van Leeuwe, MA, et al. 2018 Microalgal community structure and primary production in Arctic and Antarctic sea ice: A synthesis. Elem Sci Anth, 6: 4. DOI: https://doi.org/10.1525/elementa.267 IntroductionSea ice is one of the largest biomes on earth. The area covered by Arctic (15.6 × 10 6 km 2 ) and Antarctic (18.8 × 10 6 km 2 ) sea ice is roughly 4 and 5% of the global ocean surface (361.9 × 10 6 km 2 ) at their respective maximum extents (Meier, 2017;Stammerjohn and Maksym, 2017). Sea ice is a very diverse and potentially very productive habitat, with primary production estimated to amount to 2-24% of total production in sea-ice covered marine areas (Arrigo, 2017). Sea ice is especially productive in spring and summer when, locally, carbon biomass can be ten times higher in the bottom ice than in the seawater, with values greater than 3 mg chlorophyll a) in bottom layers (e.g., Corneau et al., 2013). On some occasions, ice algae may contribute up to 50-60% of total primary production McMinn et al., 2010; Fernandez-Mendez et al., 2015). Sympagic (ice-associated) microalgae (see Horner et al., 1992, for terminology) are REVIEWMicroalgal community structure and primary production in Arctic and Antarctic sea ice: A synthesis Sea ice is one the largest biomes on earth, yet it is poorly described by biogeochemical and climate models. In this paper, published and unpublished data on sympagic (ice-associated) algal biodiversity and productivity have been compiled from more than 300 sea-ice cores and organized into a systematic framework. Significant patterns in microalgal community structure emerged from this framework. Autotrophic flagellates characterize surface communities, interior communities consist of mixed microalgal populations and pennate diatoms dominate bottom communities. There is overlap between landfast and pack-ice communities, which supports the hypothesis that sympagic microalgae originate from the pelagic environment. Distribution in the Arctic is sometimes quite different compared to the Antarctic. This difference may be related to the time of sampling or lack of dedicated studies. Seasonality has a significant impact on species distribution, with a potentially greater role for flagellates and centric diatoms in early spring. The role of sea-ice algae in seeding pelagic blooms remains uncertain. Photosynthesis in sea ice is mainly controlled by environmental factors on a small scale and therefore cannot be linked to specific ice types. Overall, sea-ice communities show a high capacity for photoacclimation but low maximum productivity compared to pelagic phytoplankton. Low carbon assimilation rates probably result from adaptation to extreme conditions of reduced light and temperature in winter. We hypothesize that in the near future, bottom communities will develop earlier in the season and develop more biomass over a shorter period of time as light penetration increases due to the thinning of sea ice. The Arctic is already witnessing changes. The shift forward in time of the algal bloom can resu...

The Arctic sea-ice-scape is rapidly transforming. Increasing light penetration will initiate earlier seasonal primary production. This earlier growing season may be accompanied by an increase in ice algae and phytoplankton biomass, augmenting the emission of dimethylsulfide and capture of carbon dioxide. Secondary production may also increase on the shelves, although the loss of sea ice exacerbates the demise of sea-ice fauna, endemic fish and megafauna. Sea-ice loss may also deliver more methane to the atmosphere, but warmer ice may release fewer halogens, resulting in fewer ozone depletion events. The net changes in carbon drawdown are still highly uncertain. Despite large uncertainties in these assessments, we expect disruptive changes that warrant intensified long-term observations and modelling efforts.

Antarctic pack ice is inhabited by a diverse and active microbial community reliant on nutrients for growth. Seeking patterns and overlooked processes, we performed a large-scale compilation of macro-nutrient data (hereafter termed nutrients) in Antarctic pack ice (306 ice-cores collected from 19 research cruises). Dissolved inorganic nitrogen and silicic acid concentrations change with time, as expected from a seasonally productive ecosystem. In winter, salinity-normalized nitrate and silicic acid concentrations (C*) in sea ice are close to seawater concentrations (C w ), indicating little or no biological activity. In spring, nitrate and silicic acid concentrations become partially depleted with respect to seawater (C* < C w ), commensurate with the seasonal build-up of ice microalgae promoted by increased insolation. Stronger and earlier nitrate than silicic acid consumption suggests that a significant fraction of the primary productivity in sea ice is sustained by flagellates. By both consuming and producing ammonium and nitrite, the microbial community maintains these nutrients at relatively low concentrations in spring. With the decrease in insolation beginning in late summer, dissolved inorganic nitrogen and silicic acid concentrations increase, indicating imbalance between their production (increasing or unchanged) and consumption (decreasing) in sea ice. Unlike the depleted concentrations of both nitrate and silicic acid from spring to summer, phosphate accumulates in sea ice (C* > C w ). The phosphate excess could be explained by a greater allocation to phosphorus-rich biomolecules during ice algal blooms coupled with convective loss of excess dissolved nitrogen, preferential remineralization of phosphorus, and/or phosphate adsorption onto metal-organic complexes. Ammonium also appears to be efficiently adsorbed onto organic matter, with likely consequences to nitrogen mobility and availability. This dataset supports the view that the sea ice microbial community is highly efficient at processing nutrients but with a dynamic quite different from that in oceanic surface waters calling for focused future investigations.

The Baltic Sea is one of the world's largest brackish water basins and is traditionally considered to be species poor. Here, we assessed the diversity of the nanosized eukaryotic microbial wintertime community, using molecular ecological methods based on sequencing of small-subunit ribosomal RNA gene clone libraries. The results demonstrate that a rich community of small eukaryotes inhabits the Baltic Sea ice and water during winter. The community was dominated by alveolates and stramenopiles. Ciliates and cercozoans were the richest groups present, while in contrast to previous studies, diatoms showed a lower richness than expected. Furthermore, fungi and parasitic Syndiniales were present both in the water and in the sea ice. Some of the organisms in the sea-ice community were active, based on the RNA data, but a number of organisms were inactive or remnants from the freezing process. The results demonstrate that the sea-ice communities in the Baltic Sea are highly diverse and that water and ice of diVerent ages include diVerent protistan assemblages. Our study emphasizes the potential loss in biodiversity through diminishing ice cover as a result of climate change.

We report the seasonal and vertical variations of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) in fast ice at Cape Evans, McMurdo Sound (Antarctica) during the springsummer transition in 2011 and winter-spring transition in 2012. We compare the variations of DMS,P observed to the seasonal evolution of the ice algal biomass and of the physical properties of the ice cover, with emphasis on the ice texture and brine dynamics. Isolated DMS and DMSP maxima were found during both seasonal episodes in interior ice and corresponded to the occurrence of platelet crystals in the ice texture. We show that platelet crystals formation corresponded in time and depth to the incorporation of dinoflagellates (strong DMSP producers) in the ice cover. We also show that platelet crystals could modify the environmental stresses on algal cells and perturb the vertical redistribution of DMS,P concentrations. We show that during the winter-spring transition in 2012, the DMS,P profiles were strongly influenced by the development and decline of a diatom-dominated bloom in the bottom ice, with DMSP variations remarkably following chl a variations. During the spring-summer transition in 2011, the increase in brine volume fraction (influencing ice permeability) on warming was shown to trigger (1) an important release of DMS to the under-ice water through brine convection and (2) a vertical redistribution of DMSP across the ice.

Sea-ice samples intended for biological analyses, e.g., chlorophyll-a, cell enumeration of algae and protozoa and primary production, are affected by the sampling and sample processing methods. In this study, we compared different sample processing methods by melting Baltic Sea ice samples in different ways (direct melting, buffered melting in filtered seawater (FSW) and buffered melting in artificial seawater at two different salinities with added nutrients) at two temperatures [?4°C and room temperature (RT)]. We show that sea-ice samples intended for most commonly used biological analyses can be melted without the addition of FSW. In particular, adding artificial seawater should be avoided. To minimize biological processes, such as growth, death, predation and pigment degradation, the melting should be done rapidly at RT preferably by gently shaking the sample to keep the melt cool.

A laboratory experiment was conducted to study differences in the use of varying phosphorus sources by the bloom-forming filamentous cyanobacteria Aphanizomenon flos-aquae and Nodularia spumigena. Axenic strains were grown in typical bloom-time light regimes under phosphate-replete, added organic phosphorus phosphate-depleted and completely phosphorus-depleted conditions. Responses to the treatments differed clearly. A. flos-aquae growth was dependent on an ample supply of inorganic dissolved phosphorus, whereas N. spumigena grew equally well in all treatments. Cellular N:P ratios showed high plasticity for both species and phosphorus deficiency did not seem to affect nitrogen fixation of either species. Regarding bloom formation, the results call attention to the importance of the omnipresence, eurythermal growth and deeper vertical positioning of A. flos-aquae in comparison to N. spumigena. N. spumigena is able to form and sustain bloom biomasses relying on cellular phosphorus storage and effective remineralization of organic phosphorus compounds.

Viruses are recognized as important actors in ocean ecology and biogeochemical cycles, but many details are not yet understood. We participated in a winter expedition to the Weddell Sea, Antarctica, to isolate viruses and to measure virus-like particle abundance (flow cytometry) in sea ice. We isolated 59 bacterial strains and the first four Antarctic sea-ice viruses known (PANV1, PANV2, OANV1 and OANV2), which grow in bacterial hosts belonging to the typical sea-ice genera Paraglaciecola and Octadecabacter. The viruses were specific for bacteria at the strain level, although OANV1 was able to infect strains from two different classes. Both PANV1 and PANV2 infected 11/15 isolated Paraglaciecola strains that had almost identical 16S rRNA gene sequences, but the plating efficiencies differed among the strains, whereas OANV1 infected 3/7 Octadecabacter and 1/15 Paraglaciecola strains and OANV2 1/7 Octadecabacter strains. All the phages were cold-active and able to infect their original host at 0°C and 4°C, but not at higher temperatures. The results showed that virus-host interactions can be very complex and that the viral community can also be dynamic in the winter-sea ice.