melt in high and low light conditions. Field data revealed that algal communities in slush layers, often formed from the flooding of seawater (infiltration layers), dominated biomass distributions in the sea ice throughout the region, and showed distinct photophysiological characteristics from interior or bottom ice communities. Sea ice algal biomass reached 120 mg chl a m−2 and was often dominated by Phaeocystis antarctica. Shipboard growth experiments showed thatprior light history (ice or water column), rather than community composition (phytoplankton and ice algae were composed of similar taxa), primarily drove physiological responses to high and low light. P. antarctica generally dominated the community in growth experiments at the end of the 6 d incubation period. Settling column experiments suggested that P. antarctica’s higher sinking ratesrelative to other taxa may explain its minor contributions to the summer phytoplankton community in single-cell form and its absence in colonial form, observed in the long-term ecological record of this region.

On coral reefs, feeding by parrotfishes may prevent coral to macroalgal phase shifts by keeping algae in a cropped state. To predict the response of grazers to changing conditions, knowledge of the factors that affect feeding intensity is needed. Therefore, we studied food intake of the parrotfish Scarus ferrugineus on an inshore fringing reef in the southern Red Sea, Eritrea, where seasonality and extreme summer temperatures were expected to influence feeding rates. Bite rates (bites min(-1)), defecation rates (defecations min(-1)) and yield per bite (mass ingested bite(-1)) were estimated for 3 life-phase categories: 20 to 25 cm initial phase (IP), 30 to 35 cm terminal phase (TP) and 30 to 35 cm territorial terminal phase (TTP). The bite rates increased from low levels in the morning to peak values in the afternoon. IP bite rates increased linearly with seawater temperature. For TP, the temperature-bite rate relationship was quadratic, with maximum bite rates at similar to 32 degrees C and lower rates at higher temperatures. Yield per bite in both IP and TP did not differ with time of year. Throughout the year, the total daily bites were highest in IP and lowest in TTP, resulting in similar to 23% lower daily intake in TTP compared to similar-sized TP males. Daily intake of ash-free dry mass of epilithic algal matrix (g AFDM d(-1)) was 7.5 to 9.6 for IP, 12.4 to 17.5 for TP and 8.7 to 13.2 for TTP. Intake and defecation rates and hence gut turnover rates peaked from April to July. The quadratic temperature response of TP suggests that large individuals may be close to their upper thermal limit, implying that temperature rises are likely to negatively affect grazing by large-bodied parrotfishes.

It is expected that climate change will expand the open oligotrophic oceans by enhanced thermal stratification. Because temperature defines the geographic distribution of picophytoplankton in open-ocean ecosystems and regulates photophysiological responses, it is important to understand how temperature affects picophytoplankton growth and photophysiology. Two prokaryotic and 2 eukaryotic picophytoplankton strains were acclimated to 3 different temperatures, ranging from 16 to 24 degrees C. Temperature-dependent growth and photophysiology were assessed by measurements of specific growth rates, cell size, pigment composition, absorption and electron transport rates. Growth of Prochlorococcus marinus (eMED4), Prochlorococcus sp. (eMIT9313), Ostreococcus sp. (clade B) and Pelagomonas calceolata was positively related to temperature, especially in the prokaryotic strains. Changes in photophysiology included increased light harvesting, increased electron transport and reduced photoinhibition at elevated temperatures. However, the changes related to light harvesting and electron transport could not fully explain the observed difference in growth. This suggests that other processes, such as Calvin cycle activity, are likely to limit growth at sub-optimal temperatures in these picophytoplankton strains. The overall changes in photophysiology during temperature acclimation will possibly allow photosynthesis at higher irradiance intensities, but the genetically defined low temperature tolerances and photosynthetic characteristics of the different ecotypes will likely be more important in determining picophytoplankton (depth) distribution and community composition.

Photoacclimation strategies and sensitivity to photoinhibition were determined in natural phytoplankton assemblages during a phytoplankton bloom in the Amundsen Sea (South- ern Ocean) in relation to community composition, pigment content, light, and iron (Fe). Non- photochemical quenching (qN) was measured during recovery after surface irradiance exposure (SIE) for 20 min. The qN was separated into slow (qI, photoinhibition through damage of Photosys- tem II) and fast (qE, xanthophyll cycling) relaxing components. Phytoplankton within the upper mixed layer (UML) showed a higher ratio of photoprotective xanthophyll cycle pigments (diadino- xanthin (DD) + diatoxanthin (DT)) to chlorophyll a (chl a), indicative of acclimation to high light, which resulted in lower photoinhibition after SIE when compared to phytoplankton residing below the UML. Within the UML, we found differences in photoacclimation strategies in assem- blages dominated by Antarctic diatoms versus Phaeocystis antarctica (Haptophyta). Diatoms had a higher ratio of (DD + DT)/chl a, and the ratio tracked mean light levels within the UML, whereas this relationship was not apparent in P. antarctica, which had a lower (DD + DT)/chl a ratio. Despite these differences, diatoms and P. antarctica exhibited similar degrees of qN that were dominated by qE with very little qI. Bioassays under high and low Fe concentrations revealed an increase in the (DD + DT)/chl a ratio in Fe-limited populations dominated by diatoms and decreased photoinhibition. In experiments dominated by P. antarctica or with mixed populations, acclimation to low Fe increased the (DD + DT)/chl a ratio in most experiments; however, this did not affect photoinhibition. This study shows that under in situ conditions in the Amundsen Sea (1) phytoplankton photo acclimation efficiently minimizes photoinhibition, (2) photoinhibition does not control the relative abundances of P. antarctica or Antarctic diatoms, and (3) Fe limitation does not increase photo inhibition of either P. antarctica or Antarctic diatoms.

Seven species representative of the benthic diatom community of the tidal flats in the Dutch Wadden Sea hardly differed in their sensitivity to ultraviolet-B radiation (UVBR). Some isolates had been cultured in the laboratory for up to 20 yr. Cell numbers of all species increased at a rate similar to unexposed cultures up to a DNA-weighted daily UVBR dose of 3.5 kJ m(-2) d(-1) (biologically effective dose, normalized at 300 nm); only at higher UVB irradiance levels did the growth rate become reduced. No clear relationship between mean cell size and UVBR sensitivity was observed. The benthic diatoms that were tested are apparently adapted to the natural, high UVB irradiance incident at tidal flats during spring and summer. Thus, even a sharp UVBR increase resulting from severe stratospheric ozone reduction would hardly affect tidal flat diatom communities by influencing cell division rate. In contrast, growth of representatives of the phytoplankton community was already seriously affected by doses that were 10 times lower. This is in agreement with their natural, low UVBR exposure.

Shipboard studies were performed for testing the classical hypothesis that Antarctic phytoplankton suffers from a deficiency of Fe. In a suite of 5 experiments over 8 to 12 d periods and encompassing different water masses (Weddell Sea water proper, Weddell-Scotia Confluence water, Scotia Sea water), and various plankton communities, biomass and dynamic spring/summer (ice) conditions, we always observed Fe to stimulate chlorophyll a synthesis and nutrient assimilation. In 3 out of 5 experiments there was an immediate response to added Fe, while in the other 2 expenments an effect was observed after 3 to 6 d. In 4 out of 5 experiments final particulate organic carbon (POC) levels were also higher in Fe-enriched cultures compared to controls. However the controls were also found to outgrow steadily typical chlorophyll a and POC levels found in ambient waters. This strongly suggests that the in situ Fe concentration in itself does not hamper build-up of high biomass levels. Extrapolation to the in situ ecosystem therefore suggests that, despite enhancement of phytoplankton growth, Fe is not the major factor controlling phytoplankton in the Weddell/Scotia Seas. Marginal sediments appear to supply adequate dissolved Fe for supporting at least minimum growth of phytoplankton. More remote sectors of the Southern Ocean might be more likely candidates for occasional limitation by Fe alone.