Baltic biofilms
Biofilms are complex ecosystems formed by phototrophic and heterotrophic organisms, such as bacteria and fungi. Biofilms are hotspots for microbial activity that are crucial for the functioning of all aquatic ecosystems, but they are poorly studied and often overlooked. We investigated the composition and diversity of biofilm-forming bacteria and fungi along the ~2300km-long spatial gradient of the Baltic Sea, where salinity increases from ~1 in the north to ~28 in the south-west. We also studied the short-term temporal variation of bacterial and fungal communities during peak productive season. We found that bacteria and fungi responded differently to environmental variation. The bacterial communities were strongly controlled by salinity, and the diversity of bacteria formed a clear pattern on the salinity gradient, being lowest in intermediate salinity. Bacterial communities were also dominated by different bacterial orders in high, intermediate, and low salinity. During our temporal study of three weeks, bacterial orders in the communities remained the same, but their relative abundances changed. Fungal communities, instead, were controlled by the combination of several environmental variables, i.e., salinity, water temperature, and wind exposure, and their diversity or community composition did not form a clear pattern along the salinity gradient. Fungal communities were temporally stable. To our knowledge, this is one of the first studies to investigate the biofilm-forming communities, especially fungal communities, along large environmental gradients. Our results highlight the role of biofilms as complex and unique ecosystems where the organisms' roles and ecological requirements may not be comparable to those in other kinds of habitats.