Microbial reduction of Fe(III) in smectite minerals by thermophilic methanogen Methanothermobacter thermautotrophicus

Abstract

Clay minerals and thermophilic methanogens can co-exist in hot anoxic environments, including the continental subsurface, geysers, terrestrial hot springs, and deep-sea hydrothermal vent systems. However, it is unclear whether thermophilic methanogens are able to reduce structural Fe(III) in clay minerals. In this study, the ability of a thermophilic methanogen Methanothermobacter thermautotrophicus to reduce structural Fe(III) in iron-rich and iron-poor smectites, (nontronite NAu-2 and Wyoming montmorillonite SWy-2) and the relationship between iron reduction and methanogenesis were investigated. M. thermautotrophicus reduced Fe(III) in nontronite NAu-2 and montmorillonite SWy-2 with H2/CO2 as substrate. The extent of bioreduction was 27% for nontronite and 13–15% for montmorillonite. Anthraquinone-2,6-disulfonate (AQDS) did not enhance the extent of bioreduction, but accelerated the rate. When methanogenesis was inhibited via addition of 2-bromoethane sulfonate (BES), the extent of bioreduction decreased to 16% for NAu-2 and 9% for SWy-2. These data suggest that Fe(III) bioreduction and methanogenesis were mutually beneficial. The likely mechanism was that Fe(III) bioreduction lowered the reduction potential of the system so that methanogenesis became favorable, and methanogenesis in turn stimulated the growth of the methanogen, which enhanced Fe(III) bioreduction. NAu-2 was partly dissolved and high charge smectite and biogenic silica formed as a result of bioreduction.