Effects of acidification on mercury methylation, demethylation, and volatilization in sediments from an acid-susceptible lake

The effect of experimental acidification on mercury methylation, demethylation, and volatilization was examined in surficial sediment samples from a weakly buffered northern Wisconsin lake. All mercury transformations were measured with radioisotopic tracers. Acidification of sediment pH with H2SO4, HCl, or HNO3 significantly decreased 203Hg(II) methylation. Acidification of pH 6.1 (ambient) sediments to pH 4.5 with either H2SO4 or HCl inhibited methylation by over 65%. The decreased methylation was due to the increased hydrogen ion concentration because methylation was not affected by concentrations of Na2SO4 or NaCl equimolar to the amount of acid added. Inhibition of methylation was observed even after prolonged acidification of sediments to pH 5.0 for up to 74 days. Acidification of sediments to pH 5.5, 4.5, and 3.5 with HNO3 resulted in a near complete inhibition of methylation at each pH. Similarly, the addition of equimolar amounts of NaNO3 resulted in a near complete inhibition of methylation, indicating that the inhibition was due to the nitrate ion rather than to the acidity. Demethylation of methyl mercury was not affected by pHs between 8.0 and 4.4, but sharply decreased below pH 4.4. Volatilization of 203Hg(II) from surface sediments was less than 2% of methylation activity and was not significantly different from that in killed sediments. This study indicated that acidification of sediments inhibits mercury methylation and that the observed increase in the mercury burdens in fish from low pH lakes is not due to increased production of methylmercury in sediments.     

Methylation and demethylation of mercury under controlled redox, pH and salinity conditions.

In estuarine sediments, the microbially mediated processes of methylation, demethylation, and volatilization determine the state and overall toxicity of mercury pollutants. The effects of redox potential (Eh) and salinity on the above microbial processes were investigated in reactors constructed to allow for continuous monitoring and adjustment of the pH (6.8) and Eh of freshly collected estuarine sediments. For measurements of methylation and demethylation activity, sediment slurries adjusted to appropriate salinity were spiked with HgCl2 or CH3HgCl, respectively, and were incubated in the reactors. Methylmercury was measured by gas chromatography. Volatilized elemental mercury (Hg0) was trapped and determined by cold vapor atomic absorption spectrometry. Volatilization of Hg0 and CH3HgCH3 were found to be minimal. Methylation of Hg2+ was favored at Eh-220 mV as compared to +110 mV. At -220 mV, high salinity (2.5%) inhibited methylation, and low salinity (0.4%) favored it. At +110 mV, the salinity effect was less pronounced. Demethylation of CH3HgCl was favored at +110 mV regardless of the salinity level. Low redox potential under low salinity conditions inhibited demethylation, but high salinity reversed this inhibition. These findings are helpful for interpreting and predicting the behavior of mercury pollutants in estuarine sediments.     

HCl causes less intracellular acidification in Necturus gastric mucosa surface epithelial cells than other acids

Luminal acid causes intracellular acidification in the gastric epithelium, but the mechanism by which H(+) enters surface cells remains obscure. This study addressed the problem by assessing how different acids affect intracellular pH in gastric surface cells. Isolated Necturus maculosus antral mucosa was exposed to HCl, HNO(3), H(2)SO(4), and H(3)PO(4) at pH 2.30. Intracellular pH was measured with microelectrodes. The physicochemical interaction of a synthetic model of gastric phospholipids with the different acids was studied using Langmuir film balance. Exposure to luminal HNO(3), H(2)SO(4), or H(3)PO(4) caused significantly larger intracellular acidification than exposure to HCl. The degree of acidification was not dependent on the valence or nature of the anionic counterion of the acid but significantly correlated with the amount of molecular acid. By Langmuir film balance, subphases acidified with HNO(3), H(2)SO(4), or H(3)PO(4) caused more close packing of phospholipid molecules than those acidified with HCl, possibly allowing hydrogen bonding between head groups to facilitate H(+) movement across the phospholipid membrane. HCl causes significantly less intracellular acidification in gastric epithelium than HNO(3), H(2)SO(4), or H(3)PO(4). This may be caused by the lower amount of molecular HCl in solution and possible hydrogen bonding between the head groups of phospholipid molecules and the other acids.     

Geochemistry of mercury in an intertidal flat of the Scheldt estuary

Sediments were sampled on the ‘Groot Buitenschoor’, an intertidal flat located at about 60 km from the Scheldt's river mouth. Hg concentrations ranged from 30 to 1756 ng g⁻¹. The concentrations were strongly correlated with fine grain fraction, organic matter content and sulphide concentrations. Incubation experiments were performed in order to determine the potential methylation rate of Hg as well as biotic and abiotic factors influencing this transformation. About 1 to 2% of the added inorganic Hg is converted into methylmercury. This conversion rate points to the same equilibrium ratio as was observed in natural sediments, indicating an equilibrium between methylation and demethylation reactions in the sediments. Incubation of a sterilised sediment sample significantly decreased the methylation rate, but the methylmercury concentrations observed are still ten times higher than the natural (unspiked) sediment. This result could be due to a chemical (non-enzymatic) methylation of mercury. Sulphate reducing bacteria are the main species responsible for the methylation of Hg at this site. Addition of Na₂MoO₄, a specific inhibitor of sulphate reducing bacteria, decreased the methylation rate to the abiotic level (sterilised sediment). High sulphate reduction rates, however, lead to lower methylation rates. Increased formation of sulphides due to microbial sulphate reduction leads to enhanced HgS formation and this reaction competes with the methylation process. HgS is in fact the major product formed by the reaction of sulphate reducing bacteria with Hg species. About 50% of the Hg spiked to the sediments is transformed into HgS during the incubation experiments, and that compound is practically unavailable for methylation in contrast to other bound forms of Hg.     

Laboratory Study of Chemical Speciation of Mercury in Lake Sediment and Water under Aerobic and Anaerobic Conditions

Chemical speciation and partitioning of radiolabeled HgCl₂ were studied in model aquatic systems consisting of undisturbed eutrophic lake sediment and water in plastic cylinders. The cylinders were either gradually made anaerobic by a gentle flow of N₂-CO₂ or kept aerobic by air flow. The proportion of methylated ²⁰³Hg was significantly higher, in both water and sediment, in the anaerobic systems than in the aerobic systems. The composition and total concentration of fatty acids originating from bacterial phospholipids, as well as the concentration of vitamin B₁₂, including related cobalamins, were similar in sediments from the anaerobic and aerobic systems. Bacterial cell numbers were, on average, 3.6 times higher in the anaerobic water columns than in the aerobic ones. Volatilization of ²⁰³Hg occurred in all systems except in an autoclaved control and was of similar magnitudes in the anaerobic and aerobic systems. Incorporation of ²⁰³Hg into the sediment was significantly faster in the aerobic systems than in the anaerobic systems. These results suggest that episodes of anoxia in bottom waters and sediment cause an increase in net mercury methylation and, hence, an increase in bioavailable mercury.     

Effect of Salinity on Mercury-Methylating Activity of Sulfate-Reducing Bacteria in Estuarine Sediments

The biomethylation of mercury was measured in anoxic estuarine sediments that ranged in salinity from 0.03 to 2.4% with or without added molybdate, an inhibitor of sulfate reducers. Mercury methylation was inhibited by molybdate by more than 95%, regardless of sediment salinity. In the absence of inhibitor, high-salinity sediments methylated mercury at only 40% of the level observed in low-salinity sediments. In response to molybdate inhibition of sulfate reducers, methanogenesis increased up to 258% in high-salinity sediments but only up to 25% in low-salinity sediments. In contrast to an earlier low-salinity isolate, a Desulfovibrio desulfuricans strain from high-salinity sediment required 0.5 M sodium for optimal growth and mercury methylation activity. The formation of negatively charged mercuric chloride complexes at high salinity did not noticeably interfere with the methylation process. Results of these studies demonstrate that sulfate reducers are responsible for mercury methylation in anoxic estuarine sediments, regardless of the prevailing salinity.     

Mercury methylation by fish intestinal contents.

A new radiochemical method has been applied to the examination of mercury methylation in fish intestinal contents. Intestinal contents of six freshwater fish species were found capable of converting 203Hg2+ to CH3203Hg+. This activity was observed in fish from five of six lakes tested whether or not there was mercury pollution. Bacterial activity in the intestinal contents is most likely responsible for this methylation. Methylating activity of piscivors increased with decreasing quantity of intestinal contents. Generally, pike and walleye intestinal contents methylated a larger fraction of 203Hg2+ than those of whitefish and suckers. These data contradict the previous general conclusion that there is no mercury methylation in fish.     

Mercury Methylation and Demethylation in Anoxic Lake Sediments and by Strictly Anaerobic Bacteria

After spiking anoxic sediment slurries of three acidic oligotrophic lakes with either HgCl₂ at 1.0 μg/ml or CH₃HgI at 0.1 μg/ml, both mercury methylation and demethylation rates were measured. High mercury methylation potentials were accompanied by high demethylation potentials in the same sediment. These high potentials correlated positively with the concentrations of organic matter and dissolved sulfate in the sediment and with mercury levels in fish. Adjustment of the acidic sediment pH to neutrality failed to influence either the methylation or the demethylation rate of mercury. The opposing methylation and demethylation processes converged to establish similar Hg²⁺-CH₃Hg⁺ equilibria in all three sediments. Because of their metabolic dominance in anoxic sediments, mercury methylation and demethylation in pure cultures of sulfidogenic, methanogenic, and acetogenic bacteria were also measured. Sulfidogens both methylated and demethylated mercury, but the methanogen tested only catalyzed demethylation and the acetogen neither methylated nor demethylated mercury.     

Sediment microbial community structure and mercury methylation in mercury-polluted Clear Lake, California

Spatial and temporal variations in sediment microbial community structure in a eutrophic lake polluted with inorganic mercury were identified using polar lipid fatty acid (PLFA) analysis. Microbial community structure was strongly related to mercury methylation potential, sediment organic carbon content, and lake location. Pore water sulfate, total mercury concentrations, and organic matter C/N ratios showed no relationships with microbial community structure. Seasonal changes and changes potentially attributable to temperature regulation of bacterial membranes were detectable but were less important influences on sediment PLFA composition than were differences due to lake sampling location. Analysis of biomarker PLFAs characteristic of Desulfobacter and Desulfovibrio groups of sulfate-reducing bacteria suggests that Desulfobacter-like organisms are important mercury methylators in the sediments, especially in the Lower Arm of Clear Lake.     

Sulfate-reducing bacteria methylate mercury at variable rates in pure culture and in marine sediments

Differences in methylmercury (CH(3)Hg) production normalized to the sulfate reduction rate (SRR) in various species of sulfate-reducing bacteria (SRB) were quantified in pure cultures and in marine sediment slurries in order to determine if SRB strains which differ phylogenetically methylate mercury (Hg) at similar rates. Cultures representing five genera of the SRB (Desulfovibrio desulfuricans, Desulfobulbus propionicus, Desulfococcus multivorans, Desulfobacter sp. strain BG-8, and Desulfobacterium sp. strain BG-33) were grown in a strictly anoxic, minimal medium that received a dose of inorganic Hg 120 h after inoculation. The mercury methylation rates (MMR) normalized per cell were up to 3 orders of magnitude higher in pure cultures of members of SRB groups capable of acetate utilization (e.g., the family Desulfobacteriaceae) than in pure cultures of members of groups that are not able to use acetate (e.g., the family Desulfovibrionaceae). Little or no Hg methylation was observed in cultures of Desulfobacterium or Desulfovibrio strains in the absence of sulfate, indicating that Hg methylation was coupled to respiration in these strains. Mercury methylation, sulfate reduction, and the identities of sulfate-reducing bacteria in marine sediment slurries were also studied. Sulfate-reducing consortia were identified by using group-specific oligonucleotide probes that targeted the 16S rRNA molecule. Acetate-amended slurries, which were dominated by members of the Desulfobacterium and Desulfobacter groups, exhibited a pronounced ability to methylate Hg when the MMR were normalized to the SRR, while lactate-amended and control slurries had normalized MMR that were not statistically different. Collectively, the results of pure-culture and amended-sediment experiments suggest that members of the family Desulfobacteriaceae have a greater potential to methylate Hg than members of the family Desulfovibrionaceae have when the MMR are normalized to the SRR. Hg methylation potential may be related to genetic composition and/or carbon metabolism in the SRB. Furthermore, we found that in marine sediments that are rich in organic matter and dissolved sulfide rapid CH(3)Hg accumulation is coupled to rapid sulfate reduction. The observations described above have broad implications for understanding the control of CH(3)Hg formation and for developing remediation strategies for Hg-contaminated sediments.     

Sediment Microbial Community Structure and Mercury Methylation in Mercury-Polluted Clear Lake, California

Spatial and temporal variations in sediment microbial community structure in a eutrophic lake polluted with inorganic mercury were identified using polar lipid fatty acid (PLFA) analysis. Microbial community structure was strongly related to mercury methylation potential, sediment organic carbon content, and lake location. Pore water sulfate, total mercury concentrations, and organic matter C/N ratios showed no relationships with microbial community structure. Seasonal changes and changes potentially attributable to temperature regulation of bacterial membranes were detectable but were less important influences on sediment PLFA composition than were differences due to lake sampling location. Analysis of biomarker PLFAs characteristic of Desulfobacter and Desulfovibrio groups of sulfate-reducing bacteria suggests that Desulfobacter-like organisms are important mercury methylators in the sediments, especially in the Lower Arm of Clear Lake.     

Seasonal and Spatial Variations in Mercury Methylation and Demethylation in an Oligotrophic Lake

Microbial mercury methylation and methylmercury decomposition were examined in Lake Clara, an oligotrophic northern Wisconsin seepage lake, using radioisotopic tracers. Methylation activity was near background in the water column, was greatest in the profundal surficial sediments, and decreased with depth in sediment cores. Active demethylation occurred in the water column but was variable. Demethylation was greatest in the surficial sediments and decreased slightly with sediment depth. The methylation/demethylation ratio (M/D) was >1 in the water column, exhibited a sharp peak in surface sediments, and decreased in deeper sediments. Methylation and demethylation activity varied in surface sediments collected along a lake transect. The M/D ratio in surface sediments ranged from 1.4 to 5.8. Methylation in attached microbial communities was near background, while demethylation was high. The M/D ratios in the attached communities were all <0.20. Methylation activity in surface sediments incubated at in situ temperature increased from spring to late summer and decreased in the fall. Demethylation increased from early to midsummer and then declined. The M/D ratio in surface sediments increased from mid- to late summer, and decreased in the fall. These results indicate that the greatest potential for methylation in Lake Clara occurs in the surficial sediments and that methylation in surficial sediments is greatest from mid-July through September. In addition, the net rate of methylmercury production may be significantly affected by demethylation.     

Mercury Contamination and Dynamics in the Sediment of the Second Songhua River,China

The Second Songhua River (SSR) was subjected to a large amount of mercury discharge from petrochemical industries in Jilin City from the 1960s to 1980s. The objectives of this study were to investigate the spatial and temporal change of mercury concentration in the sediments of the river and to assess Hg pollution in sediment employing enrichment ratio. Bottom sediments sampled in 2005 were digested with various acids followed by analysis by atomic fluorescence spectrometry for Hg, ICP-MS for Cd, Pb and Sc, and ICP-OES for Co, Cr, Cu, Zn, Mn, Ni, P, Pb, Sb, Ti, V, Al, Fe, Mg, Ca, Na, and K, in order to measure the total concentrations of these elements in the sediments. Results indicated that mercury concentrations in the sediments were strongly related with distance from the historic industrial point source, decreasing at an exponential rate from 1.27 mg kg^?1 at Jilin City to 0.01 mg kg^?1 at downstream Haerbin City. In addition, mercury concentration decreased from 16.8 mg kg^{? 1} y^?1 in 1974 to 0.09 mg kg^?1 y^?1 in 2005 in the sediments at effluent discharge site, and from 0.006 mg kg^?1 y^?1in 1974 to 0.004 mg kg^?1 y^?1 in 2005 at Songyuan City 257 km downstream. In the sedimentary sections of the river, deeper sediments contained higher concentrations of mercury as compared to the surface sediments, suggesting discharges of higher levels of mercury in the past and its subsequent burial over the years by less polluted sediments. Background concentrations of mercury in the surface sediments, reconstructed by tracer Sc, were 0.011 to 0.018 mg kg^?1. Enrichment ratios of Hg in the sediments of SSR was 5 to 75, indicating moderate to extreme pollution, while the sediment of Songhua River is less contaminated, with enrichment ratios of 0.9 to 1.5. At present, the previously accumulated and buried mercury in sediments may not significantly affect water quality of the SSR, but might pose a potential ecological risk to aquatic and amphibian animals. Natural attenuation seems to be an economic remedial choice for these sediments.     

Mercury Uptake by Modified Mackinawite

This study investigated the mercury uptake capacity of synthetic mackinawite regarding its surface modification with L-cysteine. Mackinawite (FeS) is an excellent material for mercury uptake from anoxic-contaminated sediments. However, one limitation to its use is the low oxidation stability; it is easily transformed when applied to natural sediments. The modification of mackinawite with L-cysteine improves its oxidation stability, making it a promising material to be used in sediment remediation by in-situ capping. The results showed that L-cysteine does not affect the mercury-uptake capacity of mackinawite (around 490 mg/g for modified and unmodified mackinawite). The Hg (II) uptake decreased as the solution pH changed from acid to alkaline range, especially in solutions with low Hg (II) concentrations. Sorption curves for Hg (II) on modified mackinawite, as a function of initial concentration of Hg (II) and mackinawite, showed the same pattern as that on unmodified mackinawite, indicating no significant difference in the mercury-sorption mechanism. The solids before and after Hg (II) uptake were analyzed by X-ray powder diffraction (XRPD) by noting the composition of both modified and unmodified mackinawite, and the results were in agreement with the sorption experiments.     

The accumulation of inorganic mercury from sea water by the plaice, Pleuronectes platessa L

The accumulation of inorganic mercury from sea water by plaice eggs, larvae, and adult fish has been studied using ²⁰³HgCl₂ as a tracer. The isotope was rapidly accumulated and the levels of accumulation have been related to stable element concentrations. High concentration factors were attained by many organs, but the distribution of the ²⁰³Hg was markedly different from that of the stable element. Whereas the largest fraction of the body burden of mercury is contained by muscle, only a slow rate of accumulation into this tissue was observed. In addition, the mercury in fish muscle is of the methyl form: no evidence for the methylation of the tracer was obtained.     

The influence of lime and biological activity on sediment pH,redox and phosphorous dynamics

The addition of powdered limestone to intact sediment cores from oligotrophic, acid Lake Hovvatn caused pH to increase, redox potential (E₇) to drop, and permitted net precipitation of phosphorous (P) from the water column. Significant pH increase was found to a sediment depth of 6 cm and a maximum increase in pH from 4.9 to 6.5 was found at a depth of 0.5 cm when dosed with 36 g m^–2 of lime. Such pH increase creates important changes in sediment equilibrium chemistry and enhances habitat suitability. In the case of Hovvatn, however, sediments would consume only 5 kg of the 91 tons of applied limestone. Superficial sediments remained oxidized, but below 0.5 cm, E₇ in limed sediment declined significantly more than in unlimed sediments, with a maximum difference of 102 mV versus –66 mV at a depth of 6 cm in unlimed and limed cores, respectively. Abiotic reactions account for 82 ± 54% of this reduction and the remainder is due to the oxidation of organic matter by bacteria. Precipitation of CaSO₄, reduction of the sediments by organic compounds at elevated pH and inhibition of the downward diffusion of O₂ by the limestone powder are potential abiotic mechanisms which could drive E₇ down. Enhanced P release was not found at lowered E₇, and supernatent TP concentrations dropped from 11.7 to 4.4 µg P l^–1. More P was swept from solution in cores which recieved larger lime doses. The presence of chironomids caused sediment pH to increase by as much as 1.2 pH units, presumably due to NH₄ release, reduced sediment E₇ by as much as 171 mV and facilitated TP release during the first 17 d of core incubation. Field measurements of vertical distributions of sediment pH and E₇ before and after the liming of Hovvtn corroborated laboratory findings.     

模拟酸雨对农作物种子萌发和幼苗生长的影响

研究模拟酸雨对3种农作物种子萌发年和幼苗生长的影响。结果表明：不同pH值(2．5,4．5,5．6)的模拟酸雨对水稻和小麦的种子萌发没有影响,但明显抑制了玉米种子萌发。模拟酸雨条件下,3种农作物幼苗的生长受到抑制,生物量减少,叶绿素和类胡萝卜素含量下降,而叶绿素a/b的变化却不明显。pH4．5和5．6的模拟酸雨对玉米Fv／Fm、光化学猝灭(qP)的影响较小,非光化学猝灭(NPQ)却明显下降,表明酸雨伤害了植物PSⅡ天线对激发能的非辐射耗散能力。     

Impacts of Mercury on Freshwater Fish-Eating Wildlife and Humans

This paper reviews the current state of knowledge of the toxic effects of mercury on fish-eating birds, mammals, and humans associated with freshwater ecosystems, including new information on the relative risk of elevated methyl Hg exposure for fish-eating birds inhabiting aquatic ecosystems impacted by mining/smelting activities and areas characterized by high geological sources of Hg. The influence of various environmental conditions such as lake pH, DOC, and chemical speciation of Hg, on fish-Hg concentrations and Hg exposure in fish-eating wildlife, are discussed. Although a continuing global effort to decrease the release of this nonessential metal into the environment is warranted, Hg methylation and biomagnification may be limited in some environments due to chemical speciation of mercury in soils and sediments (e.g., HgS) and water quality conditions (e.g., high alkalinity and pH) that do not facilitate high methylation rates. We have shown such limitations for a lake where historic Hg mining greatly increased sediment-Hg loadings, yet Hg increases in small fish of various species are currently lower than expected, and top predators (bald eagles), despite having elevated concentrations of Hg in their blood compared with individuals from nearby lakes, exhibit no Hg-related reproductive impairment or other signs of MeHg intoxication. Recent epidemiological studies have shown that fish-eating human populations may be exposed to Hg sufficient to cause significant developmental effects. However, for humans, we conclude that the current USEPA reference dose for MeHg may be too restrictive, particularly for the less sensitive adult. The health status of indigenous peoples relying on the subsistence harvest of wild foods may be negatively affected by such restrictions.     

MMHg production and export from intertidal sediments to the water column of a tidal lagoon (Arcachon Bay,France)

Hg cycling in biologically productive coastal areas is of special importance given the potential for bioaccumulation of monomethylmercury (MMHg) into aquatic organisms. Field experiments were performed during three different seasons in Arcachon Bay, a mesotidal lagoon (SW France), to assess the variability of the water column concentrations, sediment–water exchanges and potential formation and degradation of MMHg. The objectives were to evaluate the contribution of intertidal mudflats to MMHg production and the various pathways of Hg species export. Dissolved and bulk concentrations of Hg species in the water column downstream of tidal flats were measured throughout several tidal cycles. The Hg benthic fluxes at the sediment–water interface were determined by means of benthic chambers for three different stations. Hg methylation and demethylation potentials were determined in surficial sediments and the water column using isotopic tracers. The tidal surveys demonstrated that benthic remobilization of Hg occurs primarily in association with sediment erosion and advection during ebb tide. However, elevated dissolved Hg concentrations observed at low tide were found to be caused by a combination of pore-waters seeping, benthic fluxes and methylation in the water column. Benthic fluxes were more intense during late winter conditions (median MMHg and inorganic Hg (IHg) fluxes: 64 and 179 pmol m^?2 h^?1, respectively) and subsequently decreased in spring (median 0.7 and ?5 pmol m^?2 h^?1, respectively) and fall (median ?0.4 and ?1.3 pmol m^?2 h^?1, respectively). The trends in methylation and demethylation potentials were at the opposite of the fluxes, two times lower during winter than for spring or fall conditions. In this tidal environment, MMHg production in surface sediments and its subsequent release is estimated to be the major source of MMHg to the water column during winter and spring time. However, during the more productive summer period, the Hg methylation extent in the water column may be very significant and equivalent to the sediment contribution.     

Sulfate-Reducing Bacteria Methylate Mercury at Variable Rates in Pure Culture and in Marine Sediments

Differences in methylmercury (CH₃Hg) production normalized to the sulfate reduction rate (SRR) in various species of sulfate-reducing bacteria (SRB) were quantified in pure cultures and in marine sediment slurries in order to determine if SRB strains which differ phylogenetically methylate mercury (Hg) at similar rates. Cultures representing five genera of the SRB (Desulfovibrio desulfuricans, Desulfobulbus propionicus, Desulfococcus multivorans, Desulfobacter sp. strain BG-8, and Desulfobacterium sp. strain BG-33) were grown in a strictly anoxic, minimal medium that received a dose of inorganic Hg 120 h after inoculation. The mercury methylation rates (MMR) normalized per cell were up to 3 orders of magnitude higher in pure cultures of members of SRB groups capable of acetate utilization (e.g., the family Desulfobacteriaceae) than in pure cultures of members of groups that are not able to use acetate (e.g., the family Desulfovibrionaceae). Little or no Hg methylation was observed in cultures of Desulfobacterium or Desulfovibrio strains in the absence of sulfate, indicating that Hg methylation was coupled to respiration in these strains. Mercury methylation, sulfate reduction, and the identities of sulfate-reducing bacteria in marine sediment slurries were also studied. Sulfate-reducing consortia were identified by using group-specific oligonucleotide probes that targeted the 16S rRNA molecule. Acetate-amended slurries, which were dominated by members of the Desulfobacterium and Desulfobacter groups, exhibited a pronounced ability to methylate Hg when the MMR were normalized to the SRR, while lactate-amended and control slurries had normalized MMR that were not statistically different. Collectively, the results of pure-culture and amended-sediment experiments suggest that members of the family Desulfobacteriaceae have a greater potential to methylate Hg than members of the family Desulfovibrionaceae have when the MMR are normalized to the SRR. Hg methylation potential may be related to genetic composition and/or carbon metabolism in the SRB. Furthermore, we found that in marine sediments that are rich in organic matter and dissolved sulfide rapid CH₃Hg accumulation is coupled to rapid sulfate reduction. The observations described above have broad implications for understanding the control of CH₃Hg formation and for developing remediation strategies for Hg-contaminated sediments.