Phylogenetic Structure of Bacterioplankton in Water Bodies of the Kuibyshev Reservoir Basin during Cyanobacterial Bloom

The phylogenetic structure of bacterioplankton of the Usa Bay and the adjacent area of the Kuibyshev Reservoir, as well as three hydrologically connected urban lakes of the Kaban system (Kazan), was analyzed using the results of high-throughput sequencing of the V3–V4 hypervariable region of the 16S rRNA gene. In the studied water objects, mass cyanobacterial development was observed, dominated by members of the phylogenetic lines Aphanizomenon/Dolichospermum and Cyanobium and the genus Planktothrix. Alpha- and betaproteobacteria dominated in the heterotrophic bacterioplankton of all stations. A significant proportion of its composition was made up of mixotrophic bacteria with the rhodopsin type of photosynthesis (for example, Ca. Fonsibacter, Ca. Nanopelagicus, and Ca. Planctophila). A characteristic feature of the studied samples was a high proportion of bacteria of PVC superphylum, especially Planctomycetota. The relationship between the composition and structure of bacterioplankton and the composition of the dominant cyanobacterial complexes was assessed, and heterotrophic OTUs assemblages associated with different types of cyanobacteria were identified. The most numerous assemblage is formed around Aphanizomenon–Dolichospermum–Microcystis and mainly consists of bacteria included in the phycosphere of colonial cyanobacteria, as well as representatives of the PVC superphylum. Two small assemblages form around Limnothrix redekei and Cyanobium rubescens and consist of typical planktonic bacteria belonging mainly to the order Flavobacteriales and the family Nanopelagicaceae.