GEOTHERMAL MERCURY AND MICROBIOLOGY IN WESTERN NORTH AMERICA

The close relation between natural occurrences of mercury and hydrothermal activity has been known for well over 100 years. The occurrence of mercury-reducing microbes was known by 1970 and the discovery of the mercuric reductase enzyme and the Mer operon soon followed. We have compiled data on mercury and microbiology in thermal features of western North America with an emphasis on mercury-resistant and mercury-reducing microbes. Chemical and isotopic data suggest loss of mercury through volatilization but speciation studies have been challenging because of the inability of stannous chloride to completely reduce divalent mercury complexed with sulfide or other thio groups. Furthermore, sampling for mercury in thermal features is highly sensitive to the method of sampling. In Yellowstone drillhole Y-7, a deep sample (70 m) kept under pressure was found to contain nearly 2,000 ng Hg/L (unfiltered acidified sample). If the same sample was pumped through a filtering device that used a peristaltic pump (which depressurizes the sample) the total dissolved mercury concentration was below 15 ng/L. Samples collected from 2 or 3 depths in hot springs were found to have higher concentrations at depth. The highest mercury concentrations occur in the most acid waters consistent with the greater stability of divalent mercury at low pH. The total dissolved mercury was always less than the total mercury (unfiltered acidified sample) by 6-98%. Particulate or colloidal mercury is common in thermal waters. Methymercury also occurs in several Yellowstone hot springs up to 9 ng/L. The methylation of divalent mercury by microorganisms involves the recently identified genes hqcA and hqcB while divalent mercury microbial reduction involves the merA gene. The merA gene occurs in diverse taxa of microorganisms inhabiting thermal features that span a wide range of pH (<3 to >8) and temperatures that range from slightly above ambient to boiling. The merA gene has been detected in some thermal waters without any reported aqueous mercury determinations, suggesting the presence of aqueous mercury and the occurrence of microbial mercury transformations.