Phytoplankton dynamics shape bacterioplankton community structure and metabolism during the austral summer-autumn transition in the Western Antarctic Peninsula.

Seasonal changes in Antarctic coastal waters trigger pronounced shifts in microbial community composition and function, yet sparse spatial and temporal coverage currently limits our understanding of phytoplankton-bacterioplankton coupling. This study combines metagenomic and metatranscriptomic analyses of marine bacterioplankton with environmental data to address the functional dynamics of planktonic communities off the Western Antarctic Peninsula during the austral summer-autumn transition. Diatoms dominated the phytoplankton community, with generally low biomass and abundance, yet a species-specific succession was observed. The bacterioplankton community structure shifted from dominance of copiotrophic taxa (e.g. Polaribacter) towards oligotrophic lineages (e.g. SAR11) adapted to low-nutrient conditions, accompanied by a decrease in microbial carbohydrate-degradation activity. The capacity to degrade algal-derived polysaccharides varied between community members, with ß-glucan, α-glucan, chitin, and host glycan utilization present in all, and fucose, β-galactan and trehalose degradation restricted to specific taxa. DMSP metabolism also showed taxonomic specificity and was shaped by both physical (ice melt and fluctuations in solar irradiation) and biological factors (phytoplankton succession). Together, these findings reveal a complex, taxon-specific coupling between bacterioplankton and phytoplankton communities in the Western Antarctic Peninsula, linking community structure to likely functional gene expression and highlight how Antarctic bacterioplankton drives carbon and sulfur turnover in a polar marine ecosystem.