The oxidation of soluble manganese(II to insoluble Mn(llI) and Mn(IV) oxides and oxyhydroxides has a profound effect on the environment. The highly reactive Mn oxide mineral phases that form have highly charged surfaces that scavenge numerous heavy metals and are strong oxidants capable of oxidizing a number of toxic xenobiotic, aromatic, and other organic and inorganic compounds. Mn oxides also serve as electron acceptors for growth of anaerobic bacteria. Although abiotic formation of Mn oxides can occur, in nature most Mn oxides form through the activities of microorganisms, primarily bacteria. Although Mn(lI)-oxidizing bacteria have been known from the beginning of the 20th century, the mechanisms of Mn(II) oxidation and the biological functions it serves remain enigmatic. This project focuses on the mechanism and function of Mn(II) oxidation in the marine Bacillus species strains SG-1 and PL-12 whose mature, dormant spores bind and oxidize Mn(II). Although Mn(II) oxidation is a thermodynamically favorable process under most environmental conditions, the spores increase the rate of oxidation up to five orders of magnitude compared to the abiotic rate. Spores such as these have been shown to be dominant members of the Mn(ID-oxidizing community in surficial marine sediments where they bind and precipitate a variety of other heavy metals. Previous research has demonstrated that in spores of both SG-1 and PL-12 Mn(II) is oxidized by proteins in the outermost spore layer. The amino acid sequence of the proposed Mn(II)-oxidizing protein from strain SG-1, MnxG, suggests it is a member of the family of multicopper oxidase proteins in which multiple copper ions function as redox active cofactors for the enzyme oxidation of a substrate. MnxG appears to be a bacterial homolog of cemloplasmin, a multicopper oxidase protein that is involved in iron oxidation and uptake in the blood stream of mammals. Recently two other unrelated Mn(II)-oxidizing bacteria were shown to have Mn(II)-oxidizing proteins with copper binding domains suggesting that copper may play a universal role in Mn oxidation. This research will further characterize the Mn(II)-oxidizing proteins from strains SG-1 and PL-12 to establish that they are truly multicopper oxidases and to compare their genes and proteins to those of other Bacillus species and Mn(II)-oxidizing bacteria. The primary function of the Mn(II)-oxidizing proteins for the bacteria will also be evaluated. Bacteria play a central role in the biogeochemical cycling of metals in the environment, but the molecular and biochemical mechanisms for most of these processes are not well understood. The results of this project should provide valuable information regarding the molecular basis and function of Mn oxidation, the factors that influence Mn oxidation, and the possible universal role of copper in this biogeochemically important process. This information will contribute to the knowledge base on microbial metal redox reactions in natural ecosystems and how these organisms might be utilized to benefit society. Characterizing the mechanisms of Mn oxidation in marine Bacillus spores may also provide the fundamental basis for biotechnological applications. The inherent physically tough nature of spores, combined with their unique capacity to bind and precipitate metals over a wide range of environmental conditions without having to sustain their growth, makes Mn-oxidizing spores particularly attractive for environmental remediation and metal recovery processes.
