Inorganic mercury converted to more toxic and bio-accumulative monomethylmercury in ocean waters, possibly by microbes
A team led by the University of Alberta has confirmed that inorganic mercury (Hg) found worldwide in ocean water is transformed into monomethylmercury (MMHg)—a potent and bio-accumulative neurotoxin—in the seawater.
After two years of testing water samples across the Arctic Ocean, the researchers found that inorganic mercury, released by human activities such as industry and coal burning, undergoes a process called methylation and becomes monomethylmercury. Methylation, in this case, is the addition of a methyl group to heavy metals catalyzed by certain enzymes.
In a 1991 paper discussing concerns with mercury and monomethylmercury, William F. Fitzgerald and Thomas W. Clarkson noted that:
Interferences from human-related Hg emissions within the biogeochemical cycle of Hg in natural waters have presented a complex environmental problem. Not only must we worry about the direct impact of certain poisonous Hg compounds, but we must be alert to the potential for Hg species to transform biologically and chemically to the more lethal forms, particularly monomethylmercury (MMHg). Oddly, MMHg, which is more toxic than either Hg° or other Hg2+ species, is the principal form of Hg in fish. The grave results from localized MMHg contamination in natural waters have been tragically demonstrated by the mass poisonings at Minamata and Niigata, Japan.
Lead U of A researcher Igor Lehnherr says the greatest exposure to monomethylmercury for humans is through the consumption of marine-based foods.
Unlike inorganic mercury, monomethylmercury both bio-accumulates and bio-magnifies, meaning its toxic effects are amplified as it progresses through the food chain. Humans are at the top of the food chain, so we’re getting the highest content of this neurotoxin through contaminated seafood.
The conversion of inorganic mercury to monomethyl mercury accounts for approximately 50 per cent of this neurotoxin present in polar marine waters. Those high levels could also account for a significant amount of the mercury found in Arctic marine organisms.—Igor Lehnherr
For the study, Lehnherr’s research group incubated seawater samples, collected from the Canadian Arctic Archipelago, with stable inorganic mercury. Lehnherr found that the relatively harmless inorganic mercury was converted, through methylation, into the neurotoxin monomethylmercury. Lehnherr said the group believes the methylation process is happening in oceans all over the world.
The researchers say this is the first direct evidence that inorganic mercury is methylated in seawater. The research team is now going to look at how the process works.
We’re 90 per cent sure that the methylation process is carried is carried out by microbial life forms in the ocean like algae.—Igor Lehnherr
The research was published earlier this month online in Nature Geoscience.
Igor Lehnherr, Vincent L. St. Louis, Holger Hintelmann, Jane L. Kirk (2011) Methylation of inorganic mercury in polar marine waters. Nature Geoscience doi: 10.1038/ngeo1134
William F. Fitzgerald and Thomas W. Clarkson (1991) Mercury and Monomethylmercury: Present and Future Concerns. Environmental Health Perspectives Vol. 96, pp. 159-166