Mercury resistance and merA sequences of moderately thermophilic and mesophilic bacteria from hydrothermal vents

Abstract

Hydrothermal vent microorganisms are exposed to high concentrations of mercury, a toxic heavy metal. A common mechanism for bacterial mercury resistance is via the activity of mercuric reductase (MR), the gene product of merA. We hypothesized that some hydrothermal vent bacteria are resistant to mercury and possess merA genes that facilitate life in presence of mercury in their environment. Aerobic heterotrophic Proteobacteria were isolated from hydrothermal vent fluids from the East Pacific Rise (EPR) 9° North. Although mercury was not used as a selective agent in the isolation procedure, mercury resistance assays revealed that nine out of fourteen mesophilic and moderately thermophilic bacteria were resistant to mercury. The DNA of these vent isolates was screened by PCR for the presence of merA genes. As expected, mercury resistant isolates were found to possess the merA gene. Phylogenetic analysis based on the deduced amino acid sequences of merA showed that the moderate thermophiles EPR4, EPR6, and EPR8, form a distinct cluster among known merA sequences from gram-negative bacteria, while merA from EPR3, a mesophile, clustered with those of other gram-negative marine strains. This is the first report on mercury resistance and merA genes in moderately thermophilic and mesophilic hydrothermal vent bacteria, suggesting that mercury resistance in this unique ecosystem is mediated by MR activities. In short, our data suggest that: 1) hydrothermal vent bacteria use a similar mercury resistance strategy to that of other known mercury resistant bacteria; 2) most of the mesophilic and moderately thermophilic bacteria that were naturally exposed to metal-laden hydrothermal fluids are resistant to mercury.