Dissolved organic matter and sea ice dynamics may explain changes in microbial community structure along the western Antarctic Peninsula

The western Antarctic Peninsula (WAP) experiences extreme seasonal changes in sea-ice and light, with intense phytoplankton blooms in spring and summer. Microbial communities in this region and their coupling to phytoplankton dynamics remain poorly understood. To explore microbial diversity in the WAP, we characterized bacterial, archaeal, and eukaryotic microbial community composition from two depths (near surface and below the euphotic zone) at four sites representing inshore and offshore conditions in the northern and southern WAP during austral summer, as well as a single inshore sample during winter.  We detected differences in bacterial community composition and greater bacterial and eukaryotic richness with depth at the four sites. PCR-amplification failed to enrich archaeal targets in summer surface waters but did detect populations in summer deep and winter surface water. We hypothesize that dissolved organic matter (DOM) input from phytoplankton blooms drives differences in microbial community composition and diversity between depths or seasons. Additionally, sea ice retreat in the spring releases phytoplankton, bacteria and DOM, potentially triggering phytoplankton blooms and altering bacterial community composition and activity. To examine these factors, we conducted two mesocosm experiments in the austral summer, in which we added either small amounts of filtered or unfiltered melted sea ice to 100-m water or DOM exudates (harvested from 20L of Thalassiosira wiessflogii culture) to 10-m and 100-m water. Addition of unfiltered melted sea ice caused increases in phytoplankton abundance and biomass and an early peak in bacterial production, suggesting that water column seeding may trigger phytoplankton growth and bacterial production, while filtered melted sea-ice showed little effect. Direct DOM additions had a dramatic impact on bacterial abundance and production in 100-m water but little effect in 10-m water, suggesting that different  factors shape bacterial community dynamics at different depths.