Differing patterns in community composition and diversity of biofilm-forming bacteria and fungi along a large salinity gradient

Aim: Biofilms are complex ecosystems formed by phototrophic and heterotrophic organisms, such as bacteria and fungi. Biofilms are hotspots for microbial activity and crucial for the functioning of all aquatic ecosystems, but they are overlooked and poorly studied, especially along large environmental gradients. We aimed at resolving the responses of bacterial and fungal communities to environmental variation along a gradient with strong variation in environmental factors, especially salinity. We also investigated whether the diversity of biofilm-forming bacteria and fungi follows the pattern predicted by the Remane curve, which posits that species diversity is lowest at intermediate salinity. Location: 2300-km-long spatial gradient in the Baltic Sea, Northern Europe Time period: August 2018 Major taxa studied: Biofilm-forming bacteria and fungi Methods: We investigated the composition and diversity of bacteria and fungi by collecting biofilm samples from 42 shallow benthic ecosystems. We also characterized key environmental variables, such as salinity, nutrient concentrations, wind exposure, and climatic conditions. Bacterial and fungal communities in biofilm samples were identified using Illumina MiSeq sequencing. Results: We found that bacteria and fungi responded differently to environmental variation. Bacterial communities were mainly controlled by salinity, and their diversity was lowest in the intermediate salinity. Bacterial communities were also dominated by different bacterial orders in high, intermediate, and low salinity. Fungal communities, instead, were jointly controlled by several environmental variables, i.e., salinity, water temperature, wind exposure, and nutrients, and their diversity or composition did not show a clear pattern along the salinity gradient. Main conclusions: 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 ecosystems where organisms ecological requirements are unique and variable.