Paradox reconsidered: Methane oversaturation in well‐oxygenated lake waters

Abstract

The widely reported paradox of methane oversaturation in oxygenated water challenges the prevailing paradigm that microbial methanogenesis only occurs under anoxic conditions. Using a combination of field sampling, incubation experiments, and modeling, we show that the recurring mid-water methane peak in Lake Stechlin, northeast Germany, was not dependent on methane input from the littoral zone or bottom sediment or on the presence of known micro-anoxic zones. The methane peak repeatedly overlapped with oxygen oversaturation in the seasonal thermocline. Incubation experiments and isotope analysis indicated active methane production, which was likely linked to photosynthesis and/or nitrogen fixation within the oxygenated water, whereas lessening of methane oxidation by light allowed accumulation of methane in the oxygen-rich upper layer. Estimated methane efflux from the surface water was up to 5 mmol m 22 d 21 . Mid-water methane oversaturation was also observed in nine other lakes that collectively showed a strongly negative gradient of methane concentration within 0-20% dissolved oxygen (DO) in the bottom water, and a positive gradient within $ 20% DO in the upper water column. Further investigation into the responsible organisms and biochemical pathways will help improve our understanding of the global methane cycle.

Vertical Distribution of Functional Potential and Active Microbial Communities in Meromictic Lake Kivu

“…Indeed, their presence even in the transition zone is consistent with recent studies that showed the presence of potentially active methanogens in the oxygenated water column of an oligotrophic lake attached to photoautotrophic microbes that might be enabling aerobic growth and supply of methanogenic substrates [74,75]. However, the presence of acetoclastic methanogens (Methanosarcinales) in the oxic-anoxic transition zone might also indicate the presence of syntrophy between methanogenic and fermentative microbes, through the occurrence of a coupled mutualistic interaction between hydrogen-/formate-producing and hydrogen-/formate-using microorganisms [76].…”

Emission and oxidation of methane in a meromictic, eutrophic and temperate lake (Dendre, Belgium)

“…Such a decrease cannot be related to exchange with the atmosphere since the atmospheric d 13 C-CH 4 is close to À47‰ (Quay et al, 1999). A possible explanation would be CH 4 production in oxic conditions related to primary production by pathways that remain elusive (Tang et al, 2016) as recently reported in several lakes (Grossart et al, 2011;Bogard et al, 2014;Tang et al, 2014). Such an explanation is consistent with the eutrophic nature of the Dendre Lake and should be further investigated in future.…”

A novel pathway of direct methane production and emission by eukaryotes including plants, animals and fungi: An overview

“…Grossart et al (2011) detected the attachment of methanogenic archaea to photoautotrophs, which allowed the transfer of carbon substrates to the CH 4 -producing organisms in putative anaerobic microenvironments. Subsequent study of the same lake supported their findings and suggested that microbial methanogenesis can be coupled with nitrogen fixation (Tang et al, 2014).…”