Algae-Enriched Bacterial Community Composition Varies with Stress Response Patterns in Antarctic Algal Enrichment Cultures

Perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica, are shaped by permanent stratification, extreme oligotrophy, and salinity gradients, yet these features are vulnerable to climate-driven hydrologic change. Because phytoplankton and associated bacteria regulate carbon flow and nutrient cycling, understanding how algal–bacterial consortia respond to disturbance is key to predicting ecosystem change. We used enrichment cultures from Lakes Bonney and Fryxell to test responses to nutrient deprivation and salinity alteration, two perturbations relevant to climate-driven changes in hydrologic connectivity and expansion of open water moats. Autotrophic enrichments lacking added organic carbon were used to enrich algal–bacterial consortia dependent on photosynthetically derived substrates. Community responses were assessed with 16S rRNA amplicon sequencing of size-fractionated samples, allowing comparison of particle-associated and planktonic communities. Short-term nutrient limitation produced only limited shifts in community composition, indicating resistance to transient nutrient stress. However, bacterial communities were strongly structured by size fraction: particle-associated assemblages separated clearly from planktonic communities and were enriched in taxa linked to algal surfaces and polysaccharide-rich microhabitats, including Flavobacteriales, Sphingobacteriales, Rhizobiales, and Rhodobacterales. Salinity perturbation drove stronger restructuring of bacterial communities, with shallow Lake Bonney enrichments showing greater sensitivity than deeper communities. These findings suggest that algae-associated bacterial communities help structure Antarctic algal enrichment cultures and may influence microbial responses to climate-linked disturbance.