Mn oxides are important mineral phases in the environment, where they scavenge a variety of metals and radionuclides and, because they are the strongest oxidizing agents other than oxygen found in the environment, they readily promote the oxidation of a variety of inorganic species and organic compounds (including recalcitrant natural and xenobiotic organic materials). In most natural environments, the formation of Mn oxides is primarily due to the activities of microorganisms, particularly bacteria.

My lab is interested in the mechanisms of bacterial Mn(II) oxidation, the function(s) it might serve, the potential applications of Mn(II)-oxidizing bacteria for heavy metal removal and bioremediation, Mn oxide biomineralization, and the biogeochemical cycling of Mn in the environment. Using molecular genetic approaches we and others have demonstrated that Mn(II) oxidation is enzymatically mediated by a multicopper oxidase-like protein in several phylogenetically distinct bacteria. We are also using traditional and modern spectroscopic techniques to characterize these proteins. In collaboration with Dr. John Bargar at the Stanford Synchrotron Radiation Laboratory (SSRL), we are using X-ray absorption spectroscopy (XAS) to investigate the molecular mechanisms of Mn(II) binding, electron transfer (redox reactions), and mineral formation by Mn(II) oxidizing bacteria.

In collaboration with T. Spiro (Princeton), G. Sposito (UC Berkeley), and J. Bargar (SSRL), we have recently received funding from the NSF through their Collaborative Research Activity in Environmental Molecular Science (CRAEMS) program for a multidisciplinary project "Molecular environmental Chemistry of Mn Oxide Biomineralization" investigating the bioinorganic and environmental chemistry of Mn oxide biomineralization. This research spans the disciplines of microbiology, bioinorganic chemistry, and environmental aqueous geochemistry to investigate the molecular mechanisms of manganese oxidation by bacteria and the mechanisms by which the resulting oxide solids impact the chemistry of heavy metals and organic contaminants in soils and natural waters. Please visit our website.

Research on Mn(II) oxidation is supported by grants from the National Science Foundation (NSF) and the National Institute of Environmental Health Science, Superfund Basic Research Program.

Reduction of hexavalent Cr and stability of reduced Cr. Hexavalent chromium (Cr(VI)) is a highly soluble and carcinogenic metal whereas the reduced form (Cr(III)) is less soluble and nontoxic. Cr(VI) is a byproduct of industrial activities such as ship building and metal finishing. The reduction of Cr(VI) by bacteria is widespread and is a result of direct enzymatic activity or chemical interactions with reduced compounds (e.g., sulfide or ferrous iron) produced by bacteria. Cr(VI) reduction to the less soluble and nontoxic Cr(III) is desirable as a means of detoxification and immobilization of chromium. Nonetheless, the long term stability of Cr(III) in the environment has not been definitively established. Our research efforts encompass both the bacterial reduction of Cr(VI) as well as the susceptibility of Cr(III) to oxidation. The Cr(VI) reduction aspect focuses on the mechanism of Cr(VI) reduction by bacteria at the genetic and biochemical level whereas the study of Cr(III) oxidation focuses on chemical processes governing the oxidation and solubilization of Cr(III). Both aspects of the research are crucial to designing effective bioremediation strategies for this EPA priority pollutant.

Uranium reduction. Uranium is found in ground- and surface water as a result of natural processes as well as anthropogenic activities such as nuclear weapon and fuel programs. The majority of uranium-affected waters are contaminated with the soluble hexavalent uranium found predominantly as the uranyl (UO22+) ion and its complexes. Numerous bacteria are able to directly reduce uranyl to the insoluble U(IV) ion with or without reaping energetic benefits. Our research focuses on using differential expression techniques such as 2D-polyacrylamide gel electrophoresis to identify proteins or whole genome microarrays to identify specific genes associated with uranyl reduction. The goal is to better understand the mechanism by which uranyl is reduced and energy derived for growth.

This research is funded by the National Institute of Environmental Health Science, Superfund Basic Research Program.

Selected Publications

Bacteria–Mineral Interactions

In collaboration with Dr. Barbara Ransom (SIO) we are studying the interaction between anaerobic bacteria (iron- and sulfate-reducing and methanogenic) and clay minerals commonly found in fine-grained marine sedimentary environments: montmorillonite, illite, chlorite, kaolinite, and quartz. In our first experiments we used natural microbial populations from a marine sediment near SIO to enrich for bacteria under sulfate-reducing conditions and iron-reducing conditions. Using standard molecular biological techniques (PCR amplification of the 16S small subunit ribosomal genes, denaturing gradient gel electrophoresis (DGGE), DNA sequencing, and phylogenetic analysis) we have determined the composition of the resulting microbial communities and have compared community composition and diversity between conditions. Results show a definite preference for different members of the original microbial populations to thrive in different mono-mineralogic substrates. We are interested in the relationship between these consortia and the mineral properties, whether minerals can be used to select for specific types of microbes, and how these microbe-mineral interactions affect the biotransformation and partitioning of organic and metallic pollutants in sediments.

This research is funded by the Office of Naval Research.

Selected Publications

Microbial Weathering of Ocean Basalts

To date, it has been challenging to determine the extent to which microbes are involved in low temperature (<100¡C) basalt weathering reactions, although textural, chemical and biological evidence suggests that bacterial activity may be widespread. Our research, in collaboration with Dr. Hubert Staudigel (SIO), is designed to identify which microorganisms are involved in the initial stages of basalt weathering, their phylogeny and modes of metabolic activity, and how transient these microbial communities may be. The project is a blend of field work and laboratory culturing efforts combined with molecular microbiological methods. The field portion will be conducted during two cruises to Loihi seamount and Puna Ridge, the submarine extension of the Kilauea Volcano (Hawaii), during October 2002 and 2003. We will sample tholeiitic basalts, vent fluids and ambient seawater, as well as deploy and retrieve basalt and mono-mineralogic charges, using the PISCES V submersible. From these samples we are developing enrichment cultures of mesophilic and thermophilic Fe(II)-oxidizing microorganisms as well as anaerobic consortia of Fe(III) and sulfate reducers and methanogens. Molecular methods, such as PCR amplification of the 16S small subunit ribosomal genes, Denaturing Gradient Gel Electrophoresis (DGGE) and/or direct cloning, and DNA sequencing, are used to identify the major colonizers of the basalts.

Selected Publications