Multiple roles of DNA methylation in sea-ice bacterial communities and associated viruses

Despite growing evidence for the role of DNA methylation in bacterial acclimation to environmental stress, this epigenetic mechanism remains unexplored in sea-ice microbial communities known to tolerate multiple stressors. This study presents a first analysis of DNA methylation patterns in bacterial communities and associated viruses across the vertical thickness of sea ice. Using a novel stepped-sackhole method, we collected sea-ice brines from distinct horizons of an Arctic ice floe, capturing microbial communities that had been exposed to different environmental conditions. Through Oxford Nanopore sequencing, we characterized methylation patterns in bacterial and associated viral DNA, analysing for methylation motifs and differences between ice horizons. We identified 22 unique bacterial methylation motifs and 27 viral motifs across three nucleotide methylation types (5mC, 6mA, and 4mC), with evidence of differential methylation between upper and lower ice. Analysis of metagenome-assembled genomes revealed the regulatory potential of methylation in both ice-adapted (Psychromonas and Polaribacter) and non-adapted bacteria (Pelagibacter); e.g., in Pelagibacter, differential methylation of the GANTC motif between upper and lower ice affected genes involved in core cellular processes. Viral methylation patterns showed evidence of recent infection. We also identified orphan methyltransferases in sea-ice phages, suggesting a mechanism for bypassing host restriction-modification systems and regulating host genes. Our findings reveal that DNA methylation serves functions in sea ice beyond traditional restriction-modification systems that protect against foreign DNA, opening new avenues for research on the role of epigenetic mechanisms not only in acclimation to the cryosphere but also more generally in microbial ecology and evolution.