Comparative analysis of free-living and particle-associated bacterial communities of Lake Baikal during the ice-covered period

Abstract

Free-living (FLC) and particle-associated (PAC) bacterial communities contribute differentially to the cycling of matter in the environment. In this work we have for the first time studied the taxonomic composition and diversity of FLC and PAC in the oligotrophic freshwater Lake Baikal during the under-ice plankton peak. The communities on the bottom ice surface and in the water column under the ice were studied. Analysis by light microscopy showed that the bottom surface of the ice was dominated by green algae Chlorella spp. A complex of microalgae developed in the water column: green algae Monoraphidium griffithii, Monoraphidium contortum, Chlorella spp., chrysophyte Dinobryon cylindricum, haptophyte Chrysochromulina parva, and diatom Synedra acus. Algal abundance on the ice sub-surface was several orders of magnitude higher than that in the water column. According to pyrosequencing data of the bacterial 16S rRNA V3-V4 region, diversity of FLC (as measured by Shannon, Simpson, Chao1 and ACE indices) was higher than that of PAC. Proteobacteria (Burkholderiaceae) dominated the FLC associated with ice; FLC of the water column under ice were dominated by Actinobacteria (Sporichthyaceae and Ilumatobacteraceae) and Verrucomicrobia (Methylacidiphilaceae). PAC associated with ice were dominated by Bacteroidetes (Flavobacteriaceae), while water column-associated PAC were dominated by Verrucomicrobia (FukuN18_freshwater_group, Methylacidiphilaceae) and cyanobacteria (Cyanobiaceae). The difference between free-living and particle-associated, as well as between water column and ice sub-surface bacterial communities, were explored on operational taxonomic unit (OTU) and phylotype levels.