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

Abstract

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 d 13 C values as low as 270%) in the anoxic water column, as well as characteristic d 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 21 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 mmol L 21 d 21 ) 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.

Effects of alternative electron acceptors on the activity and community structure of methane-producing and consuming microbes in the sediments of two shallow boreal lakes

“…As NO 3 -, which is also rapidly reduced to NO 2 -in anoxic sediments, did not enhance CH 4 oxidation, it can be suggested that the CH 2 F 2 -induced inhibition of the potential CH 4 oxidation was not due to the inhibition of NO 2 -using NC 10 bacteria, but rather due to the inhibition of aerobic MOBs. Indeed, a risk of minor O 2 contamination from the substrate/tracer injection or due to diffusion from the rubber septa or silicon sampling ports during incubations, which could induce microaerobic CH 4 oxidation, has been acknowledged in previous incubation studies of anoxic freshwater samples (Blees et al 2014;á Norði and Thamdrup 2014). It is, therefore, possible that the diffusion of trace amounts of O 2 from the septa during the incubation period could not be fully prevented in this study, either.…”

A new cell morphotype among methane oxidizers: a spiral-shaped obligately microaerophilic methanotroph from northern low-oxygen environments

“…Recent research demonstrates that microaerobic methanotrophy is of significant ecological importance (Blees et al, 2014;Bar-Or et al, 2015;Chistoserdova, 2015;Hernandez et al, 2015). Most of these studies refer to lake sediments.…”

Oxygenic primary production below the oxycline and its importance for redox dynamics

“…Aerobic oxidation of methane is classically observed at the oxicanoxic interface of the water column (Rudd et al 1976;Schubert et al 2010), which forms in eutrophic lakes either during the stratification period in holomictic lakes or is permanently present in meromictic lakes. Recent studies indicate that aerobic methane oxidation occurs not only at and above the oxic-anoxic interface, but also in apparently anoxic zones which are virtually oxygen free (Blees et al 2014;Milucka et al 2015). We refer to zones with oxygen concentrations below the detection limit of the trace oxygen sensors (20 nmol l -1 ) as virtually anoxic zones.…”