Microbial reduction of Fe(III) in illite–smectite minerals by methanogen Methanosarcina mazei

Clay minerals are ubiquitous in soils, sediments and sedimentary rocks, and could coexist with methanogens in anoxic environments, but whether or not methanogens can reduce structural Fe(III) in clay minerals other than smectite is not known. The objective of this study was to understand the capability of a mesophilic methanogen, Methanosarcina mazei in reducing structural Fe(III) in illite–smectite clay minerals and its potential role in inducing mineralogical changes. Bioreduction experiments were performed in batch systems, where four different clay minerals (nontronite NAu-2, mixed-layer illite–smectite RAr-1 and ISCz-1, and illite IMt-1) were exposed to M. mazei in growth medium with and without anthraquinone-2,6-disulfonate (AQDS) as electron shuttle. The extent and rate of bioreduction were quantified via wet chemistry and mineralogical transformed by X-ray diffraction and scanning and transmission electron microscopy. Our results demonstrated that M. mazei was able to reduce structural Fe(III) in clay minerals with Fe(III) in smectite being the most reducible and illite the least. AQDS enhanced the reduction rate and extent. The bioavailability of structural Fe(III) to M. mazei was correlated to the smectite proportion in each clay mineral . Methanogenesis was inhibited by Fe(III) bioreduction, possibly due to diversion of electrons from the methanogenesis pathway to structural Fe(III) in clay minerals. Bioreduction of Fe(III) in clay minerals induced formation of biogenic mixed-layer illite–smectite, silica, and vivianite. These data collectively showed that mesophilic methanogen is capable of reducing structural Fe(III) in illite–smectite minerals and of inducing a number of mineralogical changes.