Sea Ice Meltwater Alters Prokaryotic Community Structure and Interaction Networks in the Amundsen Sea, Antarctica

The Amundsen Sea is increasingly influenced by meltwater input from retreating glaciers, ice-shelves, and seasonal sea-ice, yet the responses of Antarctic prokaryotic communities to meltwater-associated environmental change remain poorly understood. Directly resolving microbial responses in situ is challenging because meltwater signals are rapidly diluted, advected, and mixed with surrounding seawater. To address this limitation, we conducted controlled microcosm experiments using seawater communities collected from surface and mesopelagic layers of the Amundsen Sea and amended them with melted sea-ice across a dilution gradient of 0–50%. Prokaryotic community composition and potential ecological associations were examined using 16S rRNA gene sequencing and co-occurrence network analysis. Increasing sea-ice meltwater addition reduced prokaryotic diversity and shifted community composition, with strong enrichment of opportunistic taxa such as Shewanella . Comparison with theoretical passive-mixing expectations indicated that the increase in Shewanella could not be explained by sea-ice inoculation alone. In contrast, ammonia-oxidizing prokaryotes, including Nitrosopumilales and Nitrosococcales, declined along the dilution gradient despite ammonium input from sea-ice, suggesting that meltwater-associated environmental changes may offset the potential stimulatory effect of ammonium availability. Co-occurrence networks became smaller and less connected with increasing meltwater addition, with reduced cohesion and fewer keystone taxa, although particle-associated communities retained greater structural stability than free-living communities. These findings suggest that acute sea-ice meltwater input can rapidly restructure Antarctic prokaryotic communities and weaken their potential association networks.