Stable Isotope Approach to Assess the Production and Consumption of Methylphosphonate and Its Contribution to Oxic Methane Formation in Surface Waters.

Recent studies in aquatic environments have indicated that microbial methane production is not limited to strictly anoxic conditions and is widespread in the oxic water column. Based on recent investigations proposing linkage between the microbial turnover of methylphosphonate (MPn) and the widespread methane oversaturation in surface waters, we conducted an MPn/13C-MPn tracer approach that combines liquid chromatography-mass spectrometry and gas chromatography-stable isotope ratio mass spectrometry to assess concentrations of the MPn tracer and its contribution to oxic methane formation. In our study, conducted during summer 2020 in the Baltic Sea, we show that MPn is a potent methanogenic substrate in the surface water. However, we found that MPn was produced within the surface and subthermocline water bodies and that its turnover was not limited to the phosphorus-stressed and cyanobacteria-rich surface water. However, our study revealed that most of the MPn was probably degraded via alternative pathways, not releasing methane. Our assessment indicates that the contribution of the MPn degradation pathway only contributed marginally to oxic methane production at the study site in the Baltic Sea and that a variety of methanogenic pathways are probably responsible for the surface-water methane enrichments.

Methane formation driven by reactive oxygen species across all living organisms.

Methane (CH4), the most abundant hydrocarbon in the atmosphere, originates largely from biogenic sources1 linked to an increasing number of organisms occurring in oxic and anoxic environments. Traditionally, biogenic CH4 has been regarded as the final product of anoxic decomposition of organic matter by methanogenic archaea. However, plants2,3, fungi4, algae5 and cyanobacteria6 can produce CH4 in the presence of oxygen. Although methanogens are known to produce CH4 enzymatically during anaerobic energy metabolism7, the requirements and pathways for CH4 production by non-methanogenic cells are poorly understood. Here, we demonstrate that CH4 formation by Bacillus subtilis and Escherichia coli is triggered by free iron and reactive oxygen species (ROS), which are generated by metabolic activity and enhanced by oxidative stress. ROS-induced methyl radicals, which are derived from organic compounds containing sulfur- or nitrogen-bonded methyl groups, are key intermediates that ultimately lead to CH4 production. We further show CH4 production by many other model organisms from the Bacteria, Archaea and Eukarya domains, including in several human cell lines. All these organisms respond to inducers of oxidative stress by enhanced CH4 formation. Our results imply that all living cells probably possess a common mechanism of CH4 formation that is based on interactions among ROS, iron and methyl donors, opening new perspectives for understanding biochemical CH4 formation and cycling.

High Spatiotemporal Dynamics of Methane Production and Emission in Oxic Surface Water.

The discovery of methane (CH4) accumulation in oxic marine and limnic waters has redefined the role of aquatic environments in the regional CH4 cycle. Although CH4 accumulation in oxic surface waters became apparent in recent years, the sources are still subject to controversial discussions. We present high-resolution in situ measurements of CH4 concentration and its stable isotope composition in a stratified mesotrophic lake. We show that CH4 accumulation in surface waters originates from a highly dynamic interplay between (oxic) CH4 production and emission to the atmosphere. Laboratory incubations of different phytoplankton types and application of stable isotope techniques provide a first unambiguous evidence that major phytoplankton classes in Lake Stechlin per se produce CH4 under oxic conditions. Combined field and lab results show that the photoautotroph community is an important driver for CH4 production and its highly dynamic accumulation in oxic surface waters.