Dissertation Defenses

Doctoral Dissertation Announcement

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Candidate: Tsigabu A. Gebrehiwet

Degree of: Doctor of Philosophy

Department: Geosciences

Title: Chemical and Isotopic Investigations on Microbial Iron Reduction and Redox Stratification in Marine and Freshwater Ecosystems: Experimental and Field Approaches

Committee:
Dr. R.V. Krishnamurthy, Chair
Dr. Carla Koretsky
Dr. Johnson Haas
Dr. Eliot Atekwana


Date: Friday, May 11, 2007 3:00 p.m. – 5:00 p.m.
1122 Rood Hall

Abstract:
Chemical and isotopic studies were conducted on experimental (biotic and abiotic) and field based (marine and freshwater ecosystems) redox processes. In the dissimilatory Fe+3-reducing bacterium, Shewanella Putrefaciens strain 200R, mediated study Fe(III) reduction and lactate oxidation at circum-neutral pH were used to characterize the metabolic pathways. Iron(II)/(III), DIC and carbon isotope ratio (d13C) were measured under bicarbonate and phosphate buffered conditions. The d13C values of lactate, biomass and respired carbon dioxide at the log phase of microbial growth curve showed that the bicarbonate buffered system enhanced the reduction process compared to the phosphate system. In both systems carbon isotope fractionations between the lactate substrate and DIC could be modeled as a Rayleigh process and the biomass produced was depleted by ~2 ‰ relative to substrate and enriched by ~5 ‰ relative to DIC. This translates to an overall isotopic fractionation of 10 – 12 ‰ between biomass and respired CO2 in accordance with the reported isotopic signatures during serine 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. Results showed that light, pH and Fe(III) had major effect in the redox processes.
Field studies were focused on spatial and seasonal cycling of organic matter and associated redox stratification at marine and freshwater ecosystems. Pore waters were analyzed for pH, alkalinity, DIC, d13CDIC, Fe(II), SO4, and sulfides; and sediments for d13CInorg, d13COrg and (C/N)Org. Depth wise increment in DIC, Fe(II) and sulfides and depletion of  SO4 values were observed. The d13COrg and (C/N)Org values range from -21 to -17 ‰ and 10 to 18 respectively indicating equal contribution from  terrestrial and marine sources. Depleted d13Cinorg (-27 to -16 ‰) indicated CH4 oxidation.
At the freshwater site DIC, Fe(II), sulfides, SO4, d13CDIC values varied with depth. d13COrg and (C/N)Org values range from -30 to -26 ‰ and 5 to 11 respectively, suggesting a bias in favor of aquatic sources. The d13CInorg values ranged from -50 to -5‰, suggesting methane oxidation at this site also.