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The impacts of climate change on the Arctic Ocean are manifesting throughout the ecosystem at an unprecedented rate. Of global importance are the impacts on heat and freshwater exchange between the Arctic and North Atlantic Oceans. An expanding Atlantic influence in the Arctic has accelerated sea-ice decline, weakened water column stability and supported the northward shift of temperate species. The only deep-water gateway connecting the Arctic and North Atlantic and thus, fundamental for these exchange processes is the Fram Strait. Previous research in this region is extensive, however, data on the ecology of microbial communities is limited, reflecting the wider bias towards temperate and tropical latitudes. Therefore, we present 14 metagenomes, 11 short-read from Illumina and three long-read from PacBio Sequel II, of the 0.2–3 µm fraction to help alleviate such biases and support future analyses on changing ecological patterns. Additionally, we provide 136 species-representative, manually refined metagenome-assembled genomes which can be used for comparative genomics analyses and addressing questions regarding functionality or distribution of taxa.

The Arctic Ocean is experiencing unprecedented changes because of climate warming, necessitating detailed analyses on the ecology and dynamics of biological communities to understand current and future ecosystem shifts. Here, we generated a four-year, high-resolution amplicon dataset along with one annual cycle of PacBio HiFi read metagenomes from the East Greenland Current (EGC), and combined this with datasets spanning different spatiotemporal scales (Tara Arctic and MOSAiC) to assess the impact of Atlantic water influx and sea-ice cover on bacterial communities in the Arctic Ocean. Densely ice-covered polar waters harboured a temporally stable, resident microbiome. Atlantic water influx and reduced sea-ice cover resulted in the dominance of seasonally fluctuating populations, resembling a process of “replacement” through advection, mixing and environmental sorting. We identified bacterial signature populations of distinct environmental regimes, including polar night and high-ice cover, and assessed their ecological roles. Dynamics of signature populations were consistent across the wider Arctic; e.g. those associated with dense ice cover and winter in the EGC were abundant in the central Arctic Ocean in winter. Population- and community-level analyses revealed metabolic distinctions between bacteria affiliated with Arctic and Atlantic conditions; the former with increased potential to use bacterial- and terrestrial-derived substrates or inorganic compounds. Our evidence on bacterial dynamics over spatiotemporal scales provides novel insights into Arctic ecology and indicates a progressing Biological Atlantification of the warming Arctic Ocean, with consequences for food webs and biogeochemical cycles.

The Arctic Ocean is experiencing unprecedented changes as a result of climate warming, necessitating detailed analyses on the ecology and dynamics of biological communities to understand current and future ecosystem shifts. Here we show the pronounced impact that variations in Atlantic water influx and sea-ice cover have on bacterial communities in the East Greenland Current (Fram Strait) using two, 2-year high-resolution amplicon datasets and an annual cycle of long-read metagenomes. Densely ice-covered polar waters harboured a temporally stable, resident microbiome. In contrast, low-ice cover and Atlantic water influx shifted community dominance to seasonally fluctuating populations enriched in genes for phytoplankton-derived organic matter degradation. We identified signature populations associated with distinct oceanographic conditions and predicted their ecological niches. Our study indicates progressing "Biological Atlantification" in the Arctic Ocean, where the niche space of Arctic bacterial populations will diminish, while communities that taxonomically and functionally resemble those in temperate oceans will become more widespread.

Marine algae annually synthesize gigatons of glycans from carbon dioxide, exporting it within sinking particles into the deep-sea and underlying sea floor, unless those glycans are digested before by bacteria. Identifying algal glycans in the ocean remains challenging with the molecular resolution of conventional analytic techniques. Whether algal glycans are digested by heterotrophic bacteria during downward transport, before they can transfer carbon dioxide from the ocean surface into the deep-sea or the sea floor, remains unknown. In the Red Sea Shaban Deep, where at 1500 m water depth a brine basin acts as a natural sediment trap, we found its high salt and low oxygen concentration accumulated and preserved exported algal glycans for the past 2500 years. By using monoclonal antibodies specific for glycan structures, we detected fucose-containing sulfated polysaccharide, β-glucan, β-mannan and arabinogalactan glycans, synthesized by diatoms, coccolithophores, dinoflagellates and other algae living in the sunlit ocean. Their presence in deep-sea sediment demonstrates these algal glycans were not digested by bacteria. Instead they moved carbon dioxide from the surface ocean into the deep-sea, where it will be locked away from the atmosphere at least for the next 1000 years. Considering their global synthesis, quantity and stability against degradation during transport through the water column, algal glycans are agents for carbon sequestration.Significance statementAlgae and plants use the greenhouse gas carbon dioxide to synthesize polymeric carbohydrates, or glycans, for energy storage, structural support and as protection against invasion by microbes. Glycans provide protection, are carbon sinks and enable carbon sequestration for as long as they are not digested by bacteria or other organisms, which releases the carbon dioxide back in to the atmosphere. In this study, we show that non-digested algal glycans sink into the deep ocean and into marine sediment. Thus, glycans are more than food for animals and prebiotics for bacteria, they are also molecules that remove carbon dioxide from the atmosphere and transfer it to the deep-sea, where it can be stored for 1000 years and longer.