Biogeochemical cycling of iron in anoxic environments: The importance of iron speciation and bacterial iron reduction

This study was undertaken to further elucidate the biogeochemical cycling of solid phase Fe in marine sediments and to assess the role of metal-reducing bacteria in the coupling of Fe and C cycles.; An improved, calibrated extraction scheme was utilized along with chemical principles to show that previous studies may have overestimated amorphous Fe(III) oxides and underestimated crystalline Fe(III) oxides in marine sediments. A seasonal cycle of amorphous and crystalline Fe(III) oxides was shown which has not been demonstrated previously. Cycling was consistent with porewater geochemistry and the storage of Fe as pyrite. Plants and bacteria played a dominant role in the cycling of Fe in coastal marine sediments through the production of labile organic matter, ezymatically catalyzed redox reactions, and by providing a source of oxidant to the sediments.; Sediment incubations, amended with microbial inhibitors, showed biological reduction of solid Fe(III) over formaldehyde-killed controls, exclusive of Fe reduction which was attributed to biogenic sulfide. The data suggest that sediment temperature and the presence of labile organic matter limit microbial Fe(III) reduction in sediment incubations.; More effective methods for monitoring "bioavailable" Fe(III) were developed. A reactive Fe(III) intermediate was produced during dissimilatory Fe(III) reduction which has not been shown previously. Production of a reactive Fe(III) intermediate indicated that bacteria can transform solid Fe(III) to a form that is more easily reduced, and crystalline Fe(III) minerals may be available for microbial reduction.; Growth data showed that a broad range of organic carbon compounds (C{dollar}sb1{dollar} to C{dollar}sb6{dollar}) may be oxidized during dissimilatory Fe(III) and thiosulfate reduction. Novel growth yields were similar for Fe-reducing bacteria with Fe(III) or S(IV) as electron acceptor, and yields were comparable to those of other anaerobic-respiring bacteria (such as sulfate-reducers or nitrate-reducers). Because of their abundance in the marine environment, their versatility, and their ability to transform large amounts of Fe and S, this group of anaerobic respirers (Shewanella putrefaciens) was suggested to play an important role in global biogeochemical cycles.