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The Antarctic marine environment is a dynamic ecosystem where microorganisms play an important role in key biogeochemical cycles. Despite the role that microbes play in this ecosystem, little is known about the genetic and metabolic diversity of Antarctic marine microbes. In this study we leveraged DNA samples collected by the Palmer Long Term Ecological Research (LTER) project to sequence shotgun metagenomes of 48 key samples collected across the marine ecosystem of the western Antarctic Peninsula (wAP). We developed an in silico metagenomics pipeline (iMAGine) for processing metagenomic data and constructing metagenome-assembled genomes (MAGs), identifying a diverse genomic repertoire related to the carbon, sulfur, and nitrogen cycles. A novel analytical approach based on gene coverage was used to understand the differences in microbial community functions across depth and region. Our results showed that microbial community functions were partitioned based on depth. Bacterial members harbored diverse genes for carbohydrate transformation, indicating the availability of processes to convert complex carbons into simpler bioavailable forms. We generated 137 dereplicated MAGs giving us a new perspective on the role of prokaryotes in the coastal wAP. In particular, the presence of mixotrophic prokaryotes capable of autotrophic and heterotrophic lifestyles indicated a metabolically flexible community, which we hypothesize enables survival under rapidly changing conditions. Overall, the study identified key microbial community functions and created a valuable sequence library collection for future Antarctic genomics research.

The extent, duration, and seasonality of sea ice and glacial discharge strongly influence Antarctic marine ecosystems. Most organisms' life cycles in this region are attuned to ice seasonality. The annual retreat and melting of sea ice in the austral spring stratifies the upper ocean, triggering large phytoplankton blooms. The magnitude of the blooms is proportional to the winter extent of ice cover, which can act as a barrier to wind mixing. Antarctic krill, one of the most abundant metazoan populations on Earth, consume phytoplankton blooms dominated by large diatoms. Krill, in turn, support a large biomass of predators, including penguins, seals, and whales. Human activity has altered even these remote ecosystems. The western Antarctic Peninsula region has warmed by 7°C over the past 50 years, and sea ice duration has declined by almost 100 days since 1978, causing a decrease in phytoplankton productivity in the northern peninsula region. Besides climate change, Antarctic marine systems have been greatly altered by harvesting of the great whales and now krill. It is unclear to what extent the ecosystems we observe today differ from the pristine state.

We investigated the dynamics of heterotrophic bacteria in the coastal western Antarctic Peninsula (WAP), using decadal (2002-2014) time series of two bacterial variables, bacterial production (BP) via 3H-leucine incorporation rates and bacterial biomass (BB) via bacterial abundance, collected at Palmer Antarctica Long Term Ecological Research (LTER) Station B (64.8°S, 64.1°W) over a full austral growing season (October-March). Strong seasonal and interannual variability in the degree of bacterial coupling with phytoplankton processes were observed with varying lags. On average, BP was only 4% of primary production (PP), consistent with low BP:PP ratios observed in polar waters. BP was more strongly correlated with chlorophyll (Chl), than with PP, implying that bacteria feed on DOC produced from a variety of trophic levels (e.g. zooplankton sloppy feeding and excretion) as well as directly on phytoplankton-derived DOC. The degree of bottom-up control on bacterial abundance was moderate and relatively consistent across entire growing seasons, suggesting that bacteria in the coastal WAP are under consistent DOC limitation. Temperature also influenced BP rates, though its effect was weaker than DOC. We established generalized linear models (GLMs) for monthly composites of BP and BB via stepwise regression to explore a set of physical and biogeochemical predictors. Physically, high BP and large BB were shaped by a stratified water-column, similar to forcing mechanisms favoring phytoplankton blooms, but high sea surface temperature (SST) also significantly promoted bacterial processes. High BP and large BB were influenced by high PP and bulk DOC concentrations. Based on these findings, we suggest an increasingly important role of marine heterotrophic bacteria in the coastal WAP food-web as climate change introduces a more favorable environmental setting for promoting BP, with increased DOC from retreating glaciers, a more stabilized upper water-column from ice-melt, and a baseline shift of water temperature due to more frequent delivery of warming Upper Circumpolar Deep Water (UCDW) onto the WAP shelf.

The West Antarctic Peninsula (WAP) is home to large breeding colonies of the ice-dependent Antarctic Adélie penguin (Pygoscelis adeliae). Although the entire inner continental shelf is highly productive, with abundant phytoplankton and krill populations, penguin colonies are distributed heterogeneously along the WAP (Ducklow et al., 2013, in this issue). This ecological conundrum targets a long-standing question of interest: what environmental factors structure the locations of Adélie penguin "hot spots" throughout the WAP? - See more at: http://tos.org/oceanography/archive/26-3_schofield.html#sthash.KsdW5rk4.dpuf

A primary research objective of the Palmer Long Term Ecological Research (LTER) program has been to identify and understand the factors that regulate the demography of Adélie penguins (Pygoscelis adeliae). In this context, our work has been focused on variability in the marine environment on which this species depends for virtually all aspects of its life history (Ainley, 2002). As we show here, however, there are patterns evident in the population dynamics of Adélie penguins that are better explained by variability in breeding habitat quality rather than by variability in the marine system. Interactions between the geomorphology of the terrestrial environment that, in turn, affect patterns of snow deposition, drive breeding habitat quality.

Abstract: We report on results of a long-term (1993–2007) time series sediment trap moored at 170m to the west of the Antarctic Peninsula in the mid-continental shelf region (350m depth; 64°30′S, 66°00′W). This is a region characterized by late spring–summer diatom blooms, moderately high seasonal primary productivity (50–150mmolCm−2 d−1 in December–February), and high phytoplankton and krill biomass in the seasonal sea-ice zone. The mass flux ranged from near 0 to over 1gm−2 d−1 and was near 0% to >30% organic carbon (mean 8%). Sedimentation from the upper ocean as estimated by the trap collections at 170m exhibited strong seasonality with high fluxes (1–10mmolCm−2 d−1) in November–March following ice retreat and very low fluxes (<0.001mmolCm−2 d−1) during the Austral winter and under sea-ice cover. An average of 85% of the annual export of 212mmolCm−2 occurred during the seasonal peak flux episodes. Over the trap record, the annual peak flux episode has tended to occur later in the Austral summer, advancing by about 40 days since 1993. The time-integrated sedimentation during the peak flux episode was <1–50% of the SeaWiFS-estimated primary production (mean 4%) at the trap site over the period 1998–2006. The elemental composition of material captured in the traps had an average C:N:P of 212:25:1, greater than the canonical Redfield values. High C:P ratios (400–600) corresponded with the annual flux peak, indicating preferential loss of P from the sinking particles in the summer, ice-free period. The composition of the exported material more closely approximated the Redfield composition during the low-flux, winter period. [Copyright &y& Elsevier]

Antarctic krill are a key species in the Southern Ocean ecosystem, and their life cycle appears to be correlated with, and by implication dependent upon, seasonal sea ice dynamics. Moving from correlations with environmental parameters to an understanding of the mechanisms that lead to these correlations may allow predictions of the consequences of climate change on the distribution of favourable habitat for Antarctic krill. During winter cruises in 2001 and 2002 in the region west of the Antarctic Peninsula, one of the most rapidly warming regions on the planet, ice camps were established for periods of 3-9 days. Timing of sea ice advance, chlorophyll a concentrations in ice cores, and growth rates and pigment content of larval krill all differed significantly between winters. Growth rates and pigment content of larval krill from the same ice floe were correlated, suggesting that growth rates in winter are a function of the biomass of the sea ice microbial community. A possible mechanism underlying the correlation between recruitment success and timing of ice advance is proposed. In conjunction with other postulated habitat requirements, this proposed mechanism allows for speculation about future changes in the geographic location of favourable habitat for Antarctic krill. [ABSTRACT FROM AUTHOR]

Abstract: The relationship between food web structure and function across two ocean biomes was investigated using an inverse method to recover solutions of food web carbon flows. We estimated the carbon exchanges between major assemblages within plankton food webs in the North Atlantic, using the JGOFS NABE data set (1989) and near the Western Antarctic Peninsula (WAP), using the Palmer Station LTER data set, two areas exhibiting strong seasonal phytoplankton blooms. The recovery of all the potential flows of carbon allowed a system-level analysis, providing insight to processes that are seldom measured in the field and a means of comparing food webs from different regions. In the NABE food web, the dominant carbon flows involved the microorganisms including bacterial carbon demand and grazing by microzooplankton and protozoans. In the WAP food web, krill had the most significant carbon flows including grazing of large phytoplankton and respiration. A comparison between the NABE and the WAP carbon-based food webs showed key differences. Recycling and the activity of the microbial food web were much greater in the NABE food web than in the WAP. However in the WAP inverse solution, the microbial food web was just as important as the classical food web (diatoms to krill to penguins) that is traditionally believed to dominate carbon flows. Carbon flows through the NABE and WAP regions were more highly dependent on recycling than would be anticipated from the size structure of the primary producers, when analyzed using a classification scheme of Legendre, L., Rassoulzadegan, F [1996. Food-web mediated export of biogenic carbon in oceans: hydrodynamic control. Marine Ecology Progress Series 145, 179–193]. [Copyright &y& Elsevier]