Anaerobic methane‐oxidizing activity in a deep underground borehole dominantly colonized by Ca. Methanoperedenaceae

The family Ca. Methanoperedenaceae archaea mediates the anaerobic oxidation of methane (AOM) in different terrestrial environments. Using a newly developed high‐pressure laboratory incubation system, we investigated 214‐ and 249‐m deep groundwater samples at Horonobe Underground Research Laboratory, Japan, where the high and low abundances of Ca. Methanoperedenaceae archaea have been shown by genome‐resolved metagenomics, respectively. The groundwater samples amended with 13C‐labelled methane and amorphous Fe(III) were incubated at a pressure of 1.6 MPa. After 3–7 days of incubation, the AOM rate was 45.8 ± 19.8 nM/day in 214‐m groundwater. However, almost no activity was detected from 249‐m groundwater. Based on the results from 16S rRNA gene analysis, the abundance of Ca. Methanoperedenaceae archaea was high in the 214‐m deep groundwater sample, whereas Ca. Methanoperedenaceae archaea was undetected in the 249‐m deep groundwater sample. These results support the in situ AOM activity of Ca. Methanoperedenaceae archaea in the 214‐m deep subsurface borehole interval. Although the presence of Fe‐bearing phyllosilicates was demonstrated in the 214‐m deep groundwater, it needs to be determined whether Ca. Methanoperedenaceae archaea use the Fe‐bearing phyllosilicates as in situ electron acceptors by high‐pressure incubation amended with the Fe‐bearing phyllosilicates.