Aerobic oxidation of methane significantly reduces global diffusive methane emissions from shallow marine waters

Abstract

Methane is supersaturated in surface seawater and shallow coastal waters dominate global ocean methane emissions to the atmosphere. Aerobic methane oxidation (MOx) can reduce atmospheric evasion, but the magnitude and control of MOx remain poorly understood. Here we investigate methane sources and fates in the East China Sea and map global MOx rates in shallow waters by training machine-learning models. We show methane is produced during methylphosphonate decomposition under phosphate-limiting conditions and sedimentary release is also source of methane. High MOx rates observed in these productive coastal waters are correlated with methanotrophic activity and biomass. By merging the measured MOx rates with methane concentrations and other variables from a global database, we predict MOx rates and estimate that half of methane, amounting to 1.8 ± 2.7 Tg, is consumed annually in near-shore waters (<50 m), suggesting that aerobic methanotrophy is an important sink that significantly constrains global methane emissions.

Dimethylated Sulfur, Methane and Aerobic Methane Production in the Yellow Sea and Bohai Sea

“…DIN was replete in the water masses found at stations H34 (8.36 μmol L −1 ) and H19 (10.56 μmol L −1 ) followed by high N/P ratios of 93 and 106 compared to offshore sites (N/P ratio of 15 at station H12 and 12 at station B21). Recently, nutrient limitation has been discussed as a possible regulator of methane production (Damm et al., 2010; Mao et al., 2022; Repeta et al., 2016). Large fresh inputs of DIN from the Yangze River (up to 200 μmol L −1 , Xu et al., 2022), may cause an imbalance of nutrients and result in the extreme high N/P ratios in the estuarine areas compared to nitrate depleted conditions with low N/P ratios (less than Redfield ratio 16) in the offshore waters (Figure 2).…”

“…Methane supersaturation with respect to the atmospheric equilibrium concentration shows up as a persistent feature of most coastal and ocean surface water, and this has been termed the “oceanic CH 4 paradox” (Reeburgh, 2007; Repeta et al., 2016). Methylphosphonate (MPn), DMSP and DMSO has been reported to contribute to methane supersaturation in the surface mixed layer (Damm et al., 2010; Karl et al., 2008; Mao et al., 2022; Repeta et al., 2016; Zindler et al., 2013). However, most evidence for CH 4 formation from DMSP remains ambiguous and is mainly based on occasional correlations between CH 4 and DMSP concentrations (Damm et al., 2010; Zindler et al., 2013).…”

Machine Learning Predicts the Methane Clumped Isotopologue (12CH2D2) Distributions Constrain Biogeochemical Processes and Estimates the Potential Budget

“…These reactions produce unique isotopes that result in methane removal, and our data sets are available to identify these processes. ,, Under aerobic conditions, CH 4 is oxidized to CO 2 by aerobic methanotrophs through the catalytic action of methane monooxygenase (MMO). AOM is the consumption of methane in the presence of anaerobic methanotrophic archaea (ANME), causing clumped isotope equilibrium trends in marine sediment settings and natural gas reservoirs. − Methanotrophic processes have been shown to consume over half of the methane produced in the aquatic environment before being released into the atmosphere. , The major sink for atmospheric methane is its oxidation by radicals, with tropospheric OH • serving as the primary sink responsible for approximately 90% of methane consumption . Both carbon monoxide (CO) and nitrogen oxides (NOx) affect the concentration of OH • in the atmosphere, and even small changes in OH • can result in a significant impact on the atmospheric concentration of CH 4 .…”

No evidence of light inhibition on aerobic methanotrophs in coastal sediments using eDNA and eRNA

“…Compared with shallow inshore waters, the findings of our study and above-mentioned further indicate that deep offshore coastal waters (>30 m) have higher CH 4 concentrations in the bottom water and sediment, as well as a higher methanotrophic activity in the sediment surface. Even though much of the CH 4 in these areas is biologically oxidized (Mao et al, 2022), large CH 4 emissions can occur during water column mixing events, when CH 4 -rich bottom waters are brought to the surface (Bonaglia et al, 2022). Likely this lower benthic methanotrophic activity in shallow areas is related the lower concentration of pore-water CH 4 limiting methanotrophic growth.…”

“…However, the CH 4 dynamics and factors driving spatial and temporal variability in CH 4 emissions in coastal waters are complex and not yet fully understood (Rosentreter et al, 2021; Weber et al, 2019). Aerobic CH 4 oxidation by specialized bacteria (so‐called methanotrophs) in the sediment and water can potentially remove up to half of all CH 4 in shallow coastal waters (Mao et al, 2022). Understanding how and when these organisms oxidize CH 4 is therefore essential to comprehend the dynamics of CH 4 emissions from coastal zones.…”