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The Southern Ocean (SO) harbors some of the most intense phytoplankton blooms on Earth. Changes in temperature and iron availability are expected to alter the intensity of SO phytoplankton blooms, but little is known about how these changes will influence community composition and downstream biogeochemical processes. We performed light-saturated experimental manipulations on surface ocean microbial communities from McMurdo Sound in the Ross Sea to examine the effects of increased iron availability (+2 nM) and warming (+3 and +6 °C) on nutrient uptake, as well as the growth and transcriptional responses of two dominant diatoms, Fragilariopsis and Pseudo-nitzschia . We found that community nutrient uptake and primary productivity were elevated under both warming conditions without iron addition (relative to ambient −0.5 °C). This effect was greater than additive under concurrent iron addition and warming. Pseudo-nitzschia became more abundant under warming without added iron (especially at 6 °C), while Fragilariopsis only became more abundant under warming in the iron-added treatments. We attribute the apparent advantage Pseudo-nitzschia shows under warming to up-regulation of iron-conserving photosynthetic processes, utilization of iron-economic nitrogen assimilation mechanisms, and increased iron uptake and storage. These data identify important molecular and physiological differences between dominant diatom groups and add to the growing body of evidence for Pseudo-nitzschia ’s increasingly important role in warming SO ecosystems. This study also suggests that temperature-driven shifts in SO phytoplankton assemblages may increase utilization of the vast pool of excess nutrients in iron-limited SO surface waters and thereby influence global nutrient distribution and carbon cycling. 

Productivity in the Arctic is expected to increase as temperatures and the number of open water days rise. With this increased productivity, the coastal shelves of the Arctic Ocean may act as a sink for atmospheric carbon. However, this storage is dependent on a sufficient nitrogen (N) supply and current literature on biogeochemical rates of N uptake in this region is severely limited. Here, we report the spatial extent and rate at which the aquatic microbial community utilizes inorganic and organic N substrates in the Alaskan Arctic during late summer. Uptake rates (> 0.3 μm) were measured in 2016 and 2017 using isotopically labeled ammonium (), nitrate (), urea, and mixed algal amino acids. Rates of regeneration were also measured to investigate the contribution of remineralized N to primary production. Primary production was estimated using isotopically labeled bicarbonate. We found that N species uptake varied by location. Although was the form of N taken up at the greatest rate at most sites, we also found that uptake rates of urea could be greater than and that amino acid uptake was widespread. Supporting previous studies, uptake rates were correlated with primary production rates. Variability in nutrient reservoirs and sampling conditions in the Chukchi Sea between the 2 yr were responsible for some of the variances observed in estimated recycling rates and primary production. Understanding which N sources support this late‐season primary production requires obtaining uptake rates for a diverse range of inorganic and organic N substrates.

 

Biological dinitrogen (N₂) fixation is an important source of nitrogen (N) in low-latitude open oceans. The unusual N₂-fixing unicellular cyanobacteria (UCYN-A)/haptophyte symbiosis has been found in an increasing number of unexpected environments, including northern waters of the Danish Straight and Bering and Chukchi Seas. We used nanoscale secondary ion mass spectrometry (nanoSIMS) to measure¹⁵N₂uptake into UCYN-A/haptophyte symbiosis and found that UCYN-A strains identical to low-latitude strains are fixing N₂in the Bering and Chukchi Seas, at rates comparable to subtropical waters. These results show definitively that cyanobacterial N₂fixation is not constrained to subtropical waters, challenging paradigms and models of global N₂fixation. The Arctic is particularly sensitive to climate change, and N₂fixation may increase in Arctic waters under future climate scenarios.

 

Significant amounts of methane reside in sediments along the continental margins and slope of the Arctic Ocean. Methanotrophic bacteria oxidize methane to bicarbonate and also assimilate some methane‐derived carbon into biomass. Their metabolism transforms methane to other forms of carbon and sequesters it within the system, reducing its emission to the atmosphere. Increases in water temperatures driven by global climate change may accelerate the methane flux from the benthos into the water column, potentially increasing the importance of methanotrophic consumption as a methane sink. We report methane concentrations and oxidation rates in the water column of the Chukchi Sea from August 2017. This area is characterized by seasonally high nutrient concentrations that fuel high rates of pelagic primary productivity and subsequent sedimentation of organic matter, which stimulates benthic methanogenesis. Methane concentrations in the study area ranged from 6 to 72 nmol L⁻¹, consistent with previously published measurements. Methane oxidation rates were as high as 580 pmol L⁻¹ d⁻¹, similar to the rates measured in the East Siberian Arctic Shelf. Depth‐integrated methane oxidation rates were lower than methane efflux rates, suggesting that physiochemical factors prevent the methanotrophic microbial community from efficiently removing methane from the ecosystem. Still, methanotrophic bacteria provide an ecosystem service by removing a fraction of methane prior to its efflux to the atmosphere.