High methylmercury production under ferruginous conditions in sediments impacted by sewage treatment plant discharges (original) (raw)
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Sulfide and iron control on mercury speciation in anoxic estuarine sediment slurries
Marine Chemistry, 2008
In order to understand the role of sulfate and Fe(III) reduction processes in the net production of monomethylmercury (MMHg), we amended anoxic sediment slurries collected from the Venice Lagoon, Italy, with inorganic Hg and either potential electron acceptors or metabolic byproducts of sulfate and Fe(III) reduction processes, gradually changing their concentrations. Addition of sulfide (final concentration: 0.2-6.3 mM) resulted in an exponential decrease in the sulfate reduction rate and MMHg concentration with increasing concentrations of sulfide. Based on this result, we argue that the concentration of dissolved sulfide is a critical factor controlling the sulfate reduction rate, and in turn, the net MMHg production at steady state. Addition of either Fe(II) (added concentration: 0-6.1 mM) or Fe(III) (added concentration: 0-3.5 mM) resulted in similar trends in the MMHg concentration, an increase with low levels of Fe additions and a subsequent decrease with high levels of Fe additions. The limited availability of dissolved Hg, associated with sulfide removal by precipitation of FeS, appears to inhibit the net MMHg production in high levels of Fe additions. There was a noticeable reduction in the net MMHg production in Fe(III)-amended slurries as compared to Fe(II)-amended ones, which could be caused by a decrease in the sulfate reduction rate. This agrees with the results of Hg methylation assays using the enrichment cultures of anaerobic bacteria: whereas the enrichment cultures of sulfate reducers showed significant production of MMHg (4.6% of amended Hg), those of Fe(III), Mn(IV), and nitrate reducers showed no production of MMHg. It appears that enhanced Fe(III)-reduction activities suppress the formation of MMHg in high sulfate estuarine sediments.
Environmental Science & Technology, 2012
The five distinct habitat types 1 for sampling included floodplain wetland (located at 2.6 and 13.8 km from Waynesboro, referred to hereafter as Relative River Distance-RRD2.6 and RRD13.8), bed sediment pool (RRD4.8 and RRD14.0), embedded pool (RRD7.4 and RRD11.9), island or mill race side channel pool (RRD8.4 and RRD15.9), and river pool edges with finegrained sediment deposit (RRD10.0 and RRD20.6) (See Figure S1 and Table S1). S1.2 Biogeochemical Analyses of Sediments and Porewater After transferring and tightly sealing sediments into Falcon tubes in an anaerobic chamber, porewater was extracted from the samples by centrifugation (SLA-1500 rotor in a Sorvall RC-5B plus) at 10,000 rpm for 30 min at 4 °C. Aliquots of porewater were filtered through 0.22 µm membranes, and analyzed for nitrate and sulfate concentrations using an ICS-1000 ion chromatography system (Dionex, CA) equipped with an AS-40 autosampler and an Ionpac AS9-HC analytical column (4 mm × 250 mm). The mobile phase used for ion chromatography analysis was a 9 mM Na 2 CO 3 solution. Porewater pH was measured by an Accumet 915 pH Meter (Fisher Sci.). For whole sediment samples, organic matter content was analyzed by the loss-on-ignition (LOI) method. 2 Total solids and acid volatile sulfide (AVS) in sediment were measured following the procedures described by EPA Method 160.3 3 and 821/R-91-100, 4 respectively.
Science of The Total Environment, 2011
The activity of various anaerobic microbes, including sulfate reducers (SRB), iron reducers (FeRP) and methanogens (MPA) has been linked to mercury methylation in aquatic systems, although the relative importance of each microbial group in the overall process is poorly understood in natural sediments. The present study focused on the biogeochemical factors (i.e. the relative importance of various groups of anaerobic microbes (FeRP, SRB, and MPA) that affect net monomethylmercury (MMHg) formation in contaminated sediments of the St. Lawrence River (SRL) near Cornwall (Zone 1), Ontario, Canada. Methylation and demethylation potentials were measured separately by using isotope-enriched mercury species ( 200 Hg 2+ and MM 199 Hg + ) in sediment microcosms treated with specific microbial inhibitors. Sediments were sampled and incubated in the dark at room temperature in an anaerobic chamber for 96 h. The potential methylation rate constants (K m ) and demethylation rates (K d ) were found to differ significantly between microcosms. The MPA-inhibited microcosm had the highest potential methylation rate constant (0.016 d −1 ), whereas the two SRB-inhibited microcosms had comparable potential methylation rate constants (0.003 d −1 and 0.002 d −1 , respectively). The inhibition of methanogens stimulated net methylation by inhibiting demethylationand by stimulating methylation along with SRB activity. The inhibition of both methanogens and SRB was found to enhance the iron reduction rates but did not completely stop MMHg production. The strong positive correlation between K m and Sulfate Reduction Rates (SRR) and between K d and Methane Production Rates (MPR) supports the involvement of SRB in Hg methylation and MPA in MMHg demethylation in the sediments. In contrast, the strong negative correlation between K d and Iron Reduction Rates (FeRR) shows that the increase in FeRR corresponds to a decrease in demethylation, indicating that iron reduction may influence net methylation in the SLR sediments by decreasing demethylation rather than favouring methylation.
Estuarine, Coastal and Shelf Science, 2012
Mercury (Hg) transformation activities and sulfate (SO 4 2À ) reduction were studied in sediments of the Marano and Grado Lagoons in the Northern Adriatic Sea region as part of the "MIRACLE" project. The lagoons, which are sites of clam (Tapes philippinarum) farming, have been receiving excess Hg from the Isonzo River for centuries. Marano Lagoon is also contaminated from a chlor-alkali plant. Radiotracer methods were used to measure mercury methylation ( 230 Hg, 197 Hg), methylmercury (MeHg) demethylation ( 14 C-MeHg) and SO 4 2À reduction ( 35 S) in sediment cores collected in autumn, winter and summer.
Biogeochemical factors affecting mercury methylation rate in two contaminated floodplain soils
Biogeosciences, 2012
An automated biogeochemical microcosm system allowing controlled variation of redox potential (E H) in soil suspensions was used to assess the effect of various factors on the mobility of mercury (Hg) as well as on the methylation of Hg in two contaminated floodplain soils with different Hg concentrations (approximately 5 mg Hg kg −1 and >30 mg Hg kg −1). The experiment was conducted under stepwise variation from reducing (approximately −350 mV at pH 5) to oxidizing conditions (approximately 600 mV at pH 5). Results of phospholipid fatty acids (PLFA) analysis indicate the occurrence of sulfate reducing bacteria (SRB) such as Desulfobacter species (10Me16:0, cy17:0, 10Me18:0, cy19:0) or Desulfovibrio species (18:2ω6,9), which are considered to promote Hg methylation. The products of the methylation process are lipophilic, highly toxic methyl mercury species such as the monomethyl mercury ion [MeHg + ], which is named as MeHg here. The ln(MeHg/Hg t) ratio is assumed to reflect the net production of monomethyl mercury normalized to total dissolved Hg (Hg t) concentration. This ratio increases with rising dissolved organic carbon (DOC) to Hg t ratio (ln(DOC/Hg t) ratio) (R 2 = 0.39, p < 0.0001, n = 63) whereas the relation between ln(MeHg/Hg t) ratio and lnDOC is weaker (R 2 = 0.09; p < 0.05; n = 63). In conclusion, the DOC/Hg t ratio might be a more important factor for the Hg net methylation than DOC alone in the current study. Redox variations seem to affect the biogeochemical behavior of dissolved inorganic Hg species and MeHg indirectly through related changes in DOC, sulfur cycle, and microbial community structure whereas E H and pH values, as well as concentration of dissolved Fe 3+ /Fe 2+ and Cl − seem to play subordinate roles in Hg mobilization and methylation under our experimental conditions.
Applied and Environmental Microbiology, 2000
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.
Marine Pollution Bulletin, 2011
The Seine's estuary (France) waters are the receptacle of effluents originating from wastewater treatment plants (WWTP). In this estuary, mudflats are deposition zones for sediments and their associated contaminants, and play an essential role in the mercury (Hg) biogeochemical cycle mainly due to indigenous microorganisms. Microcosms were used to assess the impact of WWTP-effluents on mercury methylation by monitoring Hg species (total dissolved Hg in porewater, methylmercury and total mercury) and on microbial communities in sediments. After effluent amendment, methylmercury (MeHg) concentrations increased in relation with the total Hg and organic matter content of the WWTP-effluents. A correlation was observed between MeHg and acid-volatile-sulfides concentrations. Quantification of sulfate-reducing microorganisms involved in Hg methylation showed no increase of their abundance but their activity was probably enhanced by the organic matter supplied with the effluents. WWTP-effluent spiking modified the bacterial community fingerprint, mainly influenced by Hg contamination and the organic matter amendment.
Science of The Total Environment, 2012
Methylmercury (MeHg) is the most poisonous form of mercury (Hg) and it enters the human body primarily through consumption of Hg contaminated fish. Sulfate reducing bacteria (SRB) are major producers of MeHg in anoxic sediments. The dsrAB gene was isolated from freshwater fish pond sediments. Sequence analyses showed that the SRB in sediments was mainly composed of Desulfobulbus propionicus and Desulfovibrio vulgaris. The two species of SRB were cultured from freshwater sediments. The addition of inorganic Hg to these freshwater sediments caused an increase in MeHg concentrations at 30 days incubation. MeHg levels were sensitive to sulfate concentrations; a medium sulfate level (0.11 mg/g) produced higher levels than treatments lacking sulfate addition or when amended with 0.55 mg/g. Assessment of bacterial levels by PCR measurements of microbial DNA indicated that the MeHg levels were correlated with cell growth.
BIOGEOCHEMICAL FACTORS AFFECTING MERCURY METHYLATION IN SEDIMENTS OF THE VENICE LAGOON, ITALY
Environmental Toxicology and Chemistry, 2007
Mercury methylation and sulfate reduction rates, total Hg, and monomethyl Hg in the sediments of the Venice Lagoon (Italy) were measured in June 2005 in order to identify the factors affecting the methylation of inorganic Hg. While the rates of Hg methylation and sulfate reduction were generally higher in the surface layers (0-2.5 cm), the correlation between Hg methylation and sulfate reduction rates was not significant when considering all depths and sites. This discrepancy is discussed considering two factors: the activity of sulfate-reducing bacteria and Hg solubility. The former factor is important in determining the Hg methylation rate in comparable geochemical conditions as evidenced by similar vertical profiles of Hg methylation and sulfate reduction rates in each sediment core. The latter factor was assessed by comparing the Hg methylation rate with the particle-water partition coefficient of Hg. The Hg methylation rates normalized to sulfate reduction rates showed a negative linear correlation with the logarithm of the particle-water partition coefficient of Hg, suggesting that the availability of dissolved Hg is a critical factor affecting Hg methylation. Solid FeS seems to play an important role in controlling the solubility of Hg in Venice Lagoon sediments, where sulfate and iron reductions are the dominant electron-accepting processes. Overall, the production of monomethyl Hg in the Venice Lagoon is controlled by a fine balance between microbial and geochemical processes with key factors being the microbial sulfate reduction rate and the availability of dissolved Hg.