The microbial degradation of MeHg may proceed by two known pathways, namely, methyl-cleavage (MC) (producing CH4) or oxidative demethylation (OD) (producing CH4 and CO2). By measuring the carbon based end-products (i.e. CH4 and CO2), the fraction of MeHg degradation attributable to each of these pathways may be assessed at a particular location and/or depth. Bacteria that degrade MeHg via MC may also posses the capacity to further reduce Hg(II) to volatile Hg(0). However, it is unknown if this reduction is associated with conditions favoring OD. A clear understanding of the processes controlling both MC and OD is needed since the formation of volatile Hg(0) potentially represents a permanent loss of Hg from the system
The Everglades Water Conservation Areas (WCA's) are large sections of wetlands that act as nutrient removal zones for water draining the Everglades Agricultural Area. To the extent that nutrients affect the complex microbial dynamics in water and sediments, their direct and indirect impact on bacteria involved in the Hg cycle is of primary interest.
METHODS: Three years of field and laboratory studies research has been completed. The specific areas of investigation are outlined below.
1) Field Measurements: Sediment cores were sectioned into three to five discrete 2 to 4 cm horizons, within hours of sample collection. Homogenized sub-samples from each horizon were transferred into crimp sealed serum vials, purged with N2 gas, and injected with radiolabeled 14CH3Hg+. After incubating anaerobically for hours to days, incubations were arrested with either acid or base, depending on the method of 14C endproduct detection. End-products (14CH4 and 14CO2 ) were measured via gas proportional counting (prior to December 1996) or a CH4 combustion / CO2 trapping method (after December 1996). Degradation rate constants are calculated from the fraction of MeHg degraded per incubation time. The relative amounts of CH4 and/or CO2 produced provides an indication as to the relative importance of MC and/or OD. Degradation rates were assessed with respect to both sediment depth and site location.
2) Nutrients and Microbial Inhibitors: The affect of nutrients (NO3-, PO4-3, NH4+) and SO4-2 on the degradation of 14CH3Hg+ was assessed by amending parallel sets of incubation samples with these substrates and processing as described above. Likewise, specific microbial inhibitors of both SRB and MPB were used to determine the relative contribution of these microbial groups to MeHg degradation.
3) The Fate of Hg: Preliminary experiments were conducted exploring the fate of Hg resulting from MeHg degradation. Vapor phase Hg(0) was collected on gold traps by flushing the head-space of samples amended and incubated with MeHg. The concentration of Hg(0) was assayed by cold vapor atomic fluorescence spectroscopy. Our initial (unpublished) results suggest that very little (>> 0.01% ) of liberated Hg+2 produced as a result of MeHg degradation is reduced to volatile Hg(0).
4) Kinetic Studies: The dependence of MeHg degradation rates on MeHg amendment concentration was explored over a wide range (1-2000 ng MeH/g dry sed). The low end of this range approached natural in-situ concentrations (0.1-10 ng MeHg/g dry sed). These low levels were achieved by increasing our standard sample size from 3 to 80 cc of sediment, using a custom synthesized high-specific activity 14C-MeHg radiotracer, and employing the sensitive CH4 combustion / CO2 trapping method for 14C end-product quantification.