Methylobacter couples methane oxidation and N2O production in hypoxic wetland soil

Aerobic methanotrophs differ in metabolic mechanisms under hypoxic environments. Methane oxidation combined with nitrate/nitrite reduction is related to the emissions of greenhouse gases methane and nitrous oxide (N2O). However, the mechanisms underlying methane oxidation and N2O production remain to be elucidated through environmental and pure culture experiments. Here, microcosmic experiments of inhibition of methane oxidation and isotope tracing revealed that methane oxidation coupled to N2O production occurred in hypoxic wetland soil. RNA sequence analysis showed that Methylobacter was the dominant methane oxidizer, and a novel strain Methylobacter sp. YRD-M1 was isolated. Genomic and pure cultured results suggested that nitrite was the electron acceptor of the isolated strain under hypoxic conditions with N2O production. Despite lacking the genetic potential for dissimilatory nitrate reduction, assimilatory nitrate reductase may be involved in incomplete denitrification, leading to N2O production. In addition, strain YRD-M1 consumed dissolved oxygen to below 10 μM, showing a strong oxygen affinity. Furthermore, fermentation was induced with acetate and succinate released during hypoxic incubation. The strain YRD-M1 showed flexible and versatile metabolic processes in oxygen-limited systems. Notably, Methylobacter-like methanotrophs were widely distributed in terrestrial environments. Overall, our findings show that Methylobacter couples methane oxidation and N2O production in hypoxic environments from soil to pure culture, highlighting the potential role of related microbes in linking methane sinks and N2O sources in oxygen-limited environments.