Methane oxidation pathways and associated methanotrophic communities in the water column of a tropical lake

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We examined methane (CH4) oxidation pathways and associated methanotrophic communities in the water column of Lake Kivu using abundance and isotopic compositions of CH4 and phospholipid fatty acids (PLFA), distributions of glycerol dialkyl glycerol tetraethers, and catalyzed reporter deposition fluorescence in situ hybridization (CARD–FISH) analysis. The carbon isotopic data of CH4 indicate that aerobic CH4 oxidation is the predominant pathway of microbial CH4 consumption with an isotopic fractionation factor (α) of 1.022–1.038. A small amount of CH4 is oxidized anaerobically, with an α of 1.002–1.006. Aerobic CH4 oxidation is mediated by type II methane–oxidizing bacteria (type II MOB) based on the 13C depletion (δ13C of −40.5‰ to −43.7‰) of diagnostic C18:1ω7 fatty acids in the surface waters. CARD–FISH images and PLFA components C16:1ω7 and C16:1ω5 indicate the presence of type I MOB in the methane and nutrient–rich deep–water region. 13C depletion of C16:1ω7 and C16:1ω5 (δ13C, ∼ −40‰ to −50.6‰) in the lake water below 52 m suggests the involvement of type I MOB in methane oxidation in the anoxic deep–water regions of the lake. A novel cluster of anaerobic methane–oxidizing archaea (ANME) rather than the known ANME–1 and ANME–2 appear to be involved in anaerobic oxidation of methane (AOM). Sulfate reducing bacteria are associated with AOM in the lake based on the 13C depletion (δ13C, −38.2‰ to −45.0‰) of anteiso–methyl–C15:0 fatty acid. Methane constitutes an important carbon and energy source (up to 38%) for the heterotrophic and autotrophic communities in the lake.