Multiple sources of aerobic methane production in aquatic ecosystems include bacterial photosynthesis

Abstract

Aquatic ecosystems are globally significant sources of the greenhouse gas methane to the atmosphere. Until recently, methane production was thought to be a strictly anaerobic process confined primarily to anoxic sediments. However, supersaturation of methane in oxygenated waters has been consistently observed in lakes and the ocean (termed the ‘methane paradox’), indicating that methane can be produced under oxic conditions through unclear mechanisms. Here we show aerobic methane production from multiple sources in freshwater incubation experiments under different treatments and based on biogeochemical, metagenomic, and metatranscriptomic data. We find that aerobic methane production appears to be associated with (bacterio)chlorophyll metabolism and photosynthesis, as well as with Proteobacterial degradation of methylphosphonate. Genes encoding pathways for putative photosynthetic- and methylphosphonate-based methane production also co-occur in Proteobacterial metagenome-assembled genomes. Our findings provide insight into known mechanisms of aerobic methane production, and suggest a potential co-occurring mechanism associated with bacterial photosynthesis in aquatic ecosystems.

Aerobic methane synthesis and dynamics in a river water environment

“…CH 4 decreased during the dark period of their experiment due to off‐gassing, which no doubt occurs in open environments such as the YR. However, the minimum CH 4 concentrations during maximum solar irradiance observed in the current study are difficult to reconcile with that pure culture work (Bižić et al 2020) or lake water incubations (Perez‐Coronel and Michael Beman 2022), where CH 4 synthesis positively correlated with photosynthesis and increasing light level [17].…”

Stable Carbon Isotope Signature of Methane Released From Phytoplankton

“…Based on the stable carbon isotope mass balance of CH 4 produced and the correlation between CH 4 and chlorophyll, these research teams suggested phytoplanktonic CH 4 production as a likely source to explain the CH 4 oversaturation in the epilimnion during spring and summer. This hypothesis has recently been strongly supported by Perez‐Coronel and Beman (2022) that associated aerobic CH 4 production with (bacterio)chlorophyll metabolism and photosynthesis. δ 13 C‐CH 4_source values of oxic CH 4 production in surface water were distinct from the much more negative δ 13 C‐CH 4 values measured in sediment pore water produced by methanogenic archaea (Hartmann et al., 2020; Thottathil et al., 2022).…”

Fire transforms effects of terrestrial subsidies on aquatic ecosystem structure and function

“…CH 4 concentrations increased four‐fold across time in all tanks and were higher in the burned treatment at the end of the experiment. Natural ponds and lakes account for 67% of CH 4 emissions from inland waters (Pilla et al., 2022), and the progressive rise in methane across our experiment may be the result of an increase in anaerobic methanogenesis localized within the detritus mesh bags as well as aerobic methanogenesis produced during bacterial photosynthesis (Perez‐Coronel & Beman, 2022). The trend for higher CH 4 in the burned‐detritus mesocosms at the end of the experiment, may likewise relate to greater autochthony and NPP in burned tanks also acting to stimulate bacterial methanogenesis.…”