Methyl-mercury degradation pathways: A comparison among three mercury-impacted ecosystems

We examined microbial methylmercury(MeHg) degradation in sediment of the Florida Everglades, Carson River (NV), and San Carlos Creek (CA), three freshwater environments that differ in the extent and type of mercury contamination and sediment biogeochemistry. Degradation rate constant (k(deg)) values increased with total mercury(Hg,) contamination both among and within ecosystems. The highest k(deg)'s (2.8-5.8 d(-1)) were observed in San Carlos Creek, at acid mine drainage impacted sites immediately downstream of the former New Idria mercury mine, where Hg-t ranged from 4.5 to 21.3 ppm (dry wt). A reductive degradation pathway (presumably mer-detoxification) dominated degradation at these sites, as indicated by the nearly exclusive production of (CH4)-C-14 from C-14-MeHg, under both aerobic and anaerobic conditions. At the upstream control site, and in the less contaminated ecosystems (e.g, the Everglades), k(deg)'s were low (less than or equal to0.2 d(-1))and oxidative demethylation(OD) dominated degradation, as evident from (CO2)-C-14 production. k(deg) increased with microbial CH4 production, organic content, and reduced sulfur in the Carson River system and increased with decreasing pH in San Caries Creek. OD associated CO2 production increased with pore-water SO42- in Everglades samples but was not attributable to anaerobic methane oxidation, as has been previously proposed. This ecosystem comparison indicates that severely contaminated sediments tend to have microbial populations that actively degrade MeHg via mer-detoxification, whereas OD occurs in heavily contaminated sediments as well but dominates in those less contaminated.