Abstract
Aerobic methane-oxidizing bacteria (MOB) were suggested to play an important role in the process of anaerobic methane oxidation (AOM) in freshwater basins. The central hypothesis in the literature is that MOB conduct anaerobic respiration by using \({\text{NO}}_{2}^{ - }\) , \({\text{NO}}_{3}^{ - }\) , \({\text{SO}}_{4}^{{2 - }}\) , Fe and Mn oxides, etc. as terminal electron acceptors. However, the mechanisms underlying AOM by MOB remain poorly understood. The objective of the work was to study methane oxidation in the water column and sediments of the profundal part of a temperate freshwater lake during a period of an intense cyanobacterial bloom. Analytical, radiotracer, molecular, and incubation techniques were employed. The photic zone of the lake was oversaturated with O2, the near-bottom water was hypoxic, and the sediments were anoxic. Methane was detected throughout the water column, with its concentration in the sediments being 4 orders of magnitude higher than in the surface water. Methanotrophs of class Alphaproteobacteria dominated in the upper water layers, where both CH4 concentration and CH4 oxidation rates were minimal. The near-bottom water was characterized by the highest CH4 oxidation rates and predominance of MOB of the genus Methylobacter. In reduced sediments, MOB population retained 17–40% of its activity in the near-bottom water. Examination of the publicly available MOB genomes for the presence of the genes encoding enzymes involved in the transformation of oxidized nitrogen compounds suggested involvement of the NO-dismutation pathway in methane oxidation in reduced sediments. MOB with high affinity to O2, including some representatives of the genera Crenothrix, Methylobacter, Methylomonas, Methylomagnum, Methylovulum, and Methylocystis, could potentially be capable of this process. NO dismutation resulting in O2 production explains how these aerobic bacteria can thrive and oxidize CH4 in anaerobic sediments where they are often detected.
