Mercury Methylation Rates Research Articles

Mercury methylation rates in coastal sediments versus microbial diversity and specific activity

To investigate the role of microbial communities on the rate of mercury methylation, and in opposition, the impact of mercury on the diversity and activity of sedimentary bacteria, two different experiments were achieved. First, laboratory incubations were performed on estuarine sediments collected in the Adour estuary (Southwest France) under various conditions (oxic, anoxic, sterilized). Second, to confirm the batch experiments, in situ cores experiments were performed to investigate potential methylation of mercury in the Thau Lagoon (South France). The microbial diversity and mercury-linked activities were also analyzed using DNA fingerprinting technique (T-RLFP) and mer genes analysis. The two sets of results exhibit equivalent net methylation yields for surface sediments indicating similar methylation potentials for sediments from the Adour estuary (1% for 12 hours incubation) and the Thau Lagoon (0.9% for 1 day incubation). Methylation rates in sediment slurries decrease with the incubation time pointing out the needed of kinetic laboratory studies to complete in situ experiments. Furthermore, mercury methylation was strictly observed in anoxic slurries and correlates to anaerobic bacteria activity.

Mercury(II) sorption to two Florida Everglades peats: evidence for strong and weak binding and competition by dissolved organic matter released from the peat.

The binding of mercury(II) to two peats from Florida Everglades sites with different rates of mercury methylation was measured at pH 6.0 and 0.01 M ionic strength. The mercury(II) sorption isotherms, measured over a total mercury(II) range of 10(-7.4) to 10(-3.7) M, showed the competition for mercury(II) between the peat and dissolved organic matter released from the peat and the existence of strong and weak binding sites for mercury(II). Binding was portrayed by a model accounting for strong and weak sites on both the peat and the released DOM. The conditional binding constants (for which the ligand concentration was set as the concentration of reduced sulfur in the organic matter as measured by X-ray absorption near-edge structure spectroscopy) determined for the strong sites on the two peats were similar (Kpeat,s = 10(21.8 +/- 0.1) and 10(22.0 +/- 0.1) M-1), but less than those determined for the DOM strong sites (Kdom,s = 10(22.8 +/- 0.1) and 10(23.2 +/- 0.1) M-1), resulting in mercury(II) binding by the DOM at low mercury(II) concentrations. The magnitude of the strong site binding constant is indicative of mercury(II) interaction with organic thiol functional groups. The conditional binding constants determined for the weak peat sites (Kpeat,w = 10(11.5 +/- 0.1) and 10(11.8 +/- 0.1) M-1) and weak DOM sites (Kdom,w = 10(8.7 +/- 3.0) and 10(7.3 +/- 4.5) M-1) were indicative of mercury(II) interaction with carboxyl and phenol functional groups.

A quantitative relationship that demonstrates mercury methylation rates in marine sediments are based on the community composition and activity of sulfate-reducing bacteria.

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.

The effects of experimental reservoir creation on the bioaccumulation of methylmercury and reproductive success of tree swallows (Tachycineta bicolor).

Reservoir creation results in decomposition of flooded organic matter and increased rates of mercury methylation. Methylmercury (MeHg), the most toxic form of mercury, bioaccumulates through aquatic food webs. Our objective was to quantify the transfer of MeHg from aquatic food webs into terrestrial organisms. We examined rates of MeHg bioaccumulation in an insectivorous songbird, the tree swallow, breeding near an experimentally created reservoir. We also determined the impact of flooding and MeHg bioaccumulation on the reproductive success of these birds. Mean MeHg burdens in nestling swallows from near the experimental reservoir increased from 1,210 +/- 150 ng before flooding to 2,200 +/- 102 ng after flooding. Postflood MeHg concentrations in both the body and feathers of the birds were significantly greater than preflood MeHg concentrations. Although MeHg burdens in swallows were elevated in postflood years, we found no overt toxicological affects. An increase in dipteran productivity (the primary food source of tree swallows) after reservoir creation resulted in earlier nest initiation, larger eggs, and faster growth rates of wing and bill length in nestlings raised during postflood years.

Constants of mercury methylation and demethylation rates in sediments and comparison of tracer and ambient mercury availability

AbstractA method was developed to measure mercury methylation and demethylation rate constants simultaneously in aquatic samples. Solutions containing stable isotope tracers of 199Hg2+ and CH3202Hg+ were spiked into lake sediments at subambient concentrations. The formation of CH3199Hg+ and the decrease in CH3202Hg+ were measured simultaneously in time series experiments using gas chromatographic separation and isotope‐specific detection by inductively coupled plasma mass spectrometry. Specific rate constants for the two processes were calculated and compared to rate constants obtained by monitoring changes in concentration of the ambient methylmercury in the same sample. The inorganic mercury tracer 199Hg2+ was methylated at a faster rate compared with the ambient inorganic Hg2+, which indicates that the added tracer Hg2+ is more available for transformation reaction than the ambient Hg2+. The degradation of tracer and ambient methylmercury proceeded at a similar rate, showing no significant differences between added tracer and ambient methylmercury. The calculated half‐life for methylmercury in sediments was 1.7 d, suggesting a rapid turnover and low persistence of methylmercury in lake sediments. Different Hg species were investigated regarding their availability for methylation reactions. Compared to Hg(NO3)2, Hg‐fulvate showed reduced availability and freshly precipitated HgS was hardly available.

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.

CONSTANTS OF MERCURY METHYLATION AND DEMETHYLATION RATES IN SEDIMENTS AND COMPARISON OF TRACER AND AMBIENT MERCURY AVAILABILITY

A method was developed to measure mercury methylation and demethylation rate constants simultaneously in aquatic samples. Solutions containing stable isotope tracers of 199Hg2+ and CH3202Hg+ were spiked into lake sediments at subambient concentrations. The formation of CH3199Hg+ and the decrease in CH3202Hg+ were measured simultaneously in time series experiments using gas chromatographic separation and isotope-specific detection by inductively coupled plasma mass spectrometry. Specific rate constants for the two processes were calculated and compared to rate constants obtained by monitoring changes in concentration of the ambient methylmercury in the same sample. The inorganic mercury tracer 199Hg2+ was methylated at a faster rate compared with the ambient inorganic Hg2+, which indicates that the added tracer Hg2+ is more available for transformation reaction than the ambient Hg2+. The degradation of tracer and ambient methylmercury proceeded at a similar rate, showing no significant differences between added tracer and ambient methylmercury. The calculated half-life for methylmercury in sediments was 1.7 d, suggesting a rapid turnover and low persistence of methylmercury in lake sediments. Different Hg species were investigated regarding their availability for methylation reactions. Compared to Hg(NO3)2, Hg-fulvate showed reduced availability and freshly precipitated HgS was hardly available.

Coupling mercury methylation rates to sulfate reduction rates in marine sediments

AbstractAnoxic slurry incubations were performed to examine the relationship between mercury methylation rates (MMR) and sulfate reduction rates (SRR) in salt marsh sediments from the southeastern United States. The MMR and SRR were similarly affected by temperature, addition of low‐weight molecular organics, and microbial inhibitors. The MMR was at a maximum in the 12 h after inorganic mercury addition. Initial rates of Hg methylation are correlated with SRR. A significantly slower MMR observed after the initial 12 h following inorganic mercury addition suggested that sorption or precipitation reduced the availability of mercury for methylation. The MMR results for various concentrations of inorganic mercury indicate that saturation kinetics occur. Using this kinetic framework, a model for MMR based on SRR and inorganic Hg concentration was developed. This model was then used to predict the MMR reported in temperature and substrate/inhibitor experiments. The model provided a reasonable estimate of MMR observed in the initial 12 h of the slurry incubations. However, the sequestering of inorganic mercury into less reactive pools by sorption to surfaces or incorporation into other phases alters the amount of bioavailable Hg and hence MMR. Future models that assess the bioavailibility of Hg in the sediments must be developed to extend these observations to field sites.

Inhibition of mercury methylation in anoxic freshwater sediment by group VI anions

AbstractThe addition of group VI anions to sediment slurries resulted in the inhibition of the rate of mercury (Hg) methylation. The ranking of inhibition is as follows: tellurate (TeO) > selenate (TeO) > molybdate (MoO) > tungstate (WO). In sediment slurries treated with TeO and SeO, methylmercury (MeHg) formation was significantly inhibited (p < 0.05) at the concentrations >50 nM of TeO and >270 nM of SeO, while the significant inhibition (p < 0.05) of Hg‐methylation by MoO and WO was observed in slurries spiked at final concentrations ≥100 μM and ≥700 μM, respectively. Increasing the sulfate (SO) concentration while using fixed concentrations of inhibitors led to the partial reestablishment of some MeHg production in WO‐treated slurries, whereas, no such significant change was noticed in sediment slurries treated with MoO and TeO. These observations suggested that WO inhibits Hg methylation by a competitive mechanism, while MoO and TeO are noncompetitive inhibitors. Selenate and SO showed a qualitatively similar effect on Hg methylation at concentrations tested, in that each showed stimulation at low concentrations and inhibition at high concentrations. The depression of MeHg formation by group VI anions was not accompanied by an inhibition of general microbial activity, suggesting that only particular microorganisms, such as sulfate‐reducing bacteria, are responsible for Hg methylation. Finally, in the concentration ranges encountered in most natural aquatic environments, the inhibition of MeHg production by group VI anions is unlikely, except in systems where those elements are found in anomalously high concentrations.

INHIBITION OF MERCURY METHYLATION IN ANOXIC FRESHWATER SEDIMENT BY GROUP VI ANIONS

The addition of group VI anions to sediment slurries resulted in the inhibition of the rate of mercury (Hg) methylation. The ranking of inhibition is as follows: tellurate (TeO) > selenate (TeO) > molybdate (MoO) > tungstate (WO). In sediment slurries treated with TeO and SeO, methylmercury (MeHg) formation was significantly inhibited (p < 0.05) at the concentrations >50 nM of TeO and >270 nM of SeO, while the significant inhibition (p < 0.05) of Hg-methylation by MoO and WO was observed in slurries spiked at final concentrations ≥100 μM and ≥700 μM, respectively. Increasing the sulfate (SO) concentration while using fixed concentrations of inhibitors led to the partial reestablishment of some MeHg production in WO-treated slurries, whereas, no such significant change was noticed in sediment slurries treated with MoO and TeO. These observations suggested that WO inhibits Hg methylation by a competitive mechanism, while MoO and TeO are noncompetitive inhibitors. Selenate and SO showed a qualitatively similar effect on Hg methylation at concentrations tested, in that each showed stimulation at low concentrations and inhibition at high concentrations. The depression of MeHg formation by group VI anions was not accompanied by an inhibition of general microbial activity, suggesting that only particular microorganisms, such as sulfate-reducing bacteria, are responsible for Hg methylation. Finally, in the concentration ranges encountered in most natural aquatic environments, the inhibition of MeHg production by group VI anions is unlikely, except in systems where those elements are found in anomalously high concentrations.

A simplified radiochemical technique for measurements of net mercury methylation rates in aquatic systems near goldmining areas, Amazon, Brazil : J. R. D. Guimaraes, O. Malm & W. C. Pfeiffer, Science of the Total Environment, 175(2), 1995, pp 151–162

A simplified radiochemical technique for measurements of net mercury methylation rates in aquatic systems near goldmining areas, Amazon, Brazil : J. R. D. Guimaraes, O. Malm & W. C. Pfeiffer, Science of the Total Environment, 175(2), 1995, pp 151–162

A simplified radiochemical technique for measurements of net mercury methylation rates in aquatic systems near goldmining areas, Amazon, Brazil

Current Hg 0 emissions from goldmining activities in the Amazon basin reach 100 tons·year −1, but very little is known about Hg methylation rates in local aquatic systems. Simplified versions of the Furutani and Ruddi (1980) radiochemical method were tested and applied in the determination, by in-situ incubations, of specific net Hg methylation rates in water and surface sediments of the Madeira river goldmining area. Efficiency for Me 203 Hg extraction from water and sediment was in the 75.3–96.77% range, depending on sample type and extraction procedure. Methylation was not detectable in water but rates in surface sediments ranged from 10 −5 to 10 −1% · g −1. h −1, the higher rates being found in sites in a reservoir area. Methylation was absent or strongly depressed in sterilized sediments. The effect of delays between sampling, incubation and extraction on methylation rates and Me 203Hg recovery was documented. Despite their limited spatial and temporal range, these data are further evidence that concerns about the potential toxicological impacts of Hg emissions in the Amazon basin are justified.

Measurement of mercury methylation in sediments by using enriched stable mercury isotopes combined with methylmercury determination by gas chromatography–inductively coupled plasma mass spectrometry

A novel technique for the calculation of mercury methylation rates in sediments by using enriched stable mercury isotopes is described. The method takes advantages of the ability of an inductively coupled plasma mass spectrometry (ICP-MS) instrument to measure individual isotopes. An ICP-MS instrument was used as a detector for the determination of methylmercury compounds after separation by gas chromatography (GC). CH3Hg+ was isolated from sediments by distillation, converted to methylethylmercury by sodium tetraethylborate and analysed after purge-and-trap pre-collection on a Tenax adsorber and thermodesorption onto the GC column. Detection limits were found to be ≈ 1 pg (as Hg) absolute or 0.02 ng g–1 dry sediment. The precision was ≈ 4% relative standard deviation when 250 pg of methylmercury were processed. The accuracy of the GC–ICP-MS technique was demonstrated by analysis of an International Atomic Energy Agency certified reference material (IAEA CRM 356) Harbor Sediment, giving a concentration of 5.40 ± 0.40 ng g–1, compared with the certified value of 5.46 ± 0.38 ng g–1. Mercury methylation was investigated by spiking sediments with stable enriched mercury isotopes at in situ mercury concentrations not perturbing the system. More than 3% of the mercury added to a lake sediment was methylated during a 21 d incubation period. Isotope ratios of total mercury differed significantly from isotope ratios of methylmercury at the end of the experiment, suggesting that the system was still not in equilibrium after 21 d.

Biotic and abiotic mercury methylation and demethylation in sediments

Inorganic mercury (Hg(II)) methylation and methylmercury (MeHg) demethylation may occur in the water column, sediment-water interface and subsurficial sediment of aquatic ecosystems. These transformations involve mainly microbial mechanisms, although abiotic methylation may play a more important role in the water compartment. The relative importance of biotic versus abiotic mechanisms of methylation has not been determined however, and abiotic demethylation remains unknown. Little quantitative information is available on the role of bacterial activity in mercury transformations. It has been reported that at least 16 genera of aerobic and anaerobic microorganisms are able to methylate HG(II), and that a greater number are able to demethylate MeHg. Nevertheless, not all populations of these species are capable of methyl- and demethyl-transformations. The actual concentration of MeHg in the aquatic environment is regulated by the relative production and decomposition rates. This, in turn, depends on the availability of Hg(II), MeHg, and bacteria as well as on the physico-chemical properties of the sample. The objective of this study was to compare mercury methylation and demethylation rates in sediment samples with and without active bacterial populations. We therefore performed experiments to follow bacterial evolution during the course of Hg(II) methylation and MeHg demethylation in sediment slurries containingmore » both sterile and non-sterile sediments.« less

Mercury Concentrations in Fish Related to Size of Remote Canadian Shield Lakes

Mercury concentrations in planktivorous, omnivorous, and piscivorous fishes were inversely related to lake size in six lakes in northwestern Ontario. The lakes were remote from direct anthropogenic influences and ranged in surface area from 89 to 35 000 ha. Fish mercury concentrations were not related to ratios of drainage basin area to lake size, to ratios of epilimnetic area to lake size, to lake alkalinity, or to the concentration of mercury in lake sediments. Rates of mercury methylation (M) were positively dependent on water temperature whereas rates of methyl mercury demethylation (D) were inversely related to temperature. Thus, M/D was strongly temperature dependent. Mercury concentrations in four fish species were significantly positively correlated with mean epilimnetic water temperatures (r2's ranged from 0.66 to 0.88). This suggested that higher water temperatures in smaller lakes during the open-water season influenced M/D ratios and were the cause of higher fish mercury levels. No lake size related variation was observed in the mercury concentrations in benthivorous fishes. Our hypothesis that epilimnetic temperature affects mercury concentrations in fish deserves further attention, given the possible effects of climate warming on mercury contamination of fishery resources.

Sites of Methyl Mercury Production in Remote Canadian Shield

Methylation and demethylation of mercury were studied in sediments and surface waters of several remote lakes on the Canadian Shield. Radiochemical assays of mercury methylating activity, which peaked during summer, were 20–40 times faster in epilimnetic than in hypolimnetic sediments. Demethylation rates were usually highest during winter and in hypolimnetic sediments. Epilimnetic sediments were capable of producing methyl mercury 20–40 times faster than hypolimnetic sediments sampled at the same time, with methylating activity peaking during the warm summer months. Because of the opposite pattern of methylating and demethylating activity and because epilimnetic sediments often constitute most of the surface area of these lakes, most of the net methylation (M/D) occurred in the epilimnion of the lakes during summer. Mercury methylation rates were not related to average sediment mercury concentrations, or directly to rates of microbial activity (decomposition) in sediments. It appeared that increasing temperature was an important controlling factor that stimulated methylation but retarded demethylation. Specific methylation rates, M/D, and the measured concentration of methyl mercury in the surface waters of the lakes all peaked during midsummer, suggesting that the radiochemical assays used reflected changes in the relative activities of natural methylating and demethylating microorganisms.

Influence of Dissolved Organic Carbon, pH, and Microbial Respiration Rates on Mercury Methylation and Demethylation in Lake Water

Effects of changes in DOC concentrations, pH, and microbial respiration rates on specific rates of mercury methylation and demethylation in lake water were studied using radioisotopic techniques. Increased concentrations of DOC resulted in decreased specific rates of net methylation, possibly as a result of complexation of inorganic mercury with DOC. A reduction in pH from 7.0 to 5.0 had the greatest effect, causing large to moderate increases in net methylation rate at both low and high DOC concentrations (500–2600 μM). Rates of respiration (indicative of general rates of microbial activity), which were insensitive to pH change over the range tested (5.0–7.0), had the smallest effect on net methyl mercury production rates. We propose the following explanations for three situations in which high mercury concentrations are commonly found in fish. (1) in acidified dilute clear-water lakes, high fish mercury concentrations may be a result of enhanced in-lake methylation; (2) in brown-water circumneutral lakes, where in-lake methylation is inhibited by high DOC concentrations, terrestrial inputs of methyl mercury may be most important; and (3) in brown-water, low-pH lakes, both in-lake and terrestrial sources of methyl mercury may contribute to elevated mercury concentrations in fish.

Environmental factors affecting the formation of methylmercury in low pH lakes

AbstractRecent studies have demonstrated elevated levels of mercury in fish from remote, low alkalinity and low pH lakes. The mechanisms of this enhanced bioaccumulation are poorly understood, but the amount of methylmercury produced in a lake can play a major role. Decreased pH stimulates methylmercury production at the sediment‐water interface and possibly in the aerobic water column. Decreased pH also decreases loss of volatile mercury from lake water and increases mercury binding to particulates in water – factors that may increase methylation at low pH by enhancing the bioavailability of mercury for methylation. In anoxic subsurface sediments, decreased pH decreases the rate of mercury methylation, suggesting that methylmercury formation in the water column and at the sediment‐water interface may be most important in acidified lakes. Sulfate‐reducing bacteria are important mercury methylators in acidified lakes. Whether enhanced sulfate reduction stimulates methylmercury production in low pH lakes is presently unclear, although most of the available data do not support this hypothesis.

Measurements of Specific Rates of Net Methyl Mercury Production in the Water Column and Surface Sediments of Acidified and Circumneutral Lakes

Specific rates of mercury methylation and demethylation were determined for water and surficial sediment samples taken from several lakes located in the Experimental Lakes Area, northwestern Ontario. Specific rates of mercury methylation were found to increase with decreasing pH in epilimnetic water samples in which pH was adjusted prior to incubation and in epilimnetic water samples taken from lakes of different pH. Reduction of pH also increased methyl mercury production at the sediment surface. Both increases and decreases in pH reduced specific rates of mercury demethylation. However, these changes were smaller than for methylation. Proportionally, specific rates of methylation increased faster than increasing concentrations of Hg2+, while specific rates of mercury demethylation increased linearly with increasing concentrations of methyl mercury. Overall, this study predicts that the net rate of methyl mercury production in the water column and at the sediment–water surface will increase as a result of lake acidification, and this may at least partially explain why the mercury concentration of fish appears to increase during lake acidification.

Methods for Measuring Specific Rates of Mercury Methylation and Degradation and Their Use in Determining Factors Controlling Net Rates of Mercury Methylation

A method was developed to estimate specific rates of demethylation of methyl mercury in aquatic samples by measuring the volatile C end products of CH(3)HgI demethylation. This method was used in conjunction with a Hg radiochemical method which determines specific rates of mercury methylation. Together, these methods enabled us to examine some factors controlling the net rate of mercury methylation. The methodologies were field tested, using lake sediment samples from a recently flooded reservoir in the Southern Indian Lake system which had developed a mercury contamination problem in fish. Ratios of the specific rates of methylation/demethylation were calculated. The highest ratios of methylation/demethylation were calculated. The highest ratios of methylation/demethylation occurred in the flooded shorelines of Southern Indian Lake. These results provide an explanation for the observed increases in the methyl mercury concentrations in fish after flooding.