PNNL

Abstract

A quantitative study was conducted to understand if Fe (III) in loess sediments is available for microbial respiration by using a common metal reducing bacterium, Shewanella putrefaciens, CN32. The loess samples were collected from three different sites: St. Louis (Peoria), Missouri, USA; Huanxia (HX) and Yanchang (YCH), Shanxi Province of China. Wet chemical analyses indicated that the total Fe concentration for the three samples was 1.69%, 2.76%, and 3.29%, respectively, of which 0.48%, 0.67%, and 1.27% was Fe(III). All unreduced loess sediments contained iron oxides and phyllosilicates (smectite, illite, chlorite, vermiculite), in addition to common minerals such as quartz, feldspar, plagioclase, calcite, and dolomite. Bioreduction experiments were performed at a loess concentration of 20 mg/mL using lactate as the sole electron donor, Fe(III) in loess as the sole electron acceptor in the presence and absence of anthraquinone-2, 6-disulfonate (AQDS) as an electron shuttle. Experiments were performed in non-growth (bicarbonate buffer) and growth (M1) media with a cell concentration of ~2.8 x 107 and 2.1 x 107 cells/mL, respectively. The unreduced and bioreduced solids were analyzed by X-ray diffraction (XRD), Mössbauer spectroscopy, diffuse reflection spectroscopy (DRS), and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) methods. Despite many similarities among the three loess samples, the extent and rate of Fe (III) reduction varied significantly. For example, in presence of AQDS the extent of reduction in the non-growth experiment was 25% in HX, 34% in Peoria, and 38% in YCH. The extent of reduction in the growth experiment was 72% in HX, 94% in Peoria, and 56% in YCH. The extent of bioreduction was lower in absence of AQDS. Overall, AQDS and the M1 growth medium significantly enhanced the rate and extent of bioreduction. Fe(III) in iron oxides and Fe(III)-containing phyllosilicates was bioreduced. Biogenic illite, siderite, and vivianite formed. The results of this study suggest that Fe (III) in loess sediments represents a potentially important source of electron acceptors to support microbial activity in dry environments.