765 ISSN 0026-2617, Microbiology, 2017, Vol. 86, No. 6, pp. 765–772. © Pleiades Publishing, Ltd., 2017. Original Russian Text © V.V. Kadnikov, Yu.A. Frank, A.V. Mardanov, A.V. Beletsky, D.A. Ivasenko, N.V. Pimenov, O.V. Karnachuk, N.V. Ravin, 2017, published in Mikrobiologiya, 2017, Vol. 86, No. 6, pp. 739–747. Variability of the Composition of the Microbial Community of the Deep Subsurface Thermal Aquifer in Western Siberia V. V. Kadnikov a, b , Yu. A. Frank c , A. V. Mardanov b , A. V. Beletsky b , D. A. Ivasenko c , N. V. Pimenov d , O. V. Karnachuk c , and N. V. Ravin a, b, * a Faculty of Biology, Moscow State University, Moscow, Russia b Institute of Bioengineering, Research Center of Biotechnology, Russian Academy of Sciences, Moscow, Russia c Tomsk State University, Tomsk, Russia d Winogradsky Institute of Microbiology, Research Center of Biotechnology, Russian Academy of Sciences, Moscow, Russia *e-mail: nravin@biengi.ac.ru Received June 26, 2017 Abstract⎯The deep subsurface biosphere is one of the least studied ecosystems on Earth, containing com- munities of extremophilic microorganisms. The present work was aimed at molecular genetic characteriza- tion of microbial communities of underground thermal waters in Western Siberia, lying at depths of 2–3 km. Water samples were collected from the 5P oil-exploration well, drilled to a depth of 2.8 km near the village Chazhemto (Tomsk region). The water had a temperature of about 20°C, a neutral pH and a low redox poten- tial (–304 mV). Underground aquifers have a complex structure and may contain both planktonic microor- ganisms and those immobilized on the surface of rocks in the form of biofilms, which may be washed out and detected in the water flowing out of the well. Community composition was analyzed by amplification and pyrosequencing of the 16S rRNA gene fragments in seven water samples taken at different times during 26 hours. Bacteria, which constituted about half of the community, were represented mainly by uncultured lineages of the phyla Firmicutes, Ignavibacteria, Chloroflexi, Bacteroidetes, and Proteobacteria. Archaea belonged mainly to known methanogens of the genera Methanothermobacter, Methanosaeta, and Methano- massiliicoccus. Analysis of the samples taken at different times revealed large variations in the content of most groups of bacteria, with a decrease in Firmicutes abundance accompanied by an increase in the shares of Ignavibacteria and Chloroflexi. The share of archaea of the genus Methanothermobacter varied slightly during the day, while significant variations were observed for the phylotypes assigned to Methanosaeta and Metha- nomassiliicoccus. Hydrogenotrophic archaea of the genus Methanothermobacter are probably a permanent component of the microbial community occurring in the planktonic state, while most of the identified groups of bacteria are present in biofilms or spatially localized parts of the underground water reservoir, the material of which accidentally enters the well. Keywords: subsurface biosphere, thermal waters, microbial community, molecular analysis, methanogens DOI: 10.1134/S002626171706008X The composition of microbial communities of deep subsurface ecosystems varies depending on the geo- chemical conditions (Gihring et al., 2006). These eco- systems are isolated from the surface biosphere for thousands to millions years and do not depend on sup- ply of organic matter from the surface (Edwards et al., 2012). They usually contain low concentrations of available substrates, which implies low in situ growth rate of subsurface microorganisms (Jørgensen, 2012; Lever et al., 2015). Analysis of the 16S rRNA genes from these communities revealed diverse uncultured bacterial and archaeal lineages, which often consti- tuted the majority of the community and were specific inhabitants of such environments (Takai et al., 2001a; Gihring et al., 2006; Chivian et al., 2008; Sahl et al., 2008). Depending on available energy sources, lithoauto- trophic or heterotrophic microorganisms may pre- dominate in subsurface microbial communities. Molecular hydrogen of abiotic origin is the main energy source for the autotrophs (Nealson et al., 2005; Hallbeck and Pedersen, 2008). It may be used by sul- fate-reducing prokaryotes and by methanogenic archaea. These two groups of microorganisms are often found in subsurface ecosystems (Moser et al., 2005). Other subsurface ecosystems, such as forma- tion water of oil reservoirs, may contain high amounts of organic matter preserved since their formation. In such cases, diverse organotrophic microorganisms may develop. The most easily available source of thermal subsur- face waters (from the depth of 1‒3 km) are oil explo- EXPERIMENTAL ARTICLES