Quantitative aspects of microbial crystalline iron(III) oxide reduction were examined using a dissimilatory iron(III) oxide-reducing bacterium (Shewanella alga strain BrY). The initial rate and long-term extent of reduction of a range of synthetic iron(III) oxides were linearly correlated with oxide surface area. Oxide reduction rates reached an asymptote at cell concentrations in excess of ≈1 × 109/m2 of oxide surface. Experiments with microbially reduced goethite that had been washed with pH 5 sodium acetate to remove adsorbed Fe(II) suggested that formation of a Fe(II) surface phase (adsorbed or precipitated) limited the extent of iron(III) oxide reduction. These results demonstrated explicitly that the rate and extent of microbial iron(III) oxide reduction is controlled by the surface area and site concentration of the solid phase. Strain BrY grew in media with synthetic goethite as the sole electron acceptor. The quantity of cells produced per micromole of goethite reduced (2.5 × 106) was comparable to that determined previously for growth of BrY and other dissimilatory Fe(III)-reducing bacteria coupled to amorphous iron(III) oxide reduction. BrY reduced a substantial fraction (8−18%) of the crystalline iron(III) oxide content of a variety of soil and subsurface materials, and several cultures containing these materials were transferred repeatedly with continued active Fe(III) reduction. These findings indicate that Fe(III)-reducing bacteria may be able to survive and produce significant quantities of Fe(II) in anaerobic soil and subsurface environments where crystalline iron(III) oxides (e.g., goethite) are the dominant forms of Fe(III) available for microbial reduction. Results suggest that the potential for cell growth and Fe(II) generation will be determined by the iron(III) oxide surface site concentration in the soil or sediment matrix.

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High concentrations (20-75 pmol cm-3) of amorphous Fe(III) oxide were observed in unvegetated surface and Juncus eflusus rhizosphere sediments of a freshwater wetland in the southeastern United States. Incubation experiments demonstrated that microbial Fe(III) oxide reduction suppressed sulfate reduction and methanogenesis in surface scdimcnts and mediated 240% of depth-integrated (O-10 cm) unvegetated sediment carbon metabolism, compared to I 10% for sulfate reduction. In situ CO2 and CH, flux measurements verified that nonmethanogenic pathways accounted for -50% of unvegetated sediment carbon metabolism. Lower (-1 O-fold) rates of dark/anaerobic CH, flux from experimental vegetated cores relative to unvegetated controls suggested that methanogenesis was inhibited in the Juncus rhizosphere, in which active Fe(III) oxide reduction was indicated by the presence of low but readily detectable levels of dissolved and solid-phase Fe(II). Fe(III) oxide reduction accounted for 65% of total carbon metabolism in rhizosphere sediment incubations, compared to 22% for methanogenesis. In contrast, methanogenesis dominated carbon metabolism (72% of total) in experimental unvegetatcd sediment cores. The high Fe(III) oxide concentrations and reduction rates observed in unvegetated surface and Juncus rhizosphere sediments were perpetuated by rapid Fe(III) regeneration via oxidation of Fe(II) compounds coupled to 0, input from the overlying water and plant roots, respectively. The results indicate that Fe(III) oxide reduction could mediate a considerable amount of organic carbon oxidation and significantly suppress CH, production in freshwater wetlands situated within globally extensive iron-rich tropical and subtropical soil regimes.Natural and agricultural wetlands generate up to 50% of annual CH4 input to the atmosphere (Cicerone and Orcmland 19 8 8). Understanding factors responsible for regional variations in wetland CH4 emission is important for refining global atmospheric flux estimates, and hence assessing the current and projected contribution of CH4 to atmospheric warming (Bartlett and Harriss 1993). Recent studies indicate that a variety of factors such as wetland plant productivity (Whiting and Chanton 1993), microbial CH4 oxidation (King 1993), water table height (Freeman et al. 1993;Moore and Dalva 1993), and temperature (Bartlett and Harriss 1993) affect rates of wetland CH4 production and release. Another important factor is competition among methanogenic and other anaerobic respiratory bacteria for organic substrates in wetland sediments (Kiene 199 1). In sulfate-rich marine and brackish environments, sulfate-reducing bacteria effectively outcompete methanogens (Capone and Kiene 1988), and rates of wetland CH4 production and flux are uniformly low in such environments (Bartlett and Harriss 1993). In contrast, methanogenesis is considered to be the dominant anaerobic carbon oxidation process in sulfate-poor, orAcknowledgments

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The largest Fe isotope fractionations occur during redox changes, as well as differences in bonding, but these are expressed only in natural environments in which significant quantities of Fe may be mobilized and separated. At the circumneutral pH of most low-temperature aqueous systems, Fe 2+ aq is the most common species for mobilizing Fe, and Fe 2+ aq has low 56 Fe/ 54 Fe ratios relative to Fe 3+-bearing minerals. Of the variety of abiologic and biologic processes that involve redox or bonding changes, microbial Fe 3+ reduction produces the largest quantities of isotopically distinct Fe by several orders of magnitude relative to abiologic processes and hence plays a major role in producing Fe isotope variations on Earth. In modern Earth, the mass of Fe cycled through redox boundaries is small, but in the Archean it was much larger, reflecting juxtaposition of large inventories of Fe 2+ and Fe 3+. Development of photosynthesis produced large quantities of Fe 3+ and organic carbon that fueled a major expansion in microbial Fe 3+ reduction in the late Archean, perhaps starting as early as ∼3 Ga. The Fe isotope fingerprint of microbial Fe 3+ reduction decreases in the sedimentary rock record between ∼2.4 and 2.2 Ga, reflecting increased bacterial sulfate reduction and a concomitant decrease in the availability of reactive iron to support microbial Fe 3+ reduction. The temporal C, S, and Fe isotope record therefore reflects the interplay of changing microbial metabolisms over Earth's history.

The Hg-methylating ability of dissimilatory iron-reducing bacteria in the genera Geobacter, Desulfuromonas, and Shewanella was examined. All of the Geobacter and Desulfuromonas strains tested methylated mercury while reducing Fe(III), nitrate, or fumarate. In contrast, none of the Shewanella strains produced methylmercury at higher levels than abiotic controls under similar culture conditions. Geobacter and Desulfuromonas are closely related to known Hg-methylating sulfate-reducing bacteria within the Deltaproteobacteria.

Microbial dissimilatory iron reduction (DIR) is an important pathway for carbon oxidation in anoxic sediments, and iron isotopes may distinguish between iron produced by DIR and other sources of aqueous Fe(II). Previous studies have shown that aqueous Fe(II) produced during the earliest stages of DIR has delta56Fe values that are 0.5-2.0%o lowerthan the initial Fe(III) substrate. The new experiments reported here suggest that this fractionation is controlled by coupled electron and Fe atom exchange between Fe(II) and Fe(III) at iron oxide surfaces. In hematite and goethite reduction experiments with Geobacter sulfurreducens, the 56Fe/54Fe isotopic fractionation between aqueous Fe(II) and the outermost layers of Fe(III) on the oxide surface is approximately -3%o and can be explained by equilibrium Fe isotope partitioning between reactive Fe(II) and Fe(III) pools that coexist during DIR. The results indicate that sorption of Fe(II) to Fe(III) substrates cannot account for production of low-delta56Fe values for aqueous Fe(II) during DIR.

Initial rates of biological (Shewanella putrefaciens strain CN32, pH 6.8) and chemical (ascorbate, pH 3.0) reduction of synthetic Fe(III) oxides with a broad range of crystallinity and specific surface area were examined to assess how variations in these properties are likely to influence the kinetics of bacterial Fe(III) oxide reduction in heterogeneous natural Fe(III) oxide assemblages. The results indicate that bacterial Fe(III) oxide reduction does not respond strongly to oxide crystal thermodynamic properties (∆G f ) which exert a significant impact on the kinetics of abiotic reductive dissolution. These findings suggest that oxide mineral heterogeneity in natural soils and sediments is likely to affect initial rates of bacterial reduction (e.g. during the early stages of anaerobic metabolism following the onset of anoxic conditions) mainly via an influence on reactive surface site density and that inferences regarding the competitiveness of bacterial Fe(III) oxide reduction as a pathway for organic matter oxidation in anoxic environments cannot be based on assumed thermodynamic properties of the dominant oxide phase(s) in the soil or sediment.