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

We examined microbial methylmercury(MeHg) degradation in sediment of the Fl orida Everglades, Carson River (NV), and San Carlos Creek (CA), three fresh water environments that differ in the extent and type of mercury contaminat ion and sediment biogeochemistry. Degradation rate constant (k(deg)) values increased with total mercury(Hg,) contamination both among and within ecos ystems. The highest k(deg)'s (2.8-5.8 d(-1)) were observed in San Carlos Cr eek, at acid mine drainage impacted sites immediately downstream of the for mer 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 productio n 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 o xidative demethylation(OD) dominated degradation, as evident from (CO2)-C-1 4 production. k(deg) increased with microbial CH4 production, organic conte nt, and reduced sulfur in the Carson River system and increased with decrea sing pH in San Caries Creek. OD associated CO2 production increased with po re-water SO42- in Everglades samples but was not attributable to anaerobic methane oxidation, as has been previously proposed. This ecosystem comparis on indicates that severely contaminated sediments tend to have microbial po pulations that actively degrade MeHg via mer-detoxification, whereas OD occ urs in heavily contaminated sediments as well but dominates in those less c ontaminated.