Formation of dissolved gaseous mercury in a tropical lake (Petit-Saut reservoir, French Guiana)

The enhancement and inhibition of mercury reduction by natural organic matter in the presence of Shewanella oneidensis MR-1

Reduction-oxidation (Redox) processes of mercury (Hg) are of significant importance in influencing Hg speciation, bioavailability, and fate in anoxic environments where natural organic matter (NOM) and dissimilatory metal reducing bacteria (DMRB) are widely observed. However, the redox reaction between Hg and NOM, has not yet been studied in the presence of S. oneidensis MR-1 in anoxic environments. We have found that the reduction rate of mercuric mercury [Hg(II)] in the presence of Elliott soil humic acid (ESHA) was 0.02 h^-1. It was faster than the rate (0.01 h^-1) in the direct microbial Hg(II) reduction, suggesting that ESHA acts as an electron transfer mediator between cells and Hg, which enhances Hg(II) reduction under anoxic conditions. The overall rate of Hg(II) reduction in the presence of ESHA is determined by the rate of electron transfer from S. oneidensis MR-1 to ESHA (rate-limiting step) since the rate of electron transfer from reduced ESHA to Hg(II) was so rapid. In the reaction between S. oneidensis MR-1 and a variety of NOM analogs, the production rate of elemental mercury [Hg (0)] was linearly correlated with the free radical concentrations and aromaticities in reduced NOM analogs. However, at the high ESHA concentrations or cell contents, Hg(II) reduction might be inhibited by thiol functional groups in reduced ESHA and on cells. We suggest that the presence of NOM, cell concentration and NOM source can significantly affect the redox processes of Hg and therefore, have important implications for elucidating Hg redox processes under environmentally relevant complex conditions.

Bioaccumulation and biomagnification of mercury in African lakes: the importance of trophic status

Despite the global prevalence of both mercury (Hg) contamination and anthropogenic eutrophication, relatively little is known about the behavior of Hg in eutrophic and hypereutrophic systems or the effects of lake trophic status on Hg uptake and trophodynamics. In the current study we explore Hg trophodynamics at 8 tropical East African study sites ranging from mesotrophic to hypereutrophic, in order to assess the influence of lake trophic status on Hg uptake and biomagnification. Comprehensive water, plankton and fish samples were collected for analysis of total mercury (THg) and stable carbon and nitrogen isotopic ratios. We found evidence that uptake of THg into phytoplankton tended to be lower in higher productivity systems. THg concentrations in fish were generally low, and THg trophic magnification factors (TMFs; representing the average increase in contaminant concentrations from one trophic level to the next) ranged from 1.9 to 5.6. Furthermore TMFs were significantly lower in hypereutrophic lakes than in meso- and eutrophic lakes, and were negatively related to chlorophyll a concentrations both across our study lakes, and across African lakes for which literature data were available. These observations suggest that THg concentrations were strongly influenced by trophic status, with year-round high phytoplankton and fish growth rates reducing the potential for high THg in fish in these productive tropical lakes.

Mercury reduction and cell-surface adsorption by Geobacter sulfurreducens PCA

Both reduction and surface adsorption of mercuric mercury [Hg(II)] are found to occur simultaneously on G. sulfurreducens PCA cells under dark, anaerobic conditions. Reduction of Hg(II) to elemental Hg(0) initially follows a pseudo-first order kinetics with a half-life of <2 h in the presence of 50 nM Hg(II) and 10(11) cells L(-1) in a phosphate buffer (pH 7.4). Multiple gene deletions of the outer membrane cytochromes in this organism resulted in a decrease in reduction rate from ∼0.3 to 0.05 h(-1), and reduction was nearly absent with heat-killed cells or in the cell filtrate. Adsorption of Hg(II) by cells is found to compete with, and thus inhibit, Hg(II) reduction. Depending on the Hg to cell ratio, maximum Hg(II) reduction was observed at about 5 × 10(-19) mol Hg cell(-1), but reduction terminated at a low Hg to cell ratio (<10(-20) mol Hg cell(-1)). This inhibitory effect is attributed to bonding between Hg(II) and the thiol (-SH) functional groups on cells and validated by experiments in which the sorbed Hg(II) was readily exchanged by thiols (e.g., glutathione) but not by carboxylate compounds such as ethylenediamine-tetraacetate (EDTA). We suggest that coupled Hg(II)-cell interactions, i.e., reduction and surface binding, could be important in controlling Hg species transformation and bioavailability and should therefore be considered in microbial Hg(II) uptake and methylation studies.

Methylation of mercury in earthworms and the effect of mercury on the associated bacterial communities

Methylmercury compounds are very toxic for most organisms. Here, we investigated the potential of earthworms to methylate inorganic-Hg. We hypothesized that the anaerobic and nutrient-rich conditions in the digestive tracts of earthworm's promote the methylation of Hg through the action of their gut bacteria. Earthworms were either grown in sterile soils treated with an inorganic (HgCl₂) or organic (CH₃HgCl) Hg source, or were left untreated. After 30 days of incubation, the total-Hg and methyl-Hg concentrations in the soils, earthworms, and their casts were analyzed. The impact of Hg on the bacterial community compositions in earthworms was also studied. Tissue concentrations of methyl-Hg in earthworms grown in soils treated with inorganic-Hg were about six times higher than in earthworms grown in soils without Hg. Concentrations of methyl-Hg in the soils and earthworm casts remained at significantly lower levels suggesting that Hg was mainly methylated in the earthworms. Bacterial communities in earthworms were mostly affected by methyl-Hg treatment. Terminal-restriction fragments (T-RFs) affiliated to Firmicutes were sensitive to inorganic and methyl-Hg, whereas T-RFs related to Betaproteobacteria were tolerant to the Hg treatments. Sulphate-reducing bacteria were detected in earthworms but not in soils.

Significance of submarine groundwater discharge in the coastal fluxes of mercury in Hampyeong Bay, Yellow Sea

Submarine groundwater discharge (SGD) and various solutes released with SGD have received particular attention recently; however, understanding of the impact of SGD on trace metal fluxes in the coastal ocean is limited. To understand the contribution of SGD to the coastal Hg input, the Hg mass fluxes associated with SGD were estimated from Hampyeong Bay, a coastal embayment in the Yellow Sea. Hg concentrations in filtered groundwater and seawater ranged from 1.3 to 4.4pM and from 0.83 to 2.0pM, respectively, and Hg concentrations in unfiltered seawater ranged from 1.7 to 4.6pM. The Hg flux estimation showed that SGD was the prime input source of Hg in the bay (18±12molyr(-1)), contributing 65% of the total input. Atmospheric deposition was the second dominant source of Hg (8.5±2.7molyr(-1)), contributing 31% to the total input. The results of the current study suggest that SGD can be a significant source of Hg in estuarine/coastal systems; therefore, estimating the coastal mass budgets of Hg must include SGD as a prime source of Hg.

Mercury methylation rates of biofilm and plankton microorganisms from a hydroelectric reservoir in French Guiana

The Petit-Saut ecosystem is a hydroelectric reservoir covering 365km(2) of flooded tropical forest. This reservoir and the Sinnamary Estuary downstream of the dam are subject to significant mercury methylation. The mercury methylation potential of plankton and biofilm microorganisms/components from different depths in the anoxic reservoir water column and from two different sites along the estuary was assessed. For this, reservoir water and samples of epiphytic biofilms from the trunk of a submerged tree in the anoxic water column and from submerged branches in the estuary were batch-incubated from 1h to 3 months with a nominal 1000ng/L spike of Hg(II) chloride enriched in (199)Hg. Methylation rates were determined for different reservoir and estuarine communities under natural nutrient (reservoir water, estuary freshwater) and artificial nutrient (culture medium) conditions. Methylation rates in reservoir water incubations were the highest with plankton microorganisms sampled at -9.5m depth (0.5%/d) without addition of biofilm components. Mercury methylation rates of incubated biofilm components were strongly enhanced by nutrient addition. The results suggested that plankton microorganisms strongly contribute to the total Hg methylation in the Petit-Saut reservoir and in the Sinnamary Estuary. Moreover, specific methylation efficiencies (%Me(199)Hg(net)/cell) suggested that plankton microorganisms could be more efficient methylating actors than biofilm consortia and that their methylation efficiency may be reduced in the presence of biofilm components. Extrapolation to the reservoir scale of the experimentally determined preliminary methylation efficiencies suggested that plankton microorganisms in the anoxic water column could produce up to 27mol MeHg/year. Taking into account that (i) demethylation probably occurs in the reservoir and (ii) that the presence of biofilm components may limit the methylation efficiency of plankton microorganisms, this result is highly consistent with the annual net MeHg production estimated from mass balances (8.1mol MeHg/year, Muresan et al., 2008a).

Substantial emission of gaseous monomethylmercury from contaminated water-sediment microcosms

Emission rates of gaseous monomethylmercury (CH(3)Hg(II)), as well as elemental mercury (Hg(0)) and dimethylmercury [(CH(3))(2)Hg(II)], were determined in Hg-contaminated water-sediment microcosms (duplicates of three treatments) by gaseous species-specific isotope dilution analysis (SSIDA). Incubation of approximately 500 g (wet mass) of sediments containing 30 mumol of ambient Hg with an addition of 2.6 mumol of (201)Hg(II) tracer resulted in average (n = 6) gaseous emissions of 84 +/- 26, 100 +/- 37, and 830 +/- 380 pmol of ambient CH(3)Hg(II), CH(3)(201)Hg(II), and (201)Hg(0), respectively, during 108 days of incubation. In contrast to Hg(0), a transient temporal pattern was observed for measured CH(3)Hg(II) emission rates, which peaked at day 12 and decreased to much lower levels by the end of the experiments. At day 12, CH(3)Hg(II) constituted 30-50% of the total emitted gaseous Hg, emphasizing the significance of this species to total Hg emissions from anoxic sediment-water systems. Emission rates of gaseous CH(3)Hg(II) did not reflect the accumulated CH(3)Hg(II) content in the sediment, suggesting that emissions mainly originated from newly methylated Hg(II). Speciation modeling of the pore water suggests that CH(3)Hg(II) was emitted as CH(3)HgSH(0)(g).

Importance of elemental mercury in lake sediments

Mercury (Hg) redox changes in sediments are poorly studied and understood, even though they potentially control Hg availability for methylation and can alter sediment-water Hg exchange. Elemental Hg (Hg(0)) concentrations in sediments of two Canadian Shield lakes were assessed by thermodesorption. Hg(0) concentrations in sediments varied between 6.3 and 60.3 pg g(-1) (wet weight) which represented 7.4-28.4% of total mercury (HgT) concentration. Hg(0) concentrations were similar in both lakes. Hg(0) was rapidly adsorbed on sediments in controlled adsorption experiments and surface sediments sampled in summer had a stronger affinity for Hg(0) than deeper sediments and sediments sampled in fall. This adsorption was positively correlated to organic matter content and negatively related to particle grain size, pH and oxygen concentration in overlying water. This study demonstrates that Hg(0) is a prevalent species in sediments, but not in porewater, because of the high sorptive capacity of sediments towards Hg(0). Its potential availability towards Hg methylating bacteria remains to be determined.

Novel reduction of mercury (II) by mercury-sensitive dissimilatory metal reducing bacteria

The dissimilatory metal reducing bacterium (DMRB) Shewanella oneidensis MR-1 reduces ionic mercury (Hg[II]) to elemental mercury (Hg[0]) by an activity not related to the MerA mercuric reductase. In S. oneidensis, this activity is constitutive and effective at Hg(II) concentrations too low to induce mer operon functions. Reduction of Hg(II) by MR-1 required the presence of electron donors and electron acceptors. Reduction occurred with oxygen or fumarate, but had the highest rate when ferric oxyhydroxide was used as a terminal electron acceptor. Geobacter sulfurreducens PCA and Geobacter metallireducens GS-15 reduced Hg(II) to Hg(0) with activity comparable to MR-1; however, neither the DMRB Anaeromyxobacter dehalogenans 2CP-C nor the nitrate reducer Pseudomonas stutzeri OX-1 reduced Hg(II) during growth. This discovery of constitutive mercury reduction among anaerobes has implications to the mobilization of mercury and production of methylmercury in anoxic environments.