eTopic 22.1 Global Mercury Cycle: Contamination and Bioremediation
Many toxic metals cycle in our environment, where their chemistry is mediated by living organisms, especially bacteria. A major example of metal toxicity mediated by bacteria is that of mercury (Fig. 1). Mercury is found in small quantities in nature, but human production of pesticides puts out 10,000 metric tons annually, in addition to 3,000 metric tons released by the burning of fossil fuels. Elemental mercury (Hg0) has limited toxicity; but in the environment, mercury is methylated by bacteria, forming methylmercury, CH3Hg+, which is highly toxic. When bacteria containing methylmercury are consumed by grazers and predators, the mercury concentration increases as a proportion of biomass until it may reach levels toxic to animals such as fish and shellfish. When these animals are consumed by humans, we may experience extreme toxicity, particularly neurological effects.
Note that methylation of mercury is just one of the reactions bacteria may conduct. Anaerobic respirers may reduce Hg2+, while others may demethylate methylmercury to the less toxic Hg2+. Addition of a second methyl group produces the extremely toxic and volatile compound dimethylmercury [(CH3)2Hg], which dissipates initially but can be taken up by organisms.
Uranium contamination. Increasingly, bacteria are being harnessed for metal removal and detoxification technologies. An example is the use of microbes to reduce radioactive uranium to an insoluble form, which is easier to remove from contaminated water or soil, such as that of abandoned uranium mines (Fig. 2). A strain of Desulfovibrio desulfuricans reduces soluble U(VI) to U(IV), an insoluble form. Desulfovibrio is better known for anaerobic H2S generation associated with corrosion of sewer pipes and oil wells (discussed earlier). For metal removal, however, the organism can use U(VI) oxide for anaerobic respiration, with an electron donor such as acetate or sugar. By pumping acetate or crude sugars such as molasses into contaminated soil, soluble U(VI) is reduced to insoluble U(IV), which is immobilized in the soil and prevented from migrating further into the water table.
Figure 1 Mercury conversions include microbial metabolism. A. Mercury methylated by bacteria is accumulated by consumers across the food web, including, ultimately, humans who eat contaminated fish or shellfish. B. Various reactions of mercury are mediated by bacteria. Bacteria may modify the metal through lithotrophy or use it to oxidize H2S. Methylation of ionic mercury (Hg2+) is a common response, probably to detoxify it for the bacterium. Unfortunately, methylmercury (CH3Hg+) is the most hazardous form for consumers because it sticks within sediment and dissolves in membrane lipids. Addition of a second methyl group forms dimethylmercury [(CH3)2Hg], a volatile, highly toxic compound.
Figure 2 Bacteria remove uranium contamination. A. Desulfovibrio desulfuricans reduces U(VI) with an organic electron donor (cell length 1–2 μm; SEM). Because the resulting U(IV) is insoluble, bacterial reduction can remove uranium from contaminated wastewater. B. A uranium mine in Moab, Utah, is 200 times more radioactive than the public exposure limit. Such sites may someday be decontaminated by bacteria. Sources: A. Courtesy of Judy D. Wall, U. of Missouri, Columbia. B. Kevin Moloney/The New York Times/Redux