Reductive dissolution of iron oxide minerals by Shewanella putrefaciens BrY

Understanding rates of mineral dissolution is of great importance for geology, pedology, ecology and environmental engineering. Mineral dissolution rates can be used to estimate the time required for chemical elements to be available for transport in the environment. The dissolution of iron oxide minerals provides an essential and sometimes limiting source of iron for microorganisms and plants. Reductive dissolution is a process of iron oxide weathering whereby chemical complexes adsorbed on the mineral surface transfer electrons to ferric iron (Fe(III)) in the mineral structure, leading to its destabilization and the release of iron into solution. Recent discovery of bacteria able to utilize Fe(III) as a terminal electron acceptor for respiration introduces the interesting coupling of a surface chemical process (reductive dissolution) with a biological process (iron respiration). The main objective of this dissertation research is to characterize the growth rates of iron-respiring bacteria BrY in ferric iron solutions under conditions with and without oxygen; to measure the rates of Fe(III) reduction in these solutions; and to determine the rates of dissolution of iron oxides in the presence of BrY.; A comprehensive review of the biogeochemistry of iron is presented, emphasizing reductive dissolution of iron oxide minerals. Rates of reductive dissolution measured by other researchers with organic and inorganic reductants are compared with those measured in the presence of bacteria. Methods for measuring growth and iron reduction are described. Laboratory measurements of the growth of bacterial isolate BrY in Fe(III)-citrate and Fe(III)-EDTA solutions are presented for conditions with and without oxygen and are compared with iron reduction rates measured in suspensions of hematite, goethite, and ferrihydrite.; Characterization of bacterial isolate BrY by different methods including fatty acid analysis showed that BrY is probably a strain of Shewanella putrefaciens. Bacteria BrY grew in Fe(III)-citrate solutions with and without oxygen, but the rates of growth were not significantly different. Experiments performed under oxic conditions showed that BrY could use citrate as well as lactic acid as a carbon source. The bacteria were able to grow in solutions of Fe(III)-EDTA when oxygen was present, but not when oxygen was absent. Hematite, goethite and ferrihydrite dissolved readily when bacteria were present, but not in controls without bacteria.