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.

Keywords: methane paradox; oxic methane production; cyanobacteria; phytoplankton; greenhouse gas; methane sources; biological methane production; methane paradox history; abiotic factors; biotic factors; methylated substances; phosphonates; methylamines; DMSP; methionine; diatoms; Coccolithophores; green algae

Figure 1. Increase in atmospheric concentration of methane (CH4) for the period 1984–2019. Reproduced from National Oceanic & Atmospheric Administration with permission of Ed Dlugokencky.
Figure 2. Summary of proposed abiotic (a) and biotic (b) processes contributing to the methane paradox. (1) Diffusive or water mass transfer; (2) ebullition; (3) transfer vial aquatic animals; (4) transpiration by submerged macrophytes; (5) photodegradation of plastic; (6) photodegradation of dissolved organic matter; (7) classical methanogenesis in zooplankton guts; (8) classical methanogenesis in anoxic microniches on organic matter aggregates; (9) hydrogen transfer from cyanobacteria; (10) methane production by demethylation of organic phosphonates; (11) methane production by phytoplankton; (12) demethylation of methylamines; (13) demethylation of DMSP.
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Further Reading

Bižić M, Klintzsch T, Ionescu D, et al. (2020) Aquatic and terrestrial Cyanobacteria produce methane. Science Advances 6: eaax5343.

Bižić‐Ionescu M, Ionescu D, Günthel M, Tang KW and Grossart H‐P (2019) Oxic methane cycling: new evidence for methane formation in oxic lake water. In: Biogenesis of Hydrocarbons, pp 379–400. Springer International Publishing: Cham.

Bogard MJ, del Giorgio PA, Boutet L, et al. (2014) Oxic water column methanogenesis as a major component of aquatic CH4 fluxes. Nature Communications 5: 5350.

DelSontro T, del Giorgio PA and Prairie YT (2018) No longer a paradox: the interaction between physical transport and biological processes explains the spatial distribution of surface water methane within and across lakes. Ecosystems 21: 1073–1087.

Donis D, Flury S, Stöckli A, et al. (2017) Full‐scale evaluation of methane production under oxic conditions in a mesotrophic lake. Nature Communications 8: 1661.

Grossart H‐P, Frindte K, Dziallas C, Eckert W and Tang KW (2011) Microbial methane production in oxygenated water column of an oligotrophic lake. Proceedings of the National Academy of Sciences 108: 19657–19661.

Tang KW, McGinnis DF, Ionescu D and Grossart H‐P (2016) Methane production in oxic lake waters potentially increases aquatic methane flux to air. Environmental Science & Technology Letters 3: 227–233.

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Bižić, Mina, Grossart, Hans‐Peter, and Ionescu, Danny(Feb 2020) Methane Paradox. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0028892]