Light-Dependent Aerobic Methane Oxidation Reduces Methane Emissions from Seasonally Stratified Lakes

Abstract

Lakes are a natural source of methane to the atmosphere and contribute significantly to total emissions compared to the oceans. Controls on methane emissions from lake surfaces, particularly biotic processes within anoxic hypolimnia, are only partially understood. Here we investigated biological methane oxidation in the water column of the seasonally stratified Lake Rotsee. A zone of methane oxidation extending from the oxic/anoxic interface into anoxic waters was identified by chemical profiling of oxygen, methane and δ13C of methane. Incubation experiments with 13C-methane yielded highest oxidation rates within the oxycline, and comparable rates were measured in anoxic waters. Despite predominantly anoxic conditions within the zone of methane oxidation, known groups of anaerobic methanotrophic archaea were conspicuously absent. Instead, aerobic gammaproteobacterial methanotrophs were identified as the active methane oxidizers. In addition, continuous oxidation and maximum rates always occurred under light conditions. These findings, along with the detection of chlorophyll a, suggest that aerobic methane oxidation is tightly coupled to light-dependent photosynthetic oxygen production both at the oxycline and in the anoxic bottom layer. It is likely that this interaction between oxygenic phototrophs and aerobic methanotrophs represents a widespread mechanism by which methane is oxidized in lake water, thus diminishing its release into the atmosphere.

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

“…Concurrent with the present work, Oswald et al (2015) found the presence of aerobic methane oxidizers in the anoxic zone of Lake Rot, a highly eutrophic, dimictic lake situated north of Lucerne (Switzerland), while no known groups of anaerobic methane oxidizing archaea were found in the water column. Laboratory incubations with water samples taken from the zone below the oxycline revealed that irradiation with light stimulated methane oxidation significantly, which suggests that methane oxidation in the virtually anoxic water column is tightly coupled to oxygenic primary production.…”

“…The uncorrected oxygenic as well as the anoxygenic PP rates indicate that primary producers in the aphotic zone are still capable of PP in the presence of light. This potential for oxygenic PP was also confirmed by Oswald et al (2015) who observed a strong stimulation of methane oxidation in samples taken between 9 and 11 m on the 5th of August when they were irradiated by light in the laboratory (see their Fig. 3).…”

“…Dissolved iron and manganese were determined as the fraction smaller than 0.22 lm using inductively coupled plasma mass spectrometry ICP-MS (Element2, Thermo-Fisher). Methane In-situ primary production N PIA profiles recorded in-situ using PIA, S samples taken with the syringe sampler, N samples taken using the Niskin bottle, CTD measurements taken using an additional CTD with submersible PAR sensor and fluorescence probe for Chl-a, M data recorded by a meteorological station a Details on the laboratory incubations to determine methane oxidation dynamics and their results can be found in Oswald et al (2015) Oxygenic primary production below the oxycline and its importance for redox dynamics 729 was determined using headspace gas chromatography using a gas chromatograph with flame ionization detector (Agilent 6890 N, Agilent technologies). Phytoplankton composition was determined down to the genus level based on the nomenclature published in the Algaebase database (Guiry and Guiry 2013) using light microscopy.…”

“…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.…”

“…We refer to zones with oxygen concentrations below the detection limit of the trace oxygen sensors (20 nmol l -1 ) as virtually anoxic zones. Recently it has been demonstrated that methane oxidation can be stimulated in anoxic lake waters by light (Milucka et al 2015;Oswald et al 2015). This suggests that oxygenic primary production can be a source of electron accepting capacity in virtually anoxic environments, yet this possibility has gone largely unnoticed until now.…”