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 .

The Effects of Engineered Aeration on Atmospheric Methane Flux From a Chesapeake Bay Tidal Tributary

“…Rock Creek is a source of atmospheric methane, regardless of aeration status, or site. The air-water methane flux from Rock Creek varied between 0.2 and 1.5 mmol m −2 d −1 (note change in units to compare to literature values), which was similar to fluxes measured from several estuaries (Table 3), and higher than those from oceanic environments, which vary between 0.0001 and 0.1 mmol m −2 d −1 (Bižić et al, 2020). Rock Creek methane fluxes are on par with a shallow subarctic lake which reached almost 0.4 mmol m −2 d −1 (Jansen et al, 2020), even though at times of ice-out, these lakes can release as much as 75 mmol m −2 d −1 (McIntosh Marcek et al, 2021).…”

“…We thus explore the possibility of water column methane production under aeration. There is growing evidence for an oxic production pathway in surface waters (see review in Bižić et al, 2020) that might be important. Most notably, it was concluded that 90% of methane in surface waters of a temperate lake was formed in the oxic surface waters, and not the sediments (Donis et al, 2017).…”

“…The primary source of methane is the underlying sediments, as in most organic rich environments (Martens and Berner, 1974;Reeburgh, 2007). However, there is an increasing appreciation for alternative sources such as demethylation of organic phosphonates (Karl et al, 2008), bacterial degradation of water column dissolved organic matter (Repeta et al, 2016) and/or production by phytoplankton (Bižić et al, 2020) that have not been fully explored in estuarine systems. In shallow-water, dynamic coastal and estuarine environments, methane can also be delivered with currents or tides from lateral sources (Bižić et al, 2020).…”

“…However, there is an increasing appreciation for alternative sources such as demethylation of organic phosphonates (Karl et al, 2008), bacterial degradation of water column dissolved organic matter (Repeta et al, 2016) and/or production by phytoplankton (Bižić et al, 2020) that have not been fully explored in estuarine systems. In shallow-water, dynamic coastal and estuarine environments, methane can also be delivered with currents or tides from lateral sources (Bižić et al, 2020). While methane formed in the sediments can enter the water column through ebullition (Boudreau, 2012) or diffusion, nearly 85% of the methane produced within sediments is oxidized anaerobically before it reaches the sediment-water interface via microbially mediated reactions including sulfate reduction, nitrate reduction, and iron reduction (Froelich et al, 1979;Reeburgh, 2007).…”

On the Possibility of Aerobic Methane Production by Pelagic Microbial Communities of the Laptev Sea

“…Development of methanogenic archaea in the seas is associated mainly with anoxic sediments and (in the water column) with anoxic microniches in large suspended organic parti-cles or in the zooplankton gut and pellets (Bianchi et al, 1992;Ditchfield et al, 2012). However, the surface, oxygen-containing waters of various areas of the Global Ocean often exhibit methane oversaturation relative to the atmosphere, the phenomenon termed the methane paradox (Karl et al, 2008;Bižic et al, 2020). Detection of anoxic microniches harboring methanogenic archaea has long been used to explain this phenomenon.…”