Mineral transformations associated with goethite reduction by Methanosarcina barkeri Deng Liu a , Hongmei Wang a, ⁎, Hailiang Dong a, b, ⁎, Xuan Qiu a , Xiuzhu Dong c , Charles A. Cravotta III d a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China b Department of Geology, Miami University, Ohio 45056, USA c State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Science, Beijing 100080, China d United States Geological Survey, Pennsylvania Water Science Center, New Cumberland, Pennsylvania 17070, USA abstract article info Article history: Received 21 December 2010 Received in revised form 26 June 2011 Accepted 28 June 2011 Available online 3 July 2011 Edited by: Dr. J. Fein Keywords: Bioreduction Goethite Methanogen Mineral transformation Vivianite To investigate the interaction between methanogens and iron-containing minerals in anoxic environments, we conducted batch culture experiments with Methanosarcina barkeri in a phosphate-buffered basal medium (PBBM) to bioreduce structural Fe(III) in goethite with hydrogen as the sole substrate. Fe(II) and methane concentrations were monitored over the course of the bioreduction experiments with wet chemistry and gas chromatography, respectively. Subsequent mineralogical changes were characterized with X-ray diffraction (XRD) and scanning electron microscopy (SEM). In the presence of an electron shuttle anthraquinone-2,6- disulfonate (AQDS), 30% Fe(III) in goethite (weight basis) was reduced to Fe(II). In contrast, only 2% Fe(III) (weight basis) was bioreduced in the absence of AQDS. Most of the bioproduced Fe(II) was incorporated into secondary minerals including dufrénite and vivianite. Our data implied a dufrénite–vivianite transformation mechanism where a metastable dufrénite transformed to a more stable vivianite over extended time in anaerobic conditions. Methanogenesis was greatly inhibited by bioreduction of goethite Fe(III). These results have important implications for the methane flux associated with Fe(III) bioreduction and ferrous iron mineral precipitation in anaerobic soils and sediments. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Iron is the fourth most abundant element on the Earth surface, and the redox transition between ferric and ferrous iron contributes abundant energy flux to the Earth system. This redox process is partly mediated by microbial activity through much of the Earth history (Lovley et al., 1989; Ehrlich, 1990; Ehrlich, 1990; Vargas et al., 1998; Edwards et al., 2000; Islam et al., 2004; Kappler and Newman, 2004; Johnson and Beard, 2005; Peretyazhko et al., 2010). Since the discoveries of two genera of iron-reducing bacteria, Geobacter and Shewanella, in the 1980s (Lovley and Phillips, 1988; Myers and Nealson, 1988), the field of microbial reduction of iron has received increased attention (Lovley, 2000; Dong et al., 2009). Iron-reducing microorganisms that can utilize Fe(III) as terminal electron acceptor to couple oxidation of organic matter are termed as dissimilatory Fe(III) reducing prokaryotes (DIRP) (Lovley, 1991). Bioreducible Fe(III) in minerals is commonly present in sediments and soils with a concentration of several tens of mmol per kilogram of dry sediment (Straub et al., 2001). Ferrous iron produced by microbial reduction of Fe(III) minerals may accumulate in aquatic systems and can be sequestered into minerals, including magnetite (Lovley et al., 1987), siderite (Kim et al., 2004), vivianite (Dong et al., 2003; Zhang et al., 2009), and iron sulfide (Herbert et al., 1998), depending on the specific geochemical conditions. In anoxic soils and sediments, methane-producing archaea, i.e., methanogens, may be abundant and co-exist with Fe(III)-containing minerals. The interaction between iron reduction and methanogen- esis has been studied previously (Lovley and Phillips, 1987; Vargas et al., 1998; van Bodegom and Stams, 1999; Bond and Lovley, 2002; Roden and Wetzel, 2003; van Bodegom et al., 2004; Liu et al., 2011). Methanogenesis has been shown to be strongly inhibited in iron- rich sediments due to the competition between iron-reducing bacteria and methanogens for the same substrate (e.g. acetate and hydrogen) (Lovley and Phillips, 1987; Achtnich et al., 1995; van Bodegom and Stams, 1999; Roden and Wetzel, 2003). Certain types of methanogens, particularly those that can use H 2 as a substrate, may also directly reduce ferric iron, resulting in transfer of fewer electrons from H 2 to CO 2 and thus inhibiting methanogenesis (Vargas et al., 1998; Bond and Lovley, 2002; van Bodegom et al., 2004; Liu et al., 2011). The discovery of iron reduction by methanogens not only expands the pool of microorganisms that is capable of the iron redox cycling with important implications for microbial ecology and metal biogeochemistry but also provides a Chemical Geology 288 (2011) 53–60 ⁎ Corresponding authors at: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China. Tel.: +86 13419513876; fax: + 86 27 87436235. E-mail addresses: wanghmei04@163.com (H. Wang), dongh@muohio.edu (H. Dong). 0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.06.013 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo