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Abstract
5kg of mercury to the global atmosphere with a residence time of approximately one year. This phenomenon changes the flux of biologically available mercury in natural microbial communities where enzymatic actions, including mercuric reductase and organomercurial lyase activities, underpin the biogeochemical cycling of mercury with repercussions for human exposure to toxic forms of the element. To elucidate the impact of episodic mercury bioavailability on the response of microbial communities, the expression of microbial proteins and nucleic acids in environmental strains of Pseudomonas species were evaluated under various concentrations of mercury ranging from 0 to 500 µM. Routine cultivation of Pseudomonas aeruginosa PU21 containing the 142.5 kb plasmid Rip64 in medium containing 100 µg of Hg ++ /ml (500 µM) exhibited a prolonged lag phase survived by hyper-resistant cells able to grow in medium containing 200 µg of Hg ++ /ml. Nucleic acid analyses showed a distinct mutation in the merA gene encoding for mercuric reductase activity in cells able to grow at elevated mercury concentrations. A similar mutation was detected in the merR locus which serves as the regulator of the mer operon. Mutations were not detected in merC which encodes for a hydrophobic membrane-associated protein implicated in active mercury transport. Protein profiles of cells grown with elevated mercury concentrations were associated with a stable increase in the production of specific polypeptides. In addition, the survival and genetic response of naturally-occurring mercury resistant bacteria inoculated into contaminated environmental samples were monitored in microcosm experiments over a 30 day period. The results suggest that sudden exposure to high concentrations of mercury either decimates the bacterial population or selects for hyper-resistant strains with high level of constitutive expression of active proteins, including mercuric reductase. Methyl mercury was observed to cause a higher level of induction for mercuric reductase than the specific substrate, inorganic mercury. The selection of hyper-resistant strains is potentially useful for biotechnological strategies to control the bioavailability of mercury, and thereby potentially reducing the re-uptake of mercury into vegetation in regions frequently subjected to wildfires.
Highlights
Mercury has long been recognized as a potent and widely distributed toxicant in the global environment (Clarkson, 1990; Fitzgerald and Clarkson, 1991; Nriagu, 1979; Pirrone et al, 1996)
It is proposed that such episodic changes in biologically available mercury typify environments that are impacted by wildfires
A three tier approach was used where changes in bacterial population densities were determined under increasing concentrations of mercury; the effect of mercuric compounds on the integrity of genetic determinants were determined through mutational analysis, and the effect of mercury on the expression of genetic potential was determined through the analysis of protein molecules
Summary
Mercury has long been recognized as a potent and widely distributed toxicant in the global environment (Clarkson, 1990; Fitzgerald and Clarkson, 1991; Nriagu, 1979; Pirrone et al, 1996). Among the most consequential anthropogenic sources of mercury in the environment are mining operations, energy generation from fossil fuels, and biomass burning through wildfires (Raloff, 1991; Wilhelm, 2001). Perpetual wildfires contribute approximately 25% of the anthropogenic sources of mercury to the episodic flux of biologically available mercury. This global estimate is consistent with regional estimates that suggested that incineration and combustion activities contribute approximately 30% of the sources of mercury in Africa and with measurement of mercury concentration in the sediment, water and biota of Lake Victoria in East Africa (Pirrone et al, 1996; Ramlal et al, 1998).
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