REDUCTION OF MERCURY(II) TO MERCURY(0) BY NITRATE ENRICHMENT CULTURES ESTABLISHED BY THE SUBSURFACE SEDIMENTS FROM OAK RIDGE, TN

While microbial mercury (Hg) reduction has been extensively studied in aerobic resistant prokaryotes carrying mer operons that are capable of reducing Hg(II) to Hg(0), little is known about mer activities among anaerobes and in anoxic zones. It has been suggested that Hg-contaminated groundwater, which may be super-saturated with Hg(0), may result from stimulation of microbial activities in subsurface environments. Considering the fact that in situ bio-remedial action which conventionally relies on strategies that enhance indigenous microbial degradation and transformation in contaminated subsurface sediments might inadvertently mobilize Hg into groundwater, a better understanding of the processes mediated by anaerobic microbial consortia that control the mobility of Hg in aquifers is crucial for future environmental management and remediation efforts.

In this project, we conducted laboratory studies to investigate the mechanisms of Hg(II) reduction by denitrifying enrichments, derived from subsurface sediments taken from Oak Ridge, TN, with an artificial groundwater medium. This oligotrophic medium was constructed intentionally with principle chemical components revealed by on-site monitoring wells to simulate in situ groundwater chemistry. Data from Hg toxicity experiments showed that Hg inhibited denitrifying activities at concentrations higher than 10 μM. Under this threshold, the enrichments detoxified Hg by reducing Hg(II) to Hg(0), as indicated by the formation of Hg(0) which was trapped in oxidizing trap solution. In addition, two strains isolated from the enrichments with 5 μM Hg(II)-spike and identified as Ralstonia sp. and Bradyrhizobium sp. (99% identity, based on 16S rRNA gene sequencing) were able to reduce Hg(II). Using degenerate primers, merA, the gene encoding for mercuric reductase, was detected in both cultures and was found to be most similar to that of γ-proteobacteria (98% identity) and γ-, β- proteobacteria (83% identity) in Ralstonia sp. and Bradyrhizobium sp., respectively. Taken together, these results suggested that Hg detoxifying processes, likely resulting from the activity of Hg-resistant denitrifiers, could affect Hg speciation and hence mobilize Hg in anoxic environments.