HR: 13:55h
AN: B42C-02    [PDF]
TI: Evidence for the Occurrence of Microbial Iron Reduction in Bulk Aerobic Unsaturated Sediments
AU: * Cooper, D C
EM: coopdc@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, P.O. Box 1625 MS 2107, Idaho Falls, ID 83415 United States
AU: Kukkadapu, R K
EM: Ravi.Kukkadapu@pnl.gov
AF: Pacific Northwest National Laboratory, MS K8-96 Environmental Molecular Sciences Laboratory, Richland, WA 99352 United States
AU: Smith, W A
EM: smitwa@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, P.O. Box 1625 MS 2107, Idaho Falls, ID 83415 United States
AU: Fox, D T
EM: foxdt@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, P.O. Box 1625 MS 2107, Idaho Falls, ID 83415 United States
AU: Plummer, M A
EM: plumma@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, P.O. Box 1625 MS 2107, Idaho Falls, ID 83415 United States
AU: Hull, L C
EM: hulllc@inel.gov
AF: Idaho National Engineering and Environmental Laboratory, P.O. Box 1625 MS 2107, Idaho Falls, ID 83415 United States
AB: Radionuclide transport experiments conducted in a large, meso-scale column reactor (MSCR, 10 ft high x 3 ft dia) operated under unsaturated flow conditions with simulated rainwater influent provide evidence that microbial iron reduction can occur in bulk-aerobic vadose zone systems with a low organic carbon content (~0.5 wt%). Soil gas analyses indicate that CO2 varied between ~0.1% of soil gas (top) and 12% to 18% of soil gas (bottom). O2 varied inversely with CO2, and the ratio of (CO2 produced) / (O2 consumed) was 0.8 +/- 0.1. NO3- was present at high concentrations, and originated from soluble NO3- salts present in the packing material. Ammonia was present at low levels, and limited NO2- production was observed. There was no increase in aqueous iron, and methane and sulfide were not produced. M\H{o}ssbauer analyses of sediment iron mineralogy indicate that the sedimentary iron in the packing material is 63% illite Fe(III), 16% illite Fe(II), 13% hematite, and 8% poorly-crystalline/small-particulate (pc/sp) iron oxide. Sediments collected from the lower portion of the column (5.5 fbs, feet below surface) still contain illite and hematite, but have lost the pc/sp iron oxide component. The Fe(III)/Fe(II) ratio of the illite appears to be unchanged at this depth. Analyses of sediment extractable DNA and cell number indicate that bacterial abundances increase from the surface to 0.5 fbs, and then remain constant with depth. Initial results from DGGE and 16s rDNA clone libraries indicate that microbial community structure alters with increasing depth, decreasing O2 content, and loss of pc/sp iron oxides. These data indicate a predominance of {\it Clostridium} at the column top, with {\it Bacillus}, {\it Desulfobacterium}, and {\it Pseudomonas} also providing a significant contribution. At 0.5 fbs, {\it Clostridium} represents a larger fraction of the total community with {\it Desulfobacterium} present as the second most abundant component. By 5.5 fbs, {\it Clostridium} is a minor component and the community is dominated by microaerophiles and facultative anaerobes such as {\it Aquaspirillum}, {\it Flexibacter}, and {\it Verrucomicrobium}. {\it Desulfobacterium} and {\it Pseudomonas} are also present at relative proportions similar to that observed higher in the column. This trend continues to 6.5 fbs, the lowest depth sampled. M\H{o}ssbauer spectroscopy indicates that pc/sp iron is being lost from the system, and the shift in microbial community structure towards facultative anaerobes capable of this metabolism ({\it Pseudomonas}, {\it Clostridium}) indicates that this shift may be engendered by either assimilatory or dissimilatory iron reducing microorganisms. Thus, anaerobic microbial processes in general, and microbial iron reduction in particular, may be important within nutrient-poor, unsaturated, bulk-aerobic vadose zone environments. Beyond this evidence for the occurrence of microbial iron reduction in an emulated vadose zone system, these data are difficult to reconcile. As is detailed in an associated poster presentation, the {\it Clostridium} results may indicate the growth of an obligate anaerobe in the upper, oxic portions of the column. As the oxygen content decreases with depth, the community then shifts away from obligate anaerobes ({\it Clostridium}) toward facultative anaerobes. This is an area of current inquiry, and questions/suggestions are encouraged.
DE: 0330 Geochemical cycles
DE: 0400 Biogeosciences
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting