Unraveling the Complex Planktonic Microbial Community in the Amundsen Sea, Southern Ocean.

Spatial differences in microbial community structure and function were examined across polynyas, sea ice zones (SIZ), and ice-free waters of the Amundsen Sea, Southern Ocean, using 16 S and 18 S rRNA gene-based eDNA metabarcoding and quantitative PCR targeting nitrogen cycling and dimethylsulfoniopropionate (DMSP) degradation genes. The SIZ exhibited enrichment of psychrophilic bacteria (Colwellia spp.) and dominant eukaryotic taxa such as Diatomea and Prymnesiophyceae, likely linked to sea-ice-driven shifts in nutrient stoichiometry (elevated N: P and positive N*). Network analysis revealed interactions among primary producers, bacteria, and zooplankton, highlighting complementary roles in trophic energy transfer and nutrient recycling. Metabolic pathway predictions implied active bacterial processes related to sulfur and nitrogen cycling in the SIZ, particularly dissimilatory nitrate reduction and DMSP demethylation, suggesting coupling between carbon, nitrogen, and sulfur pathways. Quantitative PCR showed higher copy numbers of nitrogen cycling genes and DMSP degradation genes in the SIZ than in other regions, consistent with enhanced microbial denitrification, nitrogen fixation, and sulfur cycling under cold conditions shaped by sea-ice-driven nutrient dynamics. These findings demonstrate that environmental variation in Antarctic waters influences microbial diversity, reshapes ecological interactions, and modulates biogeochemical functions, with implications for nutrient cycling, food web dynamics, and ecosystem resilience in this climate-sensitive region.