ANALYSIS OF BIOGEOCHEMICAL CONDITIONS RESPONSIBLE FOR MERCURY METHYLATION IN THE SNAKE RIVER

The biogeochemical processes responsible for methylmercury production are not well characterized. Biogeochemical conditions of the environmental system that affect the availability of terminal electron acceptors and electron donors to heterotrophic methylating microorganisms change from system to system. However, it is generally accepted that mercury methylation occurs mainly under anaerobic conditions, with the highest methylation rates occurring in anoxic sediments and waters. Here, geochemical data collected from the Snake River, bordering Idaho and Oregon, was analyzed to determine the thermodynamic availability of electron acceptors to be used by methylating microorganisms. Microorganisms tend to favor redox processes that generate the maximum amount of available energy, although redox processes that are not dominant can still be thermodynamically favorable. Using the Nernst Equation, the electron potential of relevant redox processes was calculated for water samples collected from 2015 to 2018. Thermodynamic analysis found that sulfate reduction is not a dominant redox process, despite that the most studied mercury methylators in environmental systems are sulfate reducing bacteria. Instead, this research found the dominant anaerobic process to be manganese reduction, which suggests a role of metal reducers in linking mercury methylation to manganese cycling in the environment.