Potential activity of methane and ammonium oxidation by methanotrophic communities from the soda lakes of southern Transbaikal

Radioisotopic measurements of the methane consumption by mud samples taken from nine Southern Transbaikal soda lakes (pH 9.5–10.6) showed an intense oxidation of methane in the muds of Lakes Khuzhirta, Bulamai Nur, Gorbunka, and Suduntuiskii Torom, with the maximum oxidation rate in the mud of Lake Khuzhirta (33.2 nmol/(ml day)). The incorporation rate of the radioactive label from¹⁴CH₄ into¹⁴CO₂ was higher than into acid-stable metabolites. Optimum pH values for methane oxidation in water samples were 7–8, whereas mud samples exhibited two peaks of methane oxidation activity (at pH 8.15–9.4 and 5.8–6.0). The majority of samples could oxidize ammonium to nitrites; the oxidation was inhibited by methane. The PCR amplification analysis of samples revealed the presence of genes encoding soluble and paniculate methane monooxygenase and methanol dehydrogenase. Three alkaliphilic methanotrophic bacteria of morphotype I were isolated from mud samples in pure cultures, one of which, B5, was able to oxidize ammonium to nitrites at pH 7–11. The data obtained suggest that methanotrophs are widely spread in the soda lakes of Southern Transbaikal, where they can actively oxidize methane and ammonium.