Ultramicrobacteria and filterable bacteria in the plankton of Lake Baikal

Abstract

In Lake Baikal, the number, diversity, and structure of femtoplankton – bacteria passing through filters with a 0.2 μm pore size – were studied for the first time using a complex of methods. The bacterial abundance in the femtoplankton fraction was 7×10⁴ cells/mL in the 0-50 m water layer, as measured by epifluorescence microscopy, and their contribution to the total bacterial number reached an average of 4.4%. High-throughput sequencing of 16S rRNA gene fragments revealed a significant genetic and taxonomic diversity of femtobacterioplankton in the pelagic and littoral zones of the lake. Dominant and minor phyla, orders, families, genera, and phylotypes of bacteria were identified in two fractions of Lake Baikal bacterioplankton: larger and smaller than 0.2 µm. We determined the contribution of ultra-small bacteria to the taxonomic composition of microbial communities in different parts of the lake. The microbiomes of bacterioplankton and femtobacterioplankton fractions differed significantly, and we described the peculiarities of ultramicrobacteria composition. The results demonstrated an important role of ultra-small bacteria in Lake Baikal’s ecosystem.