A Quantitative Relationship that Demonstrates Mercury Methylation Rates in Marine Sediments Are Based on the Community Composition and Activity of Sulfate-Reducing Bacteria

Abstract

A quantitative framework was developed which estimates mercury methylation rates (MMR) in sediment cores based on measured sulfate reduction rates (SRR) and the community composition sulfate-reducing bacterial consortia. MMR and SRR as well as group-specific 16S rRNA concentrations (as quantified by probe signal) associated with sulfate-reducing bacteria (SRB) were measured in triplicate cores of saltmarsh sediments. Utilizing previously documented conversion factors in conjunction with field observations of sulfate reduction, MMR were calculated, and the results were compared to experimentally derived measurements of MMR. Using our novel field data collected in saltmarsh sediment where sulfate reduction activity is high, calculated and independently measured MMR results were consistently within an order of magnitude and displayed similar trends with sediment depth. In an estuarine sediment where sulfate reduction activity was low, calculated and observed MMR diverged by greater than an order of magnitude, but again trends with depth were similar. We have expanded the small database generated to date on mercury methylation in sulfur-rich marine sediments. The quantitative frameworkwe have developed further elucidates the coupling of mercury methylation to sulfate reduction by basing calculated rates of mercury methylation on the activity and community composition of sulfate-reducing bacteria. The quantitative framework may also provide a promising alternative to the difficult and hazardous determination of MMR using radiolabeled mercury.

Seasonal composition and activity of sulfate-reducing prokaryotic communities in seagrass bed sediments

“…Past studies of 16S rRNA gene sequences conducted in seagrass bed sediments (Cifuentes et al 2000) and the saltmarsh (Rooney-Varga et al 1997, Hines et al 1999, King et al 2001 showed that SRP communities in vegetated, marine sediments are predominated by members of the Desulfobacteriaceae. This observation likely reflects similarity in the microbial communities catalyzing the terminal decomposition of organic matter in saltmarsh and seagrass bed sediments.…”

“…SRP community composition has been extensively studied in marine environments, including vegetated saltmarsh sediments, by 16S rRNA gene sequence comparisons (Rooney-Varga et al 1998, Edgcomb et al 1999, Hines et al 1999, Knoblauch et al 1999, Li et al 1999, Frischer et al 2000, Ravenschlag et al 2000, Joulian et al 2001, King et al 2001, Orphan et al 2001, Pérez-Jiménez et al 2001, Thomsen et al 2001, LlobetBrossa et al 2002. Molecular investigations of seagrass bed microbial communities have focused on 16S rRNA gene sequences (Cifuentes et al 2000) and nif H sequences (Bagwell et al 2002).…”

Sulfate-Reducing Bacterium Desulfovibrio desulfuricans ND132 as a Model for Understanding Bacterial Mercury Methylation

“…They hypothesized that the relationship was due to saturation of methylating enzymes in SRB. However, this relationship does not necessarily mean that Hg methylation follows Michaelis-Menten kinetics or that Hg uptake becomes saturated at higher Hg concentrations (48). It is more plausible to attribute the relationship to the solubility and speciation of inorganic Hg in the culture medium (10).…”

Clapper rails as indicators of mercury and PCB bioavailability in a Georgia saltmarsh system

“…Further, MeHg was found to be negligible in LCP tidal creek (0.32-1.49%) and tidal pool (0.006-1.04%) sediment samples (NOAA 1998). Studies in estuaries from this region show that Hg methylation rates are correlated with sulfur reduction rates in the anoxic slurry, which as such is a better predictor of potential bioaccumulation into the food chain than quantifying sediment MeHg (King et al 1999(King et al , 2001). The (total) Hg concentrations in the sediments of the control sites used in this study were found to be within normal background levels of\1 lg g -1 (Eisler 2000a).…”

“…The fact that carbon content was not dramatically different between the two microhabitats was not too surprising since the entire marsh will flood and drain with each tide making the depositional hydraulics similar. The normalization was performed since studies have shown predictable associations of lipophilic contaminants as well as MeHg with detrital carbon (Castro and Vale 1995;King et al 2001;Nhan et al 2001). However, other studies from the southeast have shown the relationship between Hg and carbon content to be complex (Kannan et al 1998b), most likely because carbon is only one component that can influence methylation processes.…”