Methane Paradox

Abstract In contrast to common textbook knowledge, substantial amounts of the potent greenhouse gas methane are produced and emitted from oxygenated freshwater and marine systems. Since this phenomenon contradicts the belief that biological methane production occurs only under strictly anoxic conditions, it was termed the ‘Methane Paradox’. Several biotic and abiotic mechanisms have been suggested to explain the ‘Methane Paradox’. These include the transport of methane from anoxic environments, the formation of microenvironments able to support the classical anaerobic methanogenesis and novel pathways. Among the latter demethylation of methylphosphonates has been proposed as an important pathway in both marine and freshwater systems. Nevertheless, recent studies point to the ability of a broad spectrum of organisms to produce methane independent of the currently known biochemical pathways. Key Concepts Methane has roughly 30 times the global warming potential of carbon dioxide by mass over a century. Methane is emitted from oxygen‐rich environments and not only from anoxic ones as is often stated. The ‘Methane Paradox’ describes the occurrence of elevated methane concentrations in the upper oxic water column as compared to deeper waters, suggesting local production despite the strict anaerobic nature of all known Archaea ‐based methanogenic pathways. Abiotic factors can contribute to the presence of methane in oxic waters and include transport of methane produced in anoxic environments and nonbiological degradation of methylated compounds. Biotic factors contributing to the presence of methane in oxic waters include local production by microorganisms in anoxic microniches, degradation of methylated compounds and direct, photosynthesis‐related production by phytoplankton like Cyanobacteria , diatoms, green algae and Coccolithofores .