Influences of Zero-Valent Sulfur on Mercury Methylation in Bacterial Cocultures

Kampalath, Rita Ann; Lin, Chien-Cheng; Jay, Jennifer A.

doi:10.1007/s11270-012-1399-7

Cited by 13 publications

(13 citation statements)

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“…The Hg(S n ) 2 2− /HgS 2 2− ratio will increase steeply in proportion to a S 0. Microbial Hg methylation, which is the entry point of Hg into aquatic food chains, is known to be sensitive to Hg speciation, with the polysulfide complex apparently not available for methylation [ 28 ],[ 91 ].…”

Section: Discussionmentioning

confidence: 99%

“…An example of chain length’s importance is its effect on trace metal chelation by polysulfides; only S n 2− ions having n ≥ 4 are able to form chelate rings. In sulfidic waters, chelation by polysulfides is likely to influence strongly the mobility, bioavailability and toxicity of Class b, or soft metal cations (Cu + , Ag + , Au + and Hg 2+ ) [ 18 ],[ 23 ]–[ 28 ]. Another example involves polysulfide-dependent kinetic processes.…”

Section: Introductionmentioning

confidence: 99%

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Activity of zero-valent sulfur in sulfidic natural waters

Helz

2014

Geochem Trans

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BackgroundIonic and molecular carriers of dissolved (filter-passing) zero-valent sulfur (S0) in anaerobic natural waters include polysulfides, Sn2−, molecular S8(aq), organic macromolecules and certain higher valent thioanions. Because S0 is rapidly transferred among these various carriers, its biogeochemical roles in such processes as dehalogenation of organic compounds, chelation of trace metals, and anaerobic microbial metabolism are not determined solely by one ionic or molecular species. Here, S0 is treated collectively as a virtual thermodynamic component, and computational as well as graphical methods for quantifying its activity (aS0) in natural waters are presented. From aS0, concentrations of the ionic and molecular carriers of S0 can be calculated easily.ResultsConcentration ratios of any two polysulfide ions define aS0 (Method I). Unfortunately these concentrations are often too low in nature for accurate quantification with current methods. Measurements of total divalent sulfur (ΣS-II), zero-valent sulfur (ΣS0) and pH provide a more widely applicable approach (Method II). Systematic errors in ΣS0 measurements are the main limit to accuracy of this method at the present time. Alternative methods based on greigite solubility and potentiometry are discussed. A critical comparison of Methods I and II reveals inconsistencies at low ΣS0/ΣS-II that imply errors in the thermodynamic data for HS2− and S2−. For samples having low ΣS0/ΣS-II, an interim remedy is recommended: letting pKa2 = 6.3 for all HSn− ions.ConclusionsNewly assembled data for aS0 in a selection of anaerobic natural waters indicate that S0 is always metastable in the surveyed samples with respect to disproportionation to sulfide and sulfate. In all the surveyed environments, sulfur-rich minerals, such as greigite, covellite and orpiment, are stable in preference to their sulfur-poor cohorts, mackinawite, chalcocite and realgar. The aS0 values in the dataset span conditions favoring Hg-polysulfide complexes vs. Hg-sulfide complexes, implying that aS0 could affect Hg-methylation rates in nature. No support is found for the common assumption that aS0 = 1 in reducing natural waters. This paper calls attention to an urgent need for improved measurement methods, especially for total zero-valent sulfur, as well as new determinations of ionization constants for all HSn− species.Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-014-0013-x) contains supplementary material, which is available to authorized users.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Activity of zero-valent sulfur in sulfidic natural waters

Helz

2014

Geochem Trans

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“…Sulfur biogeochemistry influences Hg methylation by affecting the activity of methylating bacteria and the availability of Hg(II) for methylation. In anaerobic sediment, the presence of sulfate, an oxidized S species, generally increases Hg methylation (Kampalath et al ) because of its role as an electron acceptor for SRB. The end product of sulfate reduction is sulfide, which strongly controls the concentration of dissolved Hg(II) (Paquette and Helz ) and may either increase or decrease Hg methylation by influencing the speciation of Hg in sediment.…”

Section: Mercury Cycling In the Environmentmentioning

confidence: 99%

“…Uncharged Hg sulfide species (e.g., Hg(SH) 2 (aq)) are available for methylation (Drott et al ), because they can pass through bacterial cell membranes where they can be methylated. The formation of solid phases (e.g., HgS(s)) or charged species acts to inhibit methylation (Kampalath et al ) because both are unavailable to bacteria. The extent of Hg methylation has been linked to precipitation and dissolution reactions of HgS(s) and regulation of SRB activity by S(‐II) toxicity (Gilmour et al ; Benoit, Gilmour et al ; Gilmour et al ).…”

Section: Mercury Cycling In the Environmentmentioning

confidence: 99%

Biogeochemical controls on methylmercury in soils and sediments: Implications for site management

Bigham

Murray

Masue-Slowey

et al. 2016

Integr Environ Assess Manag

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Management of Hg-contaminated sites poses particular challenges because methylmercury (MeHg), a potent bioaccumulative neurotoxin, is formed in the environment, and concentrations are not generally predictable based solely on total Hg (THg) concentrations. In this review, we examine the state of knowledge regarding the chemical, biological, and physical controls on MeHg production and identify those most critical for contaminated site assessment and management. We provide a list of parameters to assess Hg-contaminated soils and sediments with regard to their potential to be a source of MeHg to biota and therefore a risk to humans and ecological receptors. Because some measurable geochemical parameters (e.g., DOC) can have opposing effects on Hg methylation, we recommend focusing first on factors that describe the potential for Hg bioaccumulation: site characteristics, Hg and MeHg concentrations, Hg availability, and microbial activity, where practical. At some sites, more detailed assessment of biogeochemistry may be required to develop a conceptual site model for remedial decision making. Integr Environ Assess Manag 2017;13:249-263. C 2016 SETAC

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“…19–21 For example, the coexistence of SRB with phototrophic sulfur-oxidizing bacteria may be of particular importance in MeHg formation because sulfur oxidizers consume sulfide, 4,22–25 which may maintain favorable chemical speciation for Hg uptake by SRB and thus ultimately increase Hg methylation. 26 The biofilm growth mode itself may be another determinant of a cell’s methylation rate, possibly because of different gene expression or metabolic activities that may occur in organisms growing as biofilms as opposed to unattached cultures. In fact, previous work with pure cultures of two strains of Desulfovibrio desulfuricans showed that Hg methylation rates were 1 order of magnitude higher when the cultures were grown as biofilms compared to when the same organisms were grown as planktonic cultures.…”

Section: Introductionmentioning

confidence: 99%

Investigation of Mercury Methylation Pathways in Biofilm versus Planktonic Cultures of Desulfovibrio desulfuricans

Lin

Kampalath

Lin

et al. 2013

Environ. Sci. Technol.

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Biofilms can methylate mercury (Hg) at higher rates than unattached bacteria and are increasingly recognized as important Hg methylation sites in the environment. Our previous study showed that methylation rates in biofilm cultures were up to 1 order of magnitude greater than those in planktonic cultures of a sulfate-reducing bacterium. To probe whether the differential Hg methylation rates resulted from metabolic differences between these two cultures, Hg methylation assays following molybdate or chloroform inhibition (a specific inhibitor of the acetyl-CoA pathway) were conducted on biofilm and planktonic cultures of Desulfovibrio desulfuricans strains M8 and ND132. Molybdate was as effective in inhibiting Hg methylation as well as growth in both planktonic and biofilm cultures. The addition of chloroform only impacted Hg methylation in biofilm cultures, suggesting that different pathways are used for methylation in biofilm compared to planktonic cultures. To investigate this further, expression of the cooS gene, which encodes for carbon monoxide dehydrogenase, a key enzyme in the acetyl-CoA pathway, was compared in biofilm and planktonic cultures of ND132. Biofilm cultures showed up to 4 times higher expression of cooS than planktonic cultures. On the basis of these results, the acetyl-CoA pathway appears to play an important role in methylation in biofilm cultures of this organism, possibly by supplying the methyl group to Hg methylating enzymes; methylation in planktonic cultures appears to be independent of this pathway. This observation has important implications, particularly in developing reliable models to predict Hg methylation rates in different environments and perhaps eventually in being able to control this undesirable chemical transformation.

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Influences of Zero-Valent Sulfur on Mercury Methylation in Bacterial Cocultures

Cited by 13 publications

References 79 publications

Activity of zero-valent sulfur in sulfidic natural waters

Activity of zero-valent sulfur in sulfidic natural waters

Biogeochemical controls on methylmercury in soils and sediments: Implications for site management

Investigation of Mercury Methylation Pathways in Biofilm versus Planktonic Cultures of Desulfovibrio desulfuricans

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