Evidence for facilitated uptake of Hg(II) by Vibrio anguillarum and Escherichia coli under anaerobic and aerobic conditions

Abstract

A mer-lux bioreporter was used to study uptake of inorganic mercury, Hg(II), at trace concentrations by two facultatively anaerobic bacterial species, Vibrio anguillarum and Escherichia coli. Uptake of Hg(II) by these bacteria appeared to be facilitated, rather than by passive diffusion. Three lines of evidence support this conclusion. First, under anaerobic conditions Hg(II) uptake was greatly decreased compared with aerobic conditions, even though the chemical composition of the medium was identical except for the lack of oxygen (i.e., no reducing agents were used). Second, the uptake of Hg(II) under anaerobic conditions was not proportional to the abundance of lipophilic Hg species but was dependent on the total concentration of Hg in the samples. Third, at trace Hg(II) concentrations and under anaerobic conditions, Hg(II) uptake was enhanced by the addition of yeast extract and a variety of low molecular weight organic acids. In addition to demonstrating that Hg(II) uptake by these bacteria had the characteristics of facilitated transport, these lines of evidence also support the conclusion that processes under regulatory control of the cell affected Hg(II) uptake. If these findings apply to other bacteria as well, they mean that current models of Hg(II) uptake by microorganisms in aquatic systems, which are based solely on the role of lipophilic Hg species and passive diffusion, will need to be reconsidered.Mercury undergoes a wide variety of chemical and biological transformations in aquatic ecosystems. Microorganisms play a major role in carrying out the biological transformations, many of which affect the toxicity of mercury. For example, bacteria that contain the mer operon can reduce Hg(II) to the volatile and relatively nontoxic Hg 0 (Hobman and Brown 1997). Hg(II) is transformed by bacteria in anaerobic sediments to monomethyl mercury (Compeau and Bartha 1984), which is a more toxic form of Hg(II) and is more readily bioaccumulated in the aquatic food web than inorganic mercury (Mason et al. 1996;Watras et al. 1998). For these reactions to occur, Hg(II) must first enter the cell; Acknowledgments We thank Dr. Cindy Gilmour for instruction and use of the anaerobic techniques, Dr. Bob Flett for Hg standards and analyses, and Dr. Feiyue Wang for help and discussions on metal speciation. Tamiko Hisanaga, Morris Holoka, Paul Humenchuk, Jack Switala, and Karen Scott kindly provided advice in methods development.

Bioaccumulation kinetics and exposure pathways of inorganic mercury and methylmercury in a marine fish, the sweetlips Plectorhinchus gibbosus

“…The uptakerate exhibited a log-log relationship with the dissolved concentration for both Hg species, indicating that uptake was primarily a Freundlich absorption type dominated by passive diffusion or a facilitated transport process (Golding et al 2002). Whether the dissolved uptake is active, however, cannot be concluded from this study.…”

Biological and photochemical production of dissolved gaseous mercury in a boreal lake

“…Bacterial mercuric reductase may be active under these conditions but with very low efficiencies (Golding et al 2002;Schaefer et al 2002). Also, humic substances may induce DGM pro- duction (Allard and Arsenie 1991).…”

Characterization of a marine-isolated mercury-resistant Pseudomonas putida strain SP1 and its potential application in marine mercury reduction

“…Mortazavi et al (2005) studied the effect of pH on the efficiency of removal of HgCl 2 by a P. putida strain. Other studies have indicated that pH is an important factor that significantly affects the concentration of bioavailable mercury, higher concentrations of mercury accumulating in the bacteria with the decreasing of pH (Golding et al 2002;Kelly et al 2003;Golding et al 2008;Ahn et al 2010). However, until now, the effect of pH on the expression of merA at the transcriptional level, an important process during mercury transformation, has not been examined.…”