LAUSR

In an aquatic ecosystem, the supply of nutrients is essential for the biogeochemical cycle, and it affects the taxonomic composition of the microbial communities. Here, by using high-throughput sequencing (HTS) of the 16S and 18S rRNA gene fragments, we compared free-living (FL) and particle-associated (PA) bacterial communities and microeukaryotic communities in the areas with different nutrient intakes in freshwater Lake Baikal during the ice-covered and summer periods. Samples were taken at the inflow of the Selenga River, which is the main tributary of the lake, and at several established coastal research stations. The metabolic potential of the bacterial communities was predicted using PICRUSt. Differences were found in both FL and PA communities of the river mouth compared to the photic zone of the lake. The composition of FL communities was significantly different between the sampling sites in the ice-covered period, which is most likely influenced by different hydrochemical conditions. In contrast, the PA communities were more similar during the ice-covered period, but they changed considerably from spring to summer and their diversity increased. The diversity of the microeukaryotic communities also increased in summer, which may have contributed to the increase in bacterial diversity. In co-occurrence networks analysis, the number of interconnected bacterial OTUs in FL exceeded those for PA. The FL communities were dominated by Actinobacteriota, while the major PA OTUs belonged to a mixed cluster, which were mainly assigned to the phyla Bacteroidota and Verrucomicrobiota. As a result, PA communities were enriched in pathways responsible for the metabolism of sulfur, fucose, cellulose and urea. Our results confirm the difference between the FL and PA bacterial communities in Lake Baikal. These results also highlight the complex pattern of interactions between bacteria and microeukaryotes in a natural freshwater ecosystem across spatial and temporal scales.