The neurotoxin methylmercury poses a serious risk to human health because it accumulates in the food chain. To produce the toxin, certain microbes add a few atoms (a methyl group) to a mercury atom, a process known as methylation. Identifying which microbes and how much toxin they produce has been challenging. At Oak Ridge National Laboratory, researchers developed designer probes that identify microbes that carry the genes for mercury methylation. These molecular probes also measure, or quantify, how much specific types of microbes contribute to the toxin-producing process.
The development of molecular probes is a substantial improvement over previous work. The new approach considers the different degrees of methylation potential for specific types of microbes. These findings will enable a more realistic view of possible methylmercury production in a specific setting. The resulting data enable more accurate risk management assessments.
Two genes, hgcA and hgcB, are essential for microbial mercury methylation. Detecting and estimating their abundance in microbes in conjunction with quantifying mercury species and other geochemical factors is critical in determining potential hotspots of methylmercury generation in at-risk environments. Scientists at Oak Ridge National Laboratory led a team that identified a broad range of degenerate polymerase chain reaction (PCR) primers spanning known hgcAB genes to determine the presence of both genes in diverse environments. They tested these broad-range primers against an extensive set of pure cultures with published genomes that are known to methylate mercury, including 13 Deltaproteobacteria, nine Firmicutes, and nine methanogenic Archaea. For all these types of microbes, the primers not only consistently identified the methylating genes, but they enabled the team to quantify the extent to which each type of microbe methylates mercury. The degree of methylation potential ranged from around 10 percent in the Archaea to approximately 90 percent in some Deltaproteobacterialspecies. The team used environmental samples to further validate the primers and determine corrective calculations for deoxyribonucleic acid (DNA) extraction and PCR amplification efficiencies. Taken together, these findings will enable a more realistic picture of possible methylmercury generation levels that may occur in a given environment.