There are two known pathways for microbial CH₃Hg⁺ demethylation, namely via organomercurial-lyase (OML) (Robinson and Tuovinen, 1984) and oxidative demethylation (OD) (Oremland and others, 1991). The former process represents a true detoxification response by bacteria, while the latter is thought to reflect the metabolism of a small organic molecule by heterotrophic (organic utilizing) bacteria. The mer-B operon encodes for the OML enzyme which forms CH₄ and Hg(II) from CH₃Hg⁺. This is typically followed by the conversion of Hg(II) to volatile gaseous Hg⁰, a reaction catalyzed by the mercuric-reductase enzyme encoded by the mer-A operon. Both methanogenic and sulfate-reducing bacteria have been shown to be involved in OD, the defining characteristic of which is the oxidation of the CH₃Hg⁺ methyl group to CO₂, either with or without concurrent CH₄ production (Oremland and others, 1991). The importance in distinguishing between these two modes of degradation lies in the potential fate of mercury in each case. The detoxification response of bacteria, in the case of OML and associated reductase, acts to remove mercury from the immediate environment by converting it to a final form (Hg⁰) having a greater potential for transfer back to the atmosphere. In contrast, our preliminary results suggest that no such reductase activity is associated with the OD pathway. Thus, in situations where OD dominates CH₃Hg⁺ breakdown, the final form of mercury is likely Hg(II) which is available for remethylation and arguably has a longer residence time in the aquatic ecosystem. By measuring the radiolabeled ¹⁴C gaseous endproducts of ¹⁴CH₃Hg⁺ degradation (that is,¹⁴CO₂ and ¹⁴CH₄) we are able to get a cursory measure of the relative importance of these two distinct pathways under various environmental conditions and among different spatial regions in the Everglades.

We have recently reported that CH₃Hg⁺ is degraded at in situ concentrations, at least in part, by the OD pathway in Everglades sediments (Marvin-DiPasquale and Oremland, 1998). This implies that much of the CH₃Hg⁺ formed in anoxic sediments, not transferred to the water column, may be actively retained in the sediments due to a dynamic cycle of demethylation-remethylation. We conclude that the OD pathway should be included in the mercury model being developed for the Everglades. Rate constants for CH₃Hg⁺ degradation ranged from 0.06-0.16 d^-1 in the floc layer surface sediment, decreasing with sediment depth and increasing from nutrient enriched to more pristine areas. These within-site and among-site spatial trends were similar to those observed for Hg-methylation rates and in situ sediment concentrations of both total mercury and CH₃Hg⁺ (Gilmour and others, 1998). We infer that there exists a tight coupling between processes of CH₃Hg⁺ production and consumption in this system. In comparing gross rates of these two processes it appears that CH₃Hg⁺ production exceeds degradation at all sites investigated to date. This is one factor which accounts for the accumulation of CH₃Hg⁺ in Everglades biota.

Nutrient enrichment experiments (NO^-3 and PO^3-₄) indicated that these compounds did not directly affect rates of CH₃Hg⁺ degradation, suggesting that changes in pools of porewater nutrients resulting from the ongoing construction of stormwater treatment areas will have little impact on rates of CH₃Hg⁺ degradation directly. However, it is yet unclear how secondary effects due to changes in organic deposition rates, macrophyte and periphyton community structure, resident microbial populations, and the biogeochemical cycling of organic and inorganic material, resulting from the stormwater treatment areas, will impact CH₃Hg⁺ degradation rates. Indeed, a general increase in degradation rate constants from 0.06 d^-1 to 0.14 d^-1 was observed in floc layer sediments during January 1998 along the north to south transect from Loxahatchee Wildlife Refuge to the southern reaches of Taylor Slough (unpublished data).

Since mercury input to the Everglades is primarily derived from atmospheric sources, periphyton at the water surface may intercept particle bound mercury before it can be advectively transported to the sediment. Collaborative ACME data indicates that some of these mats may be able to carry out Hgmethylation. Our unpublished results indicate that various forms of natural periphyton also have the capacity to degrade CH₃Hg⁺. The highest rates of degradation (> 0.2 d^-1) were observed with calcareous mats that are abundant in pristine areas of the Everglades. These rates were equal to or greater than those observed in surface floc sediment samples. It is assumed that this degradation is carried out in low oxygen microzones where anaerobic bacteria are able to exit. Light/dark incubation experiments suggest an interesting diel cycle, where CH₃Hg⁺ is abiotically photodegraded during the day when the photosynthetic production of oxygen inhibits anaerobic bacteria. In the evening, oxygen levels are rapidly depleted and CH₃Hg⁺ is microbially degraded by the active population of anaerobes. Although these results are preliminary, they suggest a novel dual route for CH₃Hg⁺ degradation associated with complex bacterial/algal mat matrices heretofore unexplored.

REFERENCES

Cleckner, L.B., Garrison, P.J., Hurley, J.P., Olson, M.L., Krabbenhoft, D.P., 1998, Trophic transfer of methyl mercury in the northern Florida Everglades: Biogeochemistry, v. 40, 347 p.

Compeau, G.C., Bartha, R., 1985, Sulfate-reducing bacteria--Principal methylators of mercury in anoxic estuarine sediment: Applied Environmental Microbiology, v. 50, 498 p.

Eisemann, J.D., Beyer, W.N., Bennetts, R.E., Morton, A., 1997, Mercury residues in south Florida apple snails (Pomacea paludosa): Bulletin of Environmental Contamination & Toxicology, v. 58, 739 p.

Roelke, M., Schultz, D., Facemire, C., Sundlof, S., and Royals, H., 1991, Mercury contamination in Florida panthers: Gainesville, Florida Game and Fresh Water Fish Commission.

Gilmour, C.C., Riedel, G.S., Ederington, M.C., Bell, J.T., Benoit, J.M., Gill, G.A., Stordal, M.C., 1998, Methymercury concentrations and production rates across a trophic gradient in the northern Everglades: Biogeochemistry, v. 40, 327 p.

Marvin-DiPasquale, M.C., Oremland, R.S., 1998, Bacterial methylmercury degradation in Florida Everglades peat sediment: Environmental Science & Technology, v. 32, 2556 p.

Oremland, R.S., Culbertson, C.W., Winfrey, M.R., 1991, Methylmercury decomposition in sediments and bacterial cultures--Involvement of methanogens and sulfate reducers in oxidative demethylation: Applied Environmental Microbiology, v. 57, 130 p.

Robinson, J.B., Tuovinen, O.H., 1984, Mechanisms of microbial resistance and detoxification of mercury and organomercury compounds--Physiological, biochemical, and genetic analyses: Microbiology, v. 48, 95 p.

Ware, F., Royals, H., Lange, T., 1990, Mercury contamination in Florida largemouth bass: Proceedings of the American Conference, Southeastern Association Fish and Wildlife Agency, 5 p.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)

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