Hydrothermal vent mercury detoxification

Hydrothermal vent fluids of mid-oceanic ridge systems are highly enriched with both nutrients and heavy metals, and the concentrations of these metals may reach concentrations that are toxic to most living organisms. A gradient in metal toxicity surrounding the hydrothermal vents often exists where the vent fluids are diluted with the surrounding cold, oxygenated seawater. As the fluid mixes with oxygen the metals may become oxidized, more soluble and therefore more bioavailable and toxic. Microorganisms transform mercury between its three oxidation states (0, +1 and +2) and it is therefore expected that microbes found in the areas around hydrothermal vents will have high resistance to mercury and other metals. Mercuric reductase (MR) reduces ionic mercury [Hg(II)] to elemental mercury [Hg(0)], a less toxic form, and is encoded by the mer (bacterial mercury resistance) operon. Vetriani et al. (2005) investigated the mercury resistance, mer genes, MR activities and phylogenetic relatedness of one mesophile (EPR3; T_opt 28^oC), three moderate thermophiles (EPR6, EPR7 and EPR8; T_opt 45^oC) and psychrophilic strains found nearby as controls (T_opt 4^oC), that were all isolated at various distances from diffuse flow vents on the East Pacific Rise.

Vetriani et al. (2005) found that most moderately thermophilic strains were related to the genus Alcanivorax, and were highly resistance to Hg(II) and reduced it to Hg(0). They found that their mer genes formed a unique, previously unknown cluster that was most closely related to the clade that includes the best characterized MR and they differed from each other by one to three amino acid residues. The mesophilic strains were most closely related to Pseudoalteromonas, Halomonas, Pseudomonas, Marinobacter and unclassified Rhizobiales and removed Hg(II) before any growth occurred, showing moderate resistance to Hg(II). The psychrophilic strains were related to the genus’ Moritella, Psychrobacter and Photobacterium and were very sensitive to Hg(II). The activities of the MR tested corresponded to the ambient environmental temperature in all except Escherichia coli, which was used as a control. The E. coli MR exhibited maximal activity between 55 and 65^oC when extracted from the cell, but maximum rates were at 37^oC when inside the cell. This is characteristic of a thermophilic enzyme and may be a relic of evolution in a higher temperature, supporting the hypothesis of a hyperthermophilic origination of all life. A clear spatial relationship between distance from the vent and resistance to Hg(II) was shown, as would be expected, and it is likely that the bacteria here allow other vent organisms to exist by detoxifying the environment.

It would be interesting for further study to look at whether other vent organisms are capable of surviving the toxic mercury in their environment without the presence of these microbes as to whether the microbes really are allowing the existence of the whole ecosystem. Also, if anyone else is taking the Ecotoxicology module then this paper also links well with some of the concepts there, in particular there was a comment at the end of one of the lectures about how organisms cope with such high pressure and contamination…

Vetriani, C., Chew, Y., Miller, S., Yagi, J., Coombs, J., Lutz, R. & Barkay, T. (2005) Mercury Adaptation among Bacteria from a Deep-Sea Hydrothermal Vent. Applied and Environmental Microbiology. 71, 220-226