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.

An aquatic surface microlayer covers more than 70% of the world’s surface. Our knowledge about the biology of the surface microlayer of Lake Baikal, the most ancient lake on Earth with a surface area of 31,500 km2, is still scarce. The total bacterial abundance, the number of cultured heterotrophic temporal bacteria, and the spatial distribution of bacteria in the surface microlayer and underlying waters of Lake Baikal were studied. For the first time, the chemical composition of the surface microlayer of Lake Baikal was determined. There were significant differences and a direct relationship between the total bacterial abundance in the surface microlayer and underlying waters of Lake Baikal, as well as between the number of cultured heterotrophic bacteria in studied water layers in the period of summer stratification. In the surface microlayer, the share of cultured heterotrophic bacteria was higher than in the underlying waters. The surface microlayer was characterized by enrichment with PO43−, total organic carbon and suspended particulate matter compared to underlying waters. A direct relationship was found between the number of bacteria in the surface microlayer and environmental factors, including temperature, total organic carbon and suspended particulate matter concentration.

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The status of Lake Baikal as a UNESCO World Heritage Site, due to its unique ecology and value as a global aquatic resource have resulted in strict environmental regulations to protect the watershed and biota it supports. Despite this, the ecosystem of the lake is being exposed to negative anthropogenic impact and deteriorating water quality. In this article, we describe the bioecological state of Mukhor Bay − the warmest bay of Lake Baikal and one which is actively visited by tourists. We try to highlight the environmental problems of Baikal and aim to change attitudes towards water resources in this unique ecosystem. We present data on the taxonomic, genetic, and biochemical characteristics of cyanobacteria blooms that develop in the plankton of the bay. Further, we establishing their risk to human health and provide information for the environmental state management. Toxic blooms of Dolichospermum lemmermannii (cyanobacteria) were detected in the phytoplankton of Mukhor Bay. The nutrients content and chlorophyll concentration was low and corresponded to the oligo-mesotrophic status. Microcystins were detected in the dried phytoplankton biomass and the water, coastal samples contained a higher amount of this toxin. We hypothesize that one of the factors contributing to toxic cyanobacteria growth in the Mukhor Bay is the nutrient loadings due to surface and groundwater runoff from surrounding area.

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