Long-term heating differently impacts diversity and seasonal dynamics of prokaryotes and micro-eukaryotes in Baltic Sea coastal biofilm communities.

Warming temperatures, heat waves, and altered conditions associated with climate change affect biodiversity and ecological processes across environments, with coastal zones being particularly vulnerable. Biofilm-forming organisms in shallow coastal areas are taxonomically diverse and include bacteria, fungi, and algae that contribute to energy and nutrient cycling along with providing habitats and food for species at the base of the food web. To understand how biofilm-forming organisms respond differently to spatiotemporally changing environmental conditions, seasonal sampling was performed in a Baltic Sea bay that has undergone 50 years of thermal heating, an unaffected nearby control bay, and a temperature gradient along an exposed coastline between the bays. The diversity, composition, and seasonal dynamics of the biofilm communities differed between the three environments largely due to temperature and water chemistry, with biofilms in the heated bay being more similar across seasons compared with the control bay and the gradient, and with prokaryotes exhibiting stronger spatial heterogeneity and seasonal dynamics compared to micro-eukaryotes. In the gradient, the dominating taxonomic groups were distinct, community composition was primarily influenced by seasonal turnover and wave exposure, and alpha diversity of prokaryotes decreased with increasing temperature. Seasonal shifts in the composition of micro-eukaryotic heterotrophs, phototrophs, and mixotrophs differed between environments, with heterotrophs being more dominant at higher temperatures. In conclusion, these contrasting responses indicated that climate warming may disproportionately impact different components of coastal biofilm communities, potentially decoupling key ecological processes and reducing community resilience in Baltic Sea coastal habitats.