Role of thiols in microbial metal reduction and Importance of organic-Fe(III) complexes on the benthic flux of iron from continental margin sediments
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Both Fe(III) and Mn(III,IV) oxides are ubiquitous in aquatic sediments and represent favorable terminal electron acceptors for microbial respiration. As a result, the dissimilatory reduction of Fe(III) and Mn(III,IV) contributes to the cycling of carbon in both marine and freshwater sediments and impacts a variety of other important biogeochemical and environmental processes, including the oxygen, nitrogen, phosphorus, and sulfur cycles, and the mobilization of organic and inorganic contaminants. Despite the potential impact of these processes, the microbial mechanism of metal reduction remains under debate as the organism must transfer electrons to a solid electron acceptor, a unique physiological problem in microbiology. The redox cycling of soluble electron shuttles, by which an efficient reductant of Fe(III) and Mn(III,IV) oxides is microbially recycled, has emerged as a potential solution to this problem. Indeed, addition of exogenous electron shuttles such as hydroquinones and thiols increases the rate and extent of Fe(III) and Mn(IV) oxides reduction in pure cultures of model Mn(III,IV)- and Fe(III)-reducing bacteria. In this dissertation, the ability for thiols to act as electron shuttles during dissimilatory reduction of Fe(III) and Mn(III,IV) oxides was assessed through the development of a novel voltammetric technique that simultaneously quantifies thiol and disulfide species in situ. This method was employed to investigate the kinetics and mechanism of the abiotic reaction of four low molecular weight thiols with both ferrihydrite and common Mn(III,IV) oxides at environmentally relevant concentrations and pH. To investigate the role of thiols as potential electron shuttles in the reduction of Fe(III) and Mn(IV) in sediments, depth profiles of thiols in the pore waters from a variety of locations, including subalpine freshwater wetlands in Colorado, a salt marsh in Georgia, the Rhone River delta, and the continental margins off North Carolina and Louisiana were determined along with the main redox processes in these sediments. Thiols were detected at all locations, and depth profiles demonstrated significant correlations between thiols and reduced iron (Fe2+). Sediment slurry incubations and cultures of model Fe(III)-reduction bacteria Shewanella oneidensis MR-1 confirmed endogenous production of thiols during iron reduction. Overall, these results demonstrate that thiols may be efficient electron shuttles in microbial metal reduction and could be used as tracers to differentiate dissimilatory Fe(III) reduction from the chemical reduction by dissolved sulfides in sediments, a notoriously difficult problem in marine biogeochemistry. Reduction of Fe(III) oxides in marine sediments may have important implications in chemical oceanography as it may allow dissolved iron to diffuse across the sediment water interface and the benthic iron flux may stimulate primary productivity in the water column of iron-limited high nutrient low chlorophyll (HNLC) zones. Although aeolian deposition as well as inputs from rivers, hydrothermal vents, icebergs and continental margin sediments represent significant sources of iron to the ocean, the contribution of sediments to the iron supply to the water column is not well constrained. The benthic iron flux has mostly been determined from continental shelf sediments in upwelling zones or areas exposed to large riverine inputs, overlooking slope sediments and passive continental margins. In this dissertation, the sedimentary flux of dissolved iron was determined along a transect across the North Carolina continental margin, which does not experience major upwelling or is not exposed to large riverine inputs. This study demonstrated that slope sediments produce more bioavailable iron than shelf sediments, as continental slopes are depocenters for both organic matter and terrigenous material. The upward flux of dissolved iron to the overlying waters may be suppressed and replaced by the burial of iron under the form of FeS or pyrite in sediments receiving a large flux of organic carbon that are typically dominated by sulfate reduction. In the highly active sediments of the Rhône River prodelta, complete reduction of sulfate titrated Fe2+ and increased burial of FeS. The burial of FeS resulted in the decoupling of anaerobic and aerobic processes, which allowed alkalinity to flux across the sediment water interface and potentially contribute to the buffering capacity of the waters overlying those sediments. Finally, as the stabilization of dissolved Fe(III) under the form of organic-Fe(III) complexes may be required to maintain a significant iron flux across the sediment-water interface and as these complexes are observed in most sediments not dominated by sulfate reduction, a new technique was developed to isolate the ligands responsible for stabilizing Fe(III) in solution in marine sediments. Immobilized metal affinity chromatography (IMAC) with Fe(III) successfully retained known Fe(III)-binding ligands for detection by mass spectrometry (MS), and preliminary results demonstrated the method could be used to isolate Fe(III)-ligands from both natural samples and incubations of model Fe(III)-reducing bacteria. Overall, the findings of this dissertation help redefine the mechanism of dissimilatory iron reduction, provide new chemical proxies to distinguish between microbial and chemical Fe(III) reduction in marine sediments, and provide insight into the importance of Fe(III) reduction in continental margin sediments and the biogeochemical processes controlling the release of dissolved iron from sediments.