Kinetics of microbial Fe(III) oxide reduction in freshwater wetland sediments

The kinetics of microbial amorphous Fe(III) oxide reduction was investigated in sediments from a freshwater wetland in north central Alabama, USA. Fe(III) oxide concentrations decreased exponentially with time during anaerobic incubation of sediment slurries and homogenized surface sediments. Rates of organic carbon mineralization (SCO2 1 CH4 accumulation) did not change markedly during the course of Fe(III) oxide reduction, which indicated that the exponential decline in Fe(III) oxide concentration over time resulted primarily from Fe(III) limitation rather than a decrease in organic matter decay rate. Initial rates of Fe(III) oxide reduction were linearly correlated with initial Fe(III) oxide concentrations in experiments with mixtures of Fe(III)-rich and Fe(III)-depleted sediment slurries. Similar results were obtained in experiments with sediment from various depth intervals in the upper 3 cm of freshly collected cores. These findings provide explicit evidence that microbial Fe(III) oxide reduction rates are first order with respect to amorphous Fe(III) oxide concentration in the wetland sediment. The observed first-order relationship between Fe(III) oxide concentration and reduction rate is consistent with established models of surface area–controlled mineral transformation. An experiment in which Fe(III) oxide-rich sediment slurries were amended with different amounts of labile organic matter demonstrated a direct correlation between first-order Fe(III) reduction rate constants and initial rates of organic carbon mineralization. These results provide empirical support for existing approaches to modeling organic matter decay-dependent Fe(III) oxide reduction kinetics in sediments. Fe(III) oxides together with aqueous and solid-phase Fe(II) compounds are abundant components of many hydromorphic soils and aquatic sediments, and the redox cycling of Fe exerts a wide-ranging influence on the biogeochemistry of sedimentary environments where Fe is abundant (Ponnamperuma 1972; Lovley 1991; Davison 1993). Recent studies in both freshwater (Roden and Wetzel 1996) and marine (Canfield et al. 1993; Thamdrup et al. 1994; Hines et al. 1997; Thamdrup 2000) habitats indicate that dissimilatory microbial Fe(III) oxide reduction contributes substantially to sediment carbon metabolism. Microbial Fe(III) oxide reduction can play a major role in suppressing methanogenesis in freshwater environments, both in surface sediments (Lovley and Phillips 1986b; Achtnich et al. 1995; Roden and Wetzel 1996) and in the rhizosphere of aquatic plants (Roden and Wetzel 1996; Van Der Nat and Middelburg 1998; Frenzel et al. 1999) where oxygen input from plant roots drives a dynamic Fe redox cycle. Although a large body of knowledge exists on sedimentary Fe transformations and their environmental significance (e.g., Lovley 1991; Stumm and Sulzberger 1992; Davison 1993), detailed information on the in situ kinetics of many 1 Corresponding author ([email protected]). 2 Current address: Department of Environmental Sciences and Engineering, The University of North Carolina, Chapel Hill, North Carolina 27599-7431.