| Iron is the fourth most abundant element in the Earth’s crust and the most abundant transitional metal on the Earth. Iron redox cycling between Fe(Ⅱ) and Fe(Ⅲ) profoundly impacts various geological processes and associated environmental changes. For instance, it is generally considered that iron redox affects the long-term storage of organic matter in the worldwide sediments and the nutrients migration, and acts as a key factor in controlling the mobilization and immobilization of multiple heavy metals and organic pollutions.Microorganisms have been recognized as the important agents in mediating the Fe redox cycle. Within microbial respiration, some microorganisms can harvest energy by triggering the excellular iron redox cycle, in which ferrous iron is used as an electron donor and ferric iron as an electron acceptor. Iron oxides and iron-bearing clay minerals are widely distributed in soils and sediments and often account for the majority of the iron in the sedimentary environments. The studies of microbial iron reduction lie at the heart of the microbially mediated iron redox. It is now established that iron-reducing microorganisms can utilize solid-phase Fe(Ⅲ) as terminal electron acceptors to couple oxidation of organic matter or hydrogen gas, and this microbial functional group is normally termed as dissimilatory iron reducing bacteria (DIRB). Controlled by porewater Eh and nutrient concentration, in sediments, DIRB typically are located below the denitrification zone and above the zones of sulfidogenesis and methanogenesis. Upon microbial reduction of iron-containing minerals, several new or altered minerals can possibly form, including magnetite, siderite, vivianite, green rust and others. The biogenic minerals usually exhibit different properties from their inorganic phases, such as mineral size, crystallinity and chemistry. Therefore, biogenic minerals are proposed as bio-signatures to infer past microbial activities. By contrast to the vast studies on DIRB, our knowledge of the interaction of other microbial functional groups (e.g.. sulfate-reducing bacteria and methanogens) and iron-containing minerals are still far from completion. Furthermore, the detailed information on how the elevated pressure and pH of geo-fluid influence microbial iron reduction during diagenesis is not available. To address above questions, we conducted laboratory experiments to investigated whether different microbial functional groups from anoxic environments were capable of reducing Fe(Ⅲ)-bearing clay minerals. Several anaerobic microbes were selected for the bench-scale bioreduction experiments, including Shewanella sp. belonging to DRIB, a sulfate-reducing bacterium Desulfovibrio vulgaris and a typical methanogen Methanosarcina barkeri.Exogenous electron transfer mediators employed by Fe(Ⅲ)-reducing bacteria are believed to govern the kinetics and equilibrium of bioreduction of Fe(Ⅲ) in solid phase. In contrast to the vast number of studies on humic substances and analog anthraquinone-2,6-disulfonate (AQDS), our knowledge of other potential electron shuttles involved in Fe(Ⅲ) reduction is limited. The purpose of DIRB experiments was to understand the role of cystine and cysteine in reduction of iron-rich smectite (nontronite, NAu-2) by Shewanella species. A series of abiotic or biotic experiments were conducted in non-growth media (bicarbonate buffered, pH=7.0). Fe(Ⅱ) and cysteine concentrations were monitored over the course of the bioreduction experiments with wet chemistry, and the unreduced and reduced nontronites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicated that all Shewanella species tested here were capable of reducing cystine to cysteine. Either cystine or cysteine amendments significantly stimulated the bioreduction rate and extent. The extent of bioreduction without any exogenous electron shuttles reached7.3%. However, in the presence of electron shuttles, the bioreduction extents were much greater, reachingup to18.7%to26.3%. The initial reduction rate was linearly correlated with cystine or cysteine concentration. The reduction extent calculated from bioreactor with cystine or cysteine was slightly lower than those with AQDS. Mineralogical analysis demonstrated that cystine or cysteine facilitated the reaction of smectite to illite as a result of Fe(III) bioreduction. Thus, we concluded that, in our experiments, cystine and cysteine functioned as electron carrier in the smectite reduction systems, and were favorable factor influencing smectite illitization.The objective of SRB experiments was to understand the reduction rate and capability of structural Fe(Ⅲ) in interstratified clay minerals by a mesophilic SRB. Desulfovibrio vulgaris and the potential role in catalyzing smectite illitization. Bioreduction experiments were performed in batch systems, where four different clay minerals (nontronite NAu-2, mixed-layer smectite-illite RAr-1and ISCz-1, and illite IMt-1) were exposed to D. vulgaris in non-growth medium with and without AQDS. Our results demonstrated that D. vulgaris was able to reduce structural Fe(Ⅲ) in clay minerals, and AQDS enhanced the reduction rate and extent. Our data also suggest that abiotic reduction induced by sulfide, which was generated via microbial sulfate reduction, could be one possible factor contributing to reduction enhancement. However, enzymatic pathway still was a predominant factor responsible for bioreduction of Fe(Ⅲ) in clay minerals by D. vulgaris The order of the reducibility was as follows:nontronite> mixed-layer I/S> illite. This order was correlated with smectite proportion in clay minerals and also with mineral surface area. The properties of smectite can favor electron transfer in different ways. First, higher layer expandability of smectite, also related to larger surface area and lower layer charger, provides more exposed Fe(Ⅲ) sites for microbial attachment, possible dissolution (breaking down the Fe(Ⅲ)-O bond), and transfer of electrons to Fe(Ⅲ) centers in the structures, resulting in a higher reduction extent. Second, clay minerals usually carry a net negative surface charge, and microbial surface is also commonly negatively charged (partly arising from carboxylic acids, amino and other groups), resulting in a repulsive force between themselves. However, the presence of a proton dissociation-association mechanism on the edges of clay structure can result in positive charges. Hence the edges of clay minerals are expected to attach to microorganisms. Among the S-I series, the surface charge of smectite is less negative than illite, thus edges of smectite particles might be preferably sorbed onto microbial surfaces. Such smectite-SRB association would result in electron transfer from edge sites to the interior of the smectite structure. To assess mineralogical changes after bioreduction, multiple approaches were employed including XRD and modeling using Sybilla, SEM, TEM and energy dispersive X-ray spectroscopy (EDS). These data collectively showed that D. vulgari could promote smectite illitization through reduction of structural Fe(Ⅲ) in clay minerals.The ability of methanogen Methanosarcina barkeri to reduce structural Fe(Ⅲ) in iron-containing minerals (nontronite NAu-2and goethite) and the relationship between iron reduction and methanogenesis were investigated. Bioreduction experiments using nontronite were conducted in phosphate-buffered basal medium using three types of substrate:H2/CO2, methanol, and acetate. The extent of iron reduction was measured using wet chemical methods. Time course methane production and hydrogen consumption were measured by gas chromatography. Mineralogical changes were characterized with XRD and SEM. M. barkeri was able to reduce structural Fe(Ⅲ) in NAu-2with H2/CO2and methanol as substrate, but not with acetate. The extent of bioreduction, as measured by the1,10-phenanthroline method, was7-13%with H2/CO2as substrate, depending on nontronite concentration. The extent was higher when methanol was used as a substrate, reaching25-33%. Methanogenesis was inhibited by Fe(III) reduction when H2/CO2was used as substrate, and was enhanced when methanol was used. High charge smectite formed as a result of bioreduction. M. barkeri was also able to reduce Fe(Ⅲ) in goethite with hydrogen as an electron donor, and the reduction extent was enhanced by the presence of AQDS (up to30%). Methanogenesis was inhibited by bioreduction of Fe(Ⅲ) in goethite. Along with the increase in dissolved Fe2+XRD analyses detected the formation of Fe-phosphate minerals (dufrenite and vivianite) in the AQDS treatments. An increase of particle size was observed after bioreduction, likely due to the secretion of extracellular polymeric substances (EPS) by M. barkeri.Our results suggest that methanogens play an important role in biogeochemical cycling of ferric iron and may have important implications for the global methane budget.The objective of the experiments using deep-subsurface bacteria was to understand the reduction of structural Fe(Ⅲ) and mineral transformation during the interaction between iron-rich smectite (nontronite, NAu-2) and microorganisms isolated from the subsurface environments. Fe(Ⅲ) reduction experiments were conducted in batch cultures with two bacterial strains:Shewanella piezotolerans WPS from the deep-sea sediment and an alkaliphilie strain (designated CCSD-1) from the borehole of the Chinese Continental Scientific Drilling Program (CCSD). The bioreduction of NAu-2with WPS was performed at0.1and20MPa in the presence of an electron shuttle AQDS. The experiments with CCSD-1were conducted in the growth medium with and without AQDS. Fe(II) concentrations were monitored over the course of the experiments via wet chemistry, and the unreduced and reduced nontronite were characterized by XRD, SEM and TEM. The results indicated that the tested pressures of0.1and20MPa exhibit limited effect on the bioreduction by strain WPS, and the final reduction extents by the two subsurface strains in the presence of AQDS are similar to or even higher than those reported by the surface (nearsurface) strains. Mineralogical analysis confirmed the transformation of smectite to illite within the inoculated systems with two strains, and distinct occurrence of anorthite in the experiments with CCSD-1. Our work expanded the knowledge about the microbial mediated structural Fe(Ⅲ) reduction and smectite transformation in the deep subsurface.Overall, our laboratory experiments demonstrated that DIRB, SRB and methanogens have the capability of reducing structural Fe(Ⅲ) in clay minerals. Among these three microbial functional groups, DIRB exhibited the strongest iron reduction capability, and methanogens reduced the smallest mount of Fe(Ⅲ) in clay minerals. In our studies, it further showed that different water chemistry (pH and chemical composition) resulted in different secondary mineral assemblages. Within neutral and sulfate-free experiments, high-charge smectite, mixed-layer illite-smectite, amorphous silica and calcite can be observed in bioreduced samples. Under neutral and sulfate-rich condition, amorphous iron sulfide was also found besides aforementioned secondary minerals. When bioredution occurred in alkaline condition, feldspar group minerals can be formed together with high-charge smectite, mixed-layer illite-smectite, amorphous silica and calcite. The secondary mineral assemblages observed in our bench-scale experiments have significant implications for low-temperature clay transformations in sedimentary settings. |
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