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Abstract
The response of the mercury-resistant denitrifier, Pseudomonas stutzeri strain OX, and its sensitive derivative OX1 to HgCl 2 was examined under aerobic and anaerobic conditions to evaluate the potential role of the transformations mediated by the mercury resistance operon ( mer ) in the geochemical cycling of mercury. The resistance of strain OX to increasing concentrations of Hg(II) under anaerobic conditions resulted in a tri-phasic dose-response curve. Between 0.1 and 5 w M Hg(II), OX was as sensitive to Hg(II) as anaerobically grown OX1. No further growth inhibition was observed for OX between 5 and 25 w M Hg(II) under anaerobic conditions. At concentrations > 30 w M Hg(II), OX exhibited greater tolerance to Hg(II) under anaerobic versus aerobic conditions. Similarly, the sensitive strain OX1 was able to tolerate a 6-fold higher Hg(II) concentration under anaerobic than under aerobic conditions. When grown anaerobically, the maximal apparent 203 8Hg(II) volatilization rates by strain OX were decreased relative to those seen in aerobically grown cultures. Induction of mercuric reductase (MR) under anaerobic versus aerobic conditions depended on the Hg(II) concentration; at 0.1 w M, 203 Hg volatilization was lower anaerobically, whereas at 25 w M, MR activity was similar under both conditions. Additionally, an OX1 derivative carrying a merR-lacZ fusion produced > 20 times more g -galactosidase under aerobic than under anaerobic conditions when induced with < 0.5 w M Hg(II). These results suggest that (1) anaerobiosis affects Hg(II) transport into the cell and (2) the levels of mer expression are influenced by both the redox conditions and the concentration of Hg(II). Thus, mer -mediated activities are expected to affect mercury geochemistry in anoxic environments at higher concentrations of Hg(II) than in oxic environments. The implications of these findings to methylmercury accumulation in aquatic environments are discussed.
