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Appl. Environ. Microbiol. doi:10.1128/AEM.00717-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Effect of pH on intracellular accumulation of trace concentrations of Hg (II) in Escherichia coli under anaerobic conditions as measured using a mer-lux bioreporter

Department of Microbiology, University of Manitoba, Winnipeg, Mb, R3T 2N2 Canada; Freshwater Institute, Department of Fisheries and Oceans, 501 University Crescent, Winnipeg, Mb, R3T 2N6 Canada

^* To whom correspondence should be addressed. Email: Richard_sparling{at}umanitoba.ca.

	Abstract

The effect of pH on the uptake and accumulation of Hg(II) by Escherichia coli was determined at trace, environmentally relevant, concentrations of Hg, and under anaerobic conditions. Hg(II) accumulation was measured using inducible light production from E. coli HMS174 harboring a mer-lux bioreporter plasmid (pRB28). The effect of pH on the toxicity of higher concentration of Hg(II) was measured using a constitutive lux plasmid (pRB27) in the same bacterial host. In this study, intracellular accumulation and toxicity of Hg(II) under anaerobic conditions were both significantly enhanced with decreasing pH, over the range of 8 to 5. The pH effect on Hg(II) accumulation was most pronounced at pH < 6, which substantially enhanced the Hg(II)-dependent light response. This enhanced response did not appear to be due to pH stress, as similar results were obtained whether cells were grown at the same pH as the assay, or at a different pH. The enhanced accumulation of Hg(II) was also not related to differences in the chemical speciation of Hg(II) in the external medium resulting from the changes in pH. Experiments with Cd(II), also detectable by the mer-lux bioreporter system, showed that Cd(II) accumulation responded differently to pH changes than the net accumulation of Hg(II). Potential implications of these findings for our understanding of bacterial accumulation of Hg(II) under anaerobic conditions and for bacterial mediated cycling of Hg(II) in aquatic ecosystems are discussed. Arguments are provided suggesting that this differential accumulation is due to changes in uptake of mercury.