Date of Award

6-2007

Degree Name

Doctor of Philosophy

Department

Geosciences

First Advisor

Dr. R. V. Krishnamurthy

Second Advisor

Dr. Carla Koretsky

Third Advisor

Dr. Johnson Haas

Fourth Advisor

Dr. Eliot Atekwana

Abstract

Studies using chemical and isotopic techniques have been conducted to investigate experimental microbial (biotic) iron reduction and abiotic redox processes. In the microbially mediated study Fe(III) reduction and lactate oxidation at circum-neutral pH and anaerobic conditions were used to characterize the metabolic pathways followed by dissimilatory Fe+3 -reducing bacterium Shewanella putrefaciens strain 200R. Iron (II), Iron (III), dissolved inorganic carbon (DIC) and carbon isotope ratios were measured under bicarbonate and phosphate buffered conditions. Carbon isotope disproportionation among organic carbon substrate (lactate), biomass and respired carbon dioxide at the lag to stationary phase of the microbial growth curve showed that bicarbonate buffered system has an enhancing effect in the reduction process compared to the phosphate system. Both buffered systems resulted in carbon isotope fractionations between the lactate substrate and DIC that could be modeled as a Rayleigh type process. The biomass produced under both buffer conditions was depleted on average by ~2 %0 relative to the substrate and enriched by ~5 %0 relative to the DIC. This translates to an overall isotopic fractionation of 10 - 12 %0 between the biomass and respired CO2 in both buffering systems and could explain the depleted isotopic signatures reported by previous workers during the serine metabolic pathway.

Abiotic oxidation of organic compounds as a function of electron acceptors, media composition, and pH in the presence and absence of fluorescent light was investigated. Sample collections and analytical techniques were similar to the biotic experiments. Abiotic results showed that HFO media resulted in the generation of more DIC compared to ferric citrate containing media. Type of buffer showed little effect on the redox process. Light and pH had major roles in the oxidation of citrate and lactate in the presence of ferric iron. Under dark conditions in the presence or absence of Fe (III) the DIC produced was very low in all pH conditions. The slower redox processes observed in our abiotic study, compared to previous studies, could be due to inhibiting effect of chloride ions on photochemical reactions. Chloride and bicarbonate ions have a scavenging effect on the hydroxyl radical. Isotopic results of both bicarbonate and phosphate buffered systems, at neutral pH condition, approached the δ13C values of citrate/lactate used. Rayleigh type process in the bicarbonate buffered abiotic media was modified due to the involvement of bicarbonate in the scavenging of the hydroxyl radicals in the system.

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Dissertation-Open Access

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