Researchers from the University of Ottawa studied the growth of purple bacteria under mercury exposure and found that Hg(II) can fulfill a physiological function in these bacteria by acting as an electron sink to maintain redox homeostasis during photosynthetically energy gain.
The element mercury (Hg) in its different forms (species) is extremely toxic to most organisms, including humans. It’s toxic effects are thought to be due to its ability to bind to certain key metabolic enzymes thereby blocking their function. Because of its extreme toxicity, it has long been thought that mercury had no beneficial biological functions in the living world at all. At least that was presumption until a research team published the first
evidence that a unique group of organisms can not only tolerate certain
levels of the toxin, but actually benefit by its presence. The new study
In a paper published this month in Nature Geoscience, D.S. Gregoire and A.J. Poulain from the University of Ottawa show that photosynthetic microorganisms called purple non-sulfur bacteria can use mercury as an electron acceptor during photosynthesis. These bacteria gain energy by a primitive form of photosynthesis that differs from the type common to higher plants. In the case of photosynthesis in plants, water is used as an electron donor, with carbon dioxide the electron acceptor. The result of this process is the production of sugars, the release of oxygen, and the removal of carbon dioxide from the air.
Figure: Microbial processes that convert elemental mercury into methylmercury.
Credit: Department of Energy's Oak Ridge National Laboratory
Purple non-sulfur bacteria, on the other hand, usually live in aqueous anaerobic environments where light is available to them. Instead of water they use hydrogen as the electron donor, and an organic molecule such as glycerol or fatty acids, as the electron acceptor. This also results in the production of sugars, but does not release oxygen or remove carbon dioxide from the atmosphere. This process also generates too many electrons for their organic electron donor to handle, leading to oxidative stress with the potential for damage to other molecules in the cell.
The researcher showed that purple bacteria grow better in the presence of mercury. The reason seems to be that the bacteria use the mercury to accept those extra electrons, reducing the reactive mercury species Hg(II) to elemental mercury Hg°. By this reduction, the availability of mercury for forming methylmercury by biomethylation is reduced and it partly evaporates away into the atmosphere.
It’s quite possible that the impact of mercury reduction by photosynthesis may extend far beyond the health of these unusual little microbes. Jeffry K. Schaefer, in the Department of Environmental Sciences at Rutgers University speculates that, “By limiting methyl-mercury formation and accumulation in aquatic food webs from microorganisms to fish, this process may even contribute to less toxic mercury ultimately ending up on our dinner plates.” The new study:
D.S. Gregoire, A.J. Poulain, A physiological role for HgII during phototrophic growth
, Nature Geoscience, 9/2 (2016) 121–125. DOI: 10.1038/ngeo2629 Related studies:
Haiyan Hu, Hui Lin, Wang Zheng, Stephen J. Tomanicek, Alexander Johs, Xinbin Feng, Dwayne A. Elias, Liyuan Liang, Baohua Gu, Oxidation and methylation of dissolved elemental mercury by anaerobic bacteria
, Nature Geosci., 6 (2013) 751–754. DOI: 10.1038/ngeo1894
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, Geochim. Cosmochim. Acta, 112 (2013) 166–177. DOI: 10.1016/j.gca.2013.03.001
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Link Database: Toxicity of Organo-mercury compounds Link Database: Mercury exposure through the diet Link Database: Environmental cycling of methylmercury Link Database: Environmental cycling of inorganic mercury Link Database: Environmental pollution of methylmercury Link Database: Environmental pollution of inorganic mercury Link Database: Toxicity of mercury Related EVISA News
August 6, 2013: Bacterial methylation of mercury not only starting from oxidized mercury February 8, 2013: ORNL scientists solve mystery about mercury methylation June 17, 2012: Factors Affecting Methylmercury Accumulation in the Food Chain October 9, 2006: Linking atmospheric mercury to methylmercury in fish February 17, 2006: Study shows link between clear lakes and methylmercury contamination in fish
last time modified: February 14, 2016