HR: 08:18h
AN: H41K-02 INVITED    [Abstracts]
TI: The Microbiology of Perchlorate in the Environment
AU: * Coates, J D
EM: jcoates@nature.berkeley.edu
AF: UC Berkeley, Dept of Plant and Microbial Biology 271 Koshland Hall, Berkeley, CA 94720, United States
AB: In the last decade perchlorate has been identified as an important groundwater component that poses potential health threat. Although primarily sourced anthropogenically, many recent studies have identified significant natural pools throughout the US and the natural mechanisms of its synthesis remain a mystery. As such, the true perchlorate concentrations naturally present in the environment are still unknown making its regulation problematic. Because of its solubility and non-reactivity the fate and transport of perchlorate in the environment is primarily a function of microbial activity. In the last seven years more than forty specialized perchlorate respiring organisms have been identified and characterized. These dissimilatory perchlorate reducing bacteria (DPRB) are metabolically diverse and environmental populations tend to be dominated by two primary genotypes, the Dechloromonas and the Azospira species. As such, the majority of our understanding of this metabolism is based on these organisms. These organisms are readily found in soil and sedimentary environments and often associate with the rhizosphere. Recent research has demonstrated an accumulation of these organisms along plant roots suggesting their catabolism of root exudates and molecular studies has demonstrated their existence as endophytic infections of the stem and leaves of actively growing Brachypodium grass plants although their exact role under these conditions is unknown. These microorganisms are generally not nutritionally fastidious and vitamin supplementation is unnecessary for growth although molybdenum is a required trace element for perchlorate reduction. The Dechloromonas and Azospira species generally grow optimally at pH values near neutrality in freshwater environments. Even so, recent field studies have shown that related deep-branching members of these genera often predominate in sites of adverse pH or salinity with some species being capable of growth and perchlorate respiration at pH values as low as pH 5. Although studies have demonstrated microbial perchlorate reduction in salt brines as concentrated as 11% NaCl, to date no microorganism isolated has been demonstrated to grow by perchlorate respiration in salinities greater than 2%. The metabolism is negatively regulated by oxygen and nitrate. Preference for oxygen is observed even at low oxygen partial pressures while preference for nitrate is observed regardless of the nitrate to perchlorate ratio. Even when the DPRB are pre-grown anaerobically in perchlorate, nitrate is still preferentially reduced prior to perchlorate. Molecular studies reveal that electron acceptor utilization is regulated at the genetic level and is not simply a matter of chemical kinetics. Whole genome sequencing of Dechlormonas aromatica revealed a large number of signaling proteins, a high proportion of which were two component histidine kinase systems suggesting that this organism has exquisite sensitivity to its environment. This supported by the observed ability of this organism to sense, distinguish, and chemotax towards perchlorate or nitrate depending on the growth conditions. These studies demonstrate how the concerted efforts over the last decade have resulted in significant advances in our understanding of the geobiology of microorganisms capable of reductively transforming perchlorate into innocuous chloride. Several in-situ and ex-situ bioremediative processes have been engineered and many monitoring tools based on immunology, molecular biology, and stable isotope content are now available. As such, the rapid scientific response to this emerging contaminant offers great hope for its successful elimination from contaminated environments in the future.
UR: http://pmb.berkeley.edu/~coates/
DE: 0400 BIOGEOSCIENCES
DE: 0404 Anoxic and hypoxic environments (4802, 4834)
DE: 0409 Bioavailability: chemical speciation and complexation
DE: 0410 Biodiversity
DE: 0448 Geomicrobiology
SC: Hydrology [H]
MN: 2007 Fall Meeting