Hydrogenotrophic pathway dominates methanogenesis along the river-estuary continuum of the Yangtze River.

Rivers are considered as an important source of methane (CH4) to the atmosphere, but our understanding for the methanogenic pathway in rivers and its linkage with CH4 emission is very limited. Here, we investigated the diffusive flux of CH4 and its stable carbon isotope signature (δ13C-CH4) along the river-estuary continuum of the Yangtze River. The diffusive CH4 flux was estimated to 27.9 ± 11.4 µmol/m2/d and 36.5 ± 24.4 µmol/m2/d in wet season and dry season, respectively. The δ13C-CH4 values were generally lower than -60‰, with the fractionation factor (αc) higher than 1.055 and the isotope separation factor (εc) ranged from 55 to 100. In situ microbial composition showed that hydrogenotrophic methanogens accounts for over 70% of the total reads. Moreover, the incubation test showed that the headspace CH4 concentration by adding CO2/H2 to the sediment was orders of magnitude higher than that by adding trimethylamine and sodium acetate. These results jointly verified the river-estuary continuum is a minor CH4 source and dominated by hydrogenotrophic pathway. Based on the methanogenic pathway here and previous reported data in the same region, the historical variation of diffusive CH4 flux was calculated and results showed that CH4 emission has reduced 82.5% since the construction of Three Gorges Dam (TGD). Our study verified the dominant methanogenic pathway in river ecosystems and clarified the effect and mechanism of dam construction on riverine CH4 emission.

Iron oxides act as an alternative electron acceptor for aerobic methanotrophs in anoxic lake sediments.

Conventional aerobic CH4-oxidizing bacteria (MOB) are frequently detected in anoxic environments, but their survival strategy and ecological contribution are still enigmatic. Here we explore the role of MOB in enrichment cultures under O2 gradients and an iron-rich lake sediment in situ by combining microbiological and geochemical techniques. We found that enriched MOB consortium used ferric oxides as alternative electron acceptors for oxidizing CH4 with the help of riboflavin when O2 was unavailable. Within the MOB consortium, MOB transformed CH4 to low molecular weight organic matter such as acetate for consortium bacteria as a carbon source, while the latter secrete riboflavin to facilitate extracellular electron transfer (EET). Iron reduction coupled to CH4 oxidation mediated by the MOB consortium was also demonstrated in situ, reducing 40.3% of the CH4 emission in the studied lake sediment. Our study indicates how MOBs survive under anoxia and expands the knowledge of this previously overlooked CH4 sink in iron-rich sediments.