Geographic distance and mountain ranges structure freshwater protist communities on a European scale

Protists influence ecosystems by modulating microbial population size, diversity, metabolic outputs and gene flow. In this study we used eukaryotic ribosomal amplicon diversity from 218 European freshwater lakes sampled in August 2012 to assess spatial patterns and microbial community structure in European freshwaters. The diversity of microbial communities as reflected by amplicon clusters suggested that microbial diversity in freshwaters is well-sampled at the European and at the local scale. Biodiversity and richness of high mountain lakes differ systematically from that of lowland lakes, and the taxon inventory of high-mountain lakes strongly contributes to beta-diversity despite a low taxon inventory. Beyond a certain variation of environmental factors geographic distance had a pronounced effect on community composition particularly for high mountain regions which presumably act as biogeographic islands. However, community composition in lowland lakes was also affected by geographic distance but less pronounced as in high mountain regions. In consequence protist populations are locally structured into distinct biogeographic provinces and community analyses revealed biogeographic patterns also for lowland lakes whereby European mountain ranges act as dispersal barriers in particular for short to intermediate distances whereas the effect of mountain ranges leveled off on larger scale. Together, these investigations establish a pan-European microbial freshwater inventory with pronounced local endemicity within the rare biosphere.