Micro‐aerobic bacterial methane oxidation in the chemocline and anoxic water column of deep south‐Alpine Lake Lugano (Switzerland)

We measured seasonal variations in the vertical distribution of methane concentration, methane oxidation rates, and lipid biomarkers in the northern basin of Lake Lugano. Methane consumption below the oxic—anoxic interface co‐occurred with concentration maxima of 13 C‐depleted C 16 fatty acid biomarkers (with δ 13 C values as low as −70‰) in the anoxic water column, as well as characteristic δ 13 C CH 4 profiles. We argue that the conspicuous methane concentration gradients are primarily driven by (micro‐)aerobic methane oxidation (MOx) below the chemocline. We measured a strong MOx potential throughout the anoxic water column, while MOx rates at in situ O 2 concentration > 10 nmol L −1 were undetectable. Similarly, we found MOx‐related biomarkers and gene sequences encoding the particulate methane monooxygenase in the anoxic, but not the oxic, water. The mechanism of (episodic) oxygen supply sustaining the MOx community in anoxic waters is still uncertain. Our results indicate that a bacterial methanotrophic community is responsible for the methane consumption in Lake Lugano, without detectable contribution from archaeal methanotrophs. Bacterial populations that accumulated both at the suboxic—anoxic interface and in the deeper anoxic hypolimnion, where maximum potential MOx rates were observed throughout the year (1.5–2.5 µmol L −1 d −1 ) were mainly related to Methylobacter sp. Close relatives are found in lacustrine environments throughout the world, and their potential to thrive under micro‐ and anoxic conditions in Lake Lugano may imply that micro‐aerobic methane oxidation is important in methane cycling and competition for methane and oxygen in stratified lakes worldwide.