Biological methane (CH₄) production and accumulation in oxygenated waters challenges the long-standing paradigm that microbial CH₄ production exclusively occurs under anoxic conditions. Methane production in the oxic, mixed surface layer of aquatic bodies increases CH₄ fluxes to the atmosphere, forcing us to rethink the ecology and environmental dynamics of this powerful greenhouse gas. Different autotrophic and heterotrophic microorganisms (bacteria, fungi, Cyanobacteria, eukaryotic algae and even macrophytes) produce CH₄ in oxic environments. Known processes include activity of nitrogenases, demethylation of methylphosphonates, methylamines or DMSP, while cyanobacteria and other phytoplankton produce CH₄ in oxic waters during photosynthesis - often in combination with stresses via the presence of reactive oxygen species. Yet unknown or recently published processes including purely chemical reactions likely contribute as well. It was suggested that these processes can account for most of the CH₄ flux in the oceans and in freshwaters, e.g. reaching up to 85% in stratified, temperate lakes. In addition, CH₄ oxidation in oxygen saturated surface waters may have also been underestimated. As both processes are influenced by temperature, light, nutrient availability and aquatic food web dynamics, climate change has the potential to alter CH₄ formation and consumption in oxic water layers, with profound consequences for atmospheric CH₄ flux and climate feedback. I will provide a short summary of the state of the art and discuss open questions and challenges en route to better integrating oxic CH₄ production into the global CH₄ budget.

Primary Presenter: Hans-Peter Grossart, Leibniz Inst. for Freshwater Ecol. & Inland Fish. (hgrossart@igb-berlin.de)

Authors:

Hans-Peter Grossart, Leibniz Institute of Freshwater Ecology and Inland Fisheries, Plankton and Microbial Ecology  (hgrossart@igb-berlin.de)