Anaerobic Thermodesulfovibrio and Aerobic Meiothermus Coexist in Deep Thermal Water

— Research on the microorganisms inhabiting deep aquifers is based on sampling the water released from deep wells and is seldom concerned with the physicochemical processes of the water-rock system. The issue of metabolism of aerobic prokaryotes revealed in deep habitats by molecular techniques remains unclear. Cultivation is required for direct determination of relation of prokaryotes to oxygen. In the present work, aerobic and anaerobic bacteria, which were revealed in thermal radon baths of the Belokurikha resort by molecular techniques, were isolated. Profiling by the 16S rRNA gene revealed predominance of members of the Deionococcus-Thermus group belonging to the genus Meiothermus (17.6% reads) and considered strictly aerobic. Anaerobic sulfate-reducing Thermodesulfovibrio were also present in the sample. The habitat was characterized by reductive, alkaline conditions. Target-oriented cultivation revealed aerobically growing Meiothermus sp. 1165, which was closely related to Meiothermus cerbereus . An alkaliphilic anaerobic sulfate reducer Thermodesulfovibrio sp. 1176 was also isolated. The rate of sulfate reduction measured in the Belokurikha water using yielded the value of 41.4 ± 1.06 µg S red L –1 day –1 , or 1.29 nmol S mL –1 day –1 . Analysis of the genome of strain 1176 revealed the presence of various mechanisms responsible for its relative resistance to oxygen and oxidative stress, which included superoxide reductase, rubredoxin, a Fe-Mn family superoxide dismutase, a KatG catalase-peroxidase, and a cytochrome bd ubiquinol oxidase. The low redox potential and intense anaerobic sulfate reduction provide evidence for the generally reduced conditions in the Belokurikha deep horizons. Spatial separation of aerobes and anaerobes in the water-rock system, similar to the one occurring in the terrestrial microbial mats, may be hypothesized, as well as occurring of aerobic processes in microniches.