Deep metagenomics uncovers functional adaptations and pathogenic risks in the gut microbiome of Antarctic fur seals (Arctocephalus gazella).

The Antarctic fur seal (Arctocephalus gazella) plays a key role in the Antarctic marine ecosystem by regulating krill, fish, and cephalopod populations through selective foraging, promoting Southern Ocean productivity via excretion, and influencing coastal island ecosystems during breeding season. Despite the importance of the gut microbiota in reflecting diet, health, and environmental adaptation, the gut microbiome of the Antarctic fur seal remains poorly characterized. To address this gap and evaluate its potential as a bioindicator of Antarctic marine environmental health, we employed shotgun metagenomics and 16S rRNA amplicon sequencing on fresh fecal samples collected from four Antarctic fur seals (designated S59, S62, S63, and S64) at King George Island, Western Antarctica. Despite inter-individual variation, both approaches identified Bacillota as the dominant phylum but showed genus-level discrepancies, with Fusobacterium prevailing in metagenomes and Clostridium in 16S amplicons. Viral communities constituted up to 5.3% of the microbiome, including an immunodeficiency-associated Lentivirus. Chitin-degrading capacity was ubiquitous, consistent with the host's krill-based diet. Metagenome-assembled genomes (MAGs) resolved distinct taxonomic contributions to discrete steps of chitin hydrolysis, suggesting that complete depolymerization requires metabolic cross-feeding among functionally complementary taxa. Notably, Helicobacter MAGs were abundant in individual S62, suggesting potential pathogenicity. Additionally, 16 antibiotic resistance gene types were detected, with bacitracin, polymyxin, and multidrug resistance dominating the resistome. These findings not only elucidate the community composition, functional potential, and ecological adaptation of the Antarctic fur seal gut microbiota but also establish a comprehensive baseline for assessing environmental change and human impacts on the Antarctic marine ecosystem, thereby offering valuable scientific data and methodological insights for the conservation of polar marine mammals.