MICROBIOLOGICAL AND IRON-ISOTOPIC EVIDENCE FOR DISSIMILATORY IRON REDUCTION IN RESERVOIR SEDIMENT NEAR IRON MOUNTAIN, CALIFORNIA

The goals of this study were: (1) to characterize dissimilatory iron reducing bacteria (DIRB) capable of participating in the anaerobic redox cycling of iron and (2) to investigate whether microbial iron cycling imprints a detectable isotopic biosignature in iron minerals. Bed sediment was collected from the Spring Creek Arm of Keswick Reservoir (SCAKR) in northern California, where acid mine drainage from a tributary draining the Iron Mountain Mines Superfund Site mixes with neutral-pH water from Whiskeytown Reservoir, resulting in precipitation of fine-grained Fe(III)-oxide-rich sediment in a circumneutral pH environment. Core samples from 1.5 m below the sediment-water interface were indicative of active Fe(III) reduction, as determined by the 0.5M HCl-extractable Fe(II)/[Fe(II)+Fe(III)] ratio (0.64). The sediment had pH 6.2 and contained 1,300 µmol g^-1 of 0.5M HCl-extractable Fe. Sediment pore waters had elevated concentrations of dissolved Fe(II) ranging from 650 mg L^-1 (12mM) to 2,000 mg L^-1 (36mM). Fe(III)-reducing microorganisms were enumerated (1.6±0.3 x 10⁷and 7.5±1.5 x 10⁶ CFU cm^-3 for (a) acetate- and (b) hydrogen-oxidizing bacteria, respectively) and isolated from last positive dilutions. All the isolates of (a) type were 96 to 97% identical to Geothrix fermentans, and all the isolates of (b) type were 99% identical to Clostridium magnum. A 16s rRNA gene clone library revealed a diverse microbial community, and produced clones in phyla characteristic of Fe and S biogeochemistry, such as β- and δ-proteobacteria, acidobacteria (including Geothrix fermentans) and actinobacteria. The SCAKR sediment (1.5 m core profiled at 5 depth intervals) was characterized by isotopically light dissolved Fe(II) (range: δ⁵⁶Fe = -1.54 to -0.50‰, mean: -1.04±0.44‰) and isotopically heavy amorphous or poorly crystalline Fe(III) oxides (range: δ⁵⁶Fe = +0.20 to +0.60‰, mean: +0.33±0.16‰), with the bulk sediment δ⁵⁶Fe = 0±0.07‰. The observed ⁵⁶Fe/⁵⁴Fe fractionation (about 1.3‰) is similar to fractionations observed in pure culture DIR experiments, which suggests that DIRB are reducing amorphous or poorly crystalline Fe(III) oxide minerals in the SCAKR sediment. Whether this biosignature can be replicated with the SCAKR sediment and native microbial community is under investigation.