Project Summary (2009-2011)

The overall goal of this project is to determine the role of microorganisms in the environment on the distribution, transport and fate of toxic metals. First, Dr. Young wishes to investigate the role of naturally occurring microorganisms in the release and recycling of hazardous metals from their sulfide mineral form. It is hypothesized that microbial activity affects the rate and extent of metal sulfide solubilization. To this end the researchers will first examine the aerobic solubilization of sulfide minerals (e.g. NiS, CuS, CdS, PbS) by sulfide oxidizers including those isolated and characterized in the course of this study. They will also seek additional environmental strains which can potentiate the oxidation and release of toxic metals from their sulfide mineral. The rate and extent of microbial release of metals will be compared to that which is chemically (abiotically) mediated and incorporated into the modeling taking place in Dr. Di Toro’s lab.

Second, Dr. Young’s group will examine closely the microbiological oxidation of Chromium (lll) to Chromium(VI) (Cr), the occurrence of which is extremely difficult from the Cr(OH)₃ at environmentally relevant pH because of its insolubility. The researchers have initial evidence that Cr(lll)-organic ligands can be microbiologically oxidized which provides a means by which toxic Cr(VI) can be released back into the aquatic environment. The extent to which this is found in the environment will be investigated since this can be a critical aspect to predicting Cr behavior and managing its remediation. In addition this would close the biological Cr cycle in the environment.

Third, Dr. Young will extend work on microbial transformations of arsenic in the environment by examining the regulation of expression of arsB/C and arrAB as a function of arsenate concentrations in the environment. Many microbes have the ars system with which it reduces As(V) to As(lll) and expels it from the cell. More recently the arr system, which encodes for an arsenate respiratory reductase, has been found in a series of novel anaerobes that can use As(V) as an electron acceptor for respiration. From their previous work the researchers have isolated a series of anaerobic As(V) respirers and they have determined that at least one of the isolates has both operons. This will allow the research group to examine the environmental factors, such as arsenate concentration, which regulate gene expression of the different arsenate reduction genes. This will be extended to examining the community composition and differential gene expression in response to arsenate concentration.

Fourth, Dr. Young aims to identify and characterize the pathway and genes involved in demethylation of methylarsenic compounds. Methylated arsenicals are known to be of biological origin yet microbiological demethylation has received little attention. The research group will isolate environmental microbes that demethylate arsenic. Genes will be cloned in E. coli or yeast. Genes can also be identified by homology to known demethylating enzymes (e.g. methylselenium demethylases). Putative arsenic demethylases may fall into one of these families or may be a novel enzyme. Identification of the mechanisms of demethylation would close the arsenic methylation cycle in the environment.