Microbial Metabolite Production for Accelerated Metal and Radionuclide Bioremediation (Microbial Metabolite Production Report)

Abstract

Biogeochemical activity is an ongoing and dynamic process due to bacterial activity in the subsurface. Bacteria contribute significantly to biotransformation of metals and radionuclides. As basic science reveals more information about specific mechanisms of bacterial-metal reduction, an even greater contribution of bacteria to biogeochemical activities is realized. An understanding and application of the mechanisms of metal and radionuclide reduction offers tremendous potential for development into bioremedial processes and technologies. Most bacteria are capable of biogeochemical transformation as a result of meeting nutrient requirements. These assimilatory mechanisms for metals transformation include production of small molecules that serve as electron shuttles for metal reduction. This contribution to biogeochemistry is small however due to only trace requirements for minerals by bacteria. Dissimilatory metal reducing bacteria (DMRB) reduce oxidized metals and insoluble mineral oxides as a means for biological energy production during growth. These types of bacteria offer considerable potential for bioremediation of environments contaminated with toxic metals and radionuclides because of the relatively large amount of metal biotransformation they require for growth. One of the mechanisms employed by some DMRB for electron transfer to insoluble metal oxides is melanin production. The electrochemical properties of melanin provide this polymeric, humic-type compound with electron shuttling properties. Melanin, specifically, pyomelanin, increases the rate and degree of metal reduction in DMRB as a function of pyomelanin concentration. Due to its electron shuttling behavior, only low femtogram quantities per cell are required to significantly increase metal reduction capacity of DMRB. Melanin production is not limited to DMRB. In fact melanin is one of the most common pigments produced by biological systems. Numerous soil microorganisms produce melanin, contributing to about 2-4 percent of the humic fraction of soils. Our recent work has shown that melanin production by one species of bacteria could be used by other species for metal reduction. This melanin ''sharing'' is the area of focus for this project. In addition, melanin contributes to significant increases in metal reduction by both assimilatory and dissimilatory metal reducing bacteria. Stimulation of melanin production in the subsurface offers potential for accelerating metal reduction. Another focus of this project was to determine the potential of melanin production in portions of the Tims Branch watershed as it relates to metal and radionuclide immobilization in-situ.