Phylogenetic signal of salinity and season on bacterial community compositions in the salinity gradient of the Baltic Sea. BACTERIAL SPATIO-TEMPORAL DISTRIBUTION IN THE BALTIC SEA

Understanding the principal processes that control bacterial community composition allows us to predict bacterial distributions and functions in ecosystems. In this study, we used the Baltic Sea as a model system to quantify the phylogenetic signal of salinity and season on the bacterioplankton community composition. Abundances of 16S rRNA gene amplicon sequencing reads were analyzed from samples taken on similar geographic locations in July and February along a brackish - marine salinity gradient in the Baltic Sea. We observed no pattern in bacterial richness at different salinities but a significantly increased number of bacterial phylotypes in winter. The bacterial community compositions at brackish and marine conditions, but also between July and February were significantly different. Non-metric multidimensional scaling showed that the composition of the bacterial communities was primarily separated according to salinity and secondly according to seasonal difference at all taxonomic ranks tested. Similarly, quantitative phylogenetic clustering based on phylogenetic distance and compliance in abundance profiles implicated higher taxonomic level ecological coherence for salinity compared to season. Our study therefore indicates that global patterns for salinity and season result from phylogenetically clustered preferences, whereas imprints from salinity seem to be stronger.