The diversity of aerobic anoxygenic phototrophs (AAPs) and rhodopsin-containing bacteria in the surface microlayer, water column, and epilithic biofilms of Lake Baikal was studied for the first time, employing pufM and rhodopsin genes, and compared to 16S rRNA diversity. We detected pufM-containing Alphaproteobacteria (orders Rhodobacterales, Rhizobiales, Rhodospirillales, and Sphingomonadales), Betaproteobacteria (order Burkholderiales), Gemmatimonadetes, and Planctomycetes. Rhodobacterales dominated all the studied biotopes. The diversity of rhodopsin-containing bacteria in neuston and plankton of Lake Baikal was comparable to other studied water bodies. Bacteroidetes along with Proteobacteria were the prevailing phyla, and Verrucomicrobia and Planctomycetes were also detected. The number of rhodopsin sequences unclassified to the phylum level was rather high: 29% in the water microbiomes and 22% in the epilithon. Diversity of rhodopsin-containing bacteria in epilithic biofilms was comparable with that in neuston and plankton at the phyla level. Unweighted pair group method with arithmetic mean (UPGMA) and non-metric multidimensional scaling (NMDS) analysis indicated a distinct discrepancy between epilithon and microbial communities of water (including neuston and plankton) in the 16S rRNA, pufM and rhodopsin genes.

Using the metagenomics approach and high throughput sequencing of the 16S rRNA gene amplicons, we studied the taxonomic diversity of nitrifying bacteria in microbial communities from epilithic biofilms and plankton of the littoral zone of Lake Baikal. The family Nitrosomonadaceae represents the diversity of ammonia-oxidizing bacteria; the genera (Nitrospirae) and Candidatus Nitrotoga (Gammaproteobacteria) nitrite-oxidizing bacteria; and the phyla Nitrospinae unidentified bacteria. Microbial communities from epilithic biofilms and the near-bottom water layer showed the highest diversity and those from the surface water layer the lowest one. Among ammonia oxidizers, bacteria of the uncultured genus Ellin6067 (27 phylotypes) dominated and were ubiquitous, and among nitrite oxidizers bacteria of the genus Nitrospira (7 phylotypes).

In this study, we analyzed the transcriptomes of RNA and DNA viruses from the oligotrophic water of Lake Baikal and the effluent from wastewater treatment plants (WWTPs) discharged into the lake from the towns of Severobaikalsk and Slyudyanka located on the lake shores. Given the uniqueness and importance of Lake Baikal, the issues of biodiversity conservation and the monitoring of potential virological hazards to hydrobionts and humans are important. Wastewater treatment plants discharge treated effluent directly into the lake. In this context, the identification and monitoring of allochthonous microorganisms entering the lake play an important role. Using high-throughput sequencing methods, we found that dsDNA-containing viruses of the class Caudoviricetes were the most abundant in all samples, while Leviviricetes (ssRNA(+) viruses) dominated the treated water samples. RNA viruses of the families Nodaviridae, Tombusviridae, Dicitroviridae, Picobirnaviridae, Botourmiaviridae, Marnaviridae, Solemoviridae, and Endornavirida were found in the pelagic zone of three lake basins. Complete or nearly complete genomes of RNA viruses belonging to such families as Dicistroviridae, Marnaviridae, Blumeviridae, Virgaviridae, Solspiviridae, Nodaviridae, and Fiersviridae and the unassigned genus Chimpavirus, as well as unclassified picorna-like viruses, were identified. In general, the data of sanitary/microbiological and genetic analyses showed that WWTPs inadequately purify the discharged water, but, at the same time, we did not observe viruses pathogenic to humans in the pelagic zone of the lake.

Denitrification is the major process of nitrate reduction to dinitrogen (N₂). This is the first report on detection of the nirK type denitrifying bacteria in the epilithic biofilms of the Lake Baikal littoral zone. The highest species diversity was revealed in microbial communities of the biofilms collected in the area of the Bol’shoe Goloustnoe settlement and Severobaikal’sk City, while those collected in the vicinity of Listvyanka settlement exhibited the lowest diversity. Phylogenetic analysis showed uncultured denitrifying bacteria from diverse habitats were most closely related to all cloned sequences. The closely related homologs were represented by the sequences of the orders Rhizobiales and Rhodobacterales, class Alphaproteobacteria.

The paper presents the results of the study of water quality by sanitary-microbiological characteristics in the littoral zone of Lake Baikal and its tributaries in July and September 2022. It was found that the number of studied bacteria does not correlate with the level of the lake, but is caused by the seasonal anthropogenic load. Exceedances of the standards in the summer period were detected in the area of Kultuk settlement, Baikalsk town, Listvyanka settlement, Bolshie Koty settlement, in the bays of Maloe More, Chivyrkuisky and Barguzinsky bays, and in the rivers Medlyanka, Pokhabikha, Pereyomnaya, Kuchelga, Turka, Barguzin, and in the hydrotherm of Zmeinaya Bay. In autumn, the quality of the surface water did not meet the norm near the settlement of Bolshoe Goloustnoe, in Aya Bay, in the bays Zagli and Chivyrkuisky, and in the mouths of the rivers Medlyanka, Pokhabikha, Slyudyanka, and Mysovka. In the tributaries where wastewater is discharged (Medlyanka, Pokhabikha and Mysovka), the number of sanitary-indicatory bacteria remains high both in the summer and autumn seasons.