Strategies to Quantify Mercury Biomethylation Potential in Sediments - - PowerPoint PPT Presentation

Managing Aquatic Mercury Pollution:

Strategies to Quantify Mercury Biomethylation Potential in Sediments

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Helen Hsu-Kim, DUKE UNIVERSITY

hsukim@duke.edu

Udonna Ndu, Natalia Neal-Walthall, Marc Deshusses, DUKE UNIVERSITY Dwayne Elias, Geoff Christensen, Caitlin Gionfriddo, OAK RIDGE NATIONAL LABORATORY

R01ES024344

Engstrom, 2007, PNAS

• Mercury biomagnifies in aquatic

food webs as monomethylmercury (MeHg)

• MeHg is produced by anaerobic

microorganisms

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Methylmercury: the driver of risk at Hg-contaminated sites

East Fork Poplar Creek (Tennessee) cleanup estimate : ~$3 billion Onondaga Lake (NY) cleanup estimate: ~$500 million

Management of Mercury-Contaminated sites

Penobscot River estuary (Maine) cleanup estimate : >$130 million

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Management of Mercury-Contaminated sites

Benchmarks for Site Assessment

Challenges:

• Total Hg content is a poor predictor
• f risk
• Current water quality standard:

MeHg in fish Needs:

• More functional shorter-term

target for watershed management & remediation

(e.g., Biomethylation potential of Hg)

0.001 0.01 0.1 1 10 100 1000

0.001 0.01 0.1 1 10 100 1000 10000

Sediment MeHg (ng g-1) Sediment Total Hg (µg g-1)

Urban Industrial Areas Rice/Agriculture Mining Reservoirs

103 10-3 10-2 10-1 100 101 102 100 101 102 103 104 10-1 10-2 10-3

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Hsu-Kim et al. (2018) Challenges and Opportunities in Managing Aquatic Mercury Pollution. Ambio. 47: 141-169

Why do we need a model to predict Hg methylation potential?

Hg Bioavailability

Productivity of Hg- methylating microbes low high low high

mesohaline marsh HgS contaminated tidal marsh Hg(0) discharge in low order stream Cinnabar mine drainage

Total Hg Methylation Risk Profile

high %MeHg

(d[MeHg]/dt)

low %MeHg

(d[MeHg]/dt)

moderate %MeHg

(d[MeHg]/dt)

Why do we need a model to predict Hg methylation potential?

Hg Bioavailability

Productivity of Hg- methylating microbes low high low high

mesohaline marsh HgS contaminated tidal marsh Hg(0) discharge in low order stream Cinnabar mine drainage

Total Hg Methylation Risk Profile

Site A (original status)

e.g. upland, unsat’d soil

wetland creation, flooding, sea level rise, sulfate deposition Water column aeration ?

Impacts of Remediation Activities and Other Perturbations

activated carbon amendment

high %MeHg

(d[MeHg]/dt)

low %MeHg

(d[MeHg]/dt)

moderate %MeHg

(d[MeHg]/dt)

Hg Bioavailability

low high low high Productivity of Hg- methylating microbes

Methods for Quantifying Mercury Biomethylation Potential

The conventional approach: Equilibrium speciation

weakly complexed

Hg(OH)2, HgCl2, HgCl3-

strongly complexed

Hg(HS)x, Hg-DOM

Mineral phase HgS(s) Ksp or Kd Hg2+ + xHS- ↔ Hg(HS)x2-x KHg(HS)x Hg2+ + DOM ↔ Hg-DOM KHgDOM

Dissolved Particulate Bioavailable

Sorbent

Organic matter

FeS(s)

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• All have the HgcA and HgcB proteins
• Ubiquitous in anaerobic niches

(sulfate reducers, Fe reducers, methanogens)

Parks et al. 2013 Science Gilmour et al. 2013 ES&T

Hg-methylating microbes:

MeHg

photo credit: Poulain and Barkay, 2013, Science

Sediment porewater of a freshwater lake

1000 2000 3000 3000 g 20 min 6,700 g 5 min 370,000 g 2 hr <0.2μm filter [Hg] in supernatant (ng/L)

Site 2 (lake center)

Most of the mercury in porewater is bound to particles

HgT = 700 ng/g

Hg speciation in benthic settings

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dissolved Hg(II) complexes

ligand-promoted

• r inhibited

dissolution

Stable colloid and nanoparticle suspension

bioavailable

Aggregated colloids and nanoparticles

not bioavailable

chelation by DOM cluster formation in presence of DOM ripening or aging dissolution dissociation

“free” ion, weak or labile complexes

aggregation

Heterogeneous amorphous HgS nanoparticles

precipitation with DOM sorption of DOM

DOM-capped polynuclear Hg-sulfide clusters

Aiken et al., ES&T 2011

Bioavailability of Mercury for Methylation: An Alternative Approach

Rates of these reactions at microbial interfaces determine bioavailability

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Inorganic Hg is primarily associated with particles

Slurry sample

Add GSH (1 mM) end-over-end mix anaerobic for 30 min Quantify Hg in <0.2 µm fraction

Thiol-based selective extraction

Glutathione (GSH) Extraction

Methods to Quantify Hg Bioavailability

dissolved Hg(II)

bioavailable HgSR

Ticknor et al., Env Engr Sci (2015)

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Methods to Quantify Hg Bioavailability

polypropylene cover 0.45-µm membrane filter agarose diffusion layer thiolated silica resin layer polypropylene support base

Conventional approach: derive a ‘truly dissolved’ concentration Area A

Dg

Dissolved Hg in bulk solution, Cb

Diffusive Gradient in Thin-film (DGT) samplers

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Dg

Hg concentration

depth

Soluble Hg concentration in DGT agarose diffusion layer

Sediment or surface water

Mass of Hg uptake m reactive Hg fraction

Thiolated resin interface

Cagarose

Hg uptake into DGT: Our approach:

polypropylene cover 0.45-µm membrane filter agarose diffusion layer thiolated silica resin layer polypropylene support base

Area A

Dg

Method testing: sediment microcosms

Added Hg: (100-200 ng g-1 dw per species)

dissolved 204Hg-nitrate dissolved 196Hg-humic

199Hg adsorbed to FeS

humic-coated nano-200HgS

sediment slurry with DGT

(sample origin: tidal marsh, freshwater lake)

Testing Methods of Quantifying Hg Methylation Potential

Diffusive Gradient in Thin-Films (DGT) passive samplers

Slurry sample Add GSH (1 mM) end-over-end mix anaerobic for 30 min

Glutathione (GSH) Selective Extraction

Quantify Hg in <0.2 µm fraction

Quantify over time:

• MeHg from each isotopic

endmember

• Hg on DGTs
• GSH-extractable Hg fraction
• hgcA gene copy number and

microbial community composition

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0.2 0.4 0.6 0.8 1

2 4 6

MeHg (% of total Hg)

Time (days)

²⁰⁴Hg²⁺ ¹⁹⁹Hg-FeS ¹⁹⁶Hg-humic nano-²⁰⁰HgS

Type of Hg added: Methylation of Hg added to slurries

Ndu et al., ES&T, 2018.

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Tidal marsh (mesohaline) sediment slurry

0.1 0.2 0.3 2 4 6

Hg on DGT(% of Tot Hg)

Time (days)

Uptake of total Hg in DGTs

Testing Methods of Quantifying Hg Methylation Potential

Hg uptake in DGTs correlates with MeHg production

Net MeHg production:

• correlated with uptake on the DGT sampler
• did not correlate with the <0.45 µm or the

GSH-extractable fraction

DGT Sampler Filter-passing fraction GSH-extractable fraction

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Ndu et al., ES&T, 2018.

5 10 15 20

2 4 6

MeHg (% of total Hg) Time (days)

Freshwater Lake Sediment Slurry

with 1 mM pyruvate DGT Sampler Filter-passing fraction GSH-extractable fraction

Hg uptake in DGTs correlates with MeHg production

²⁰⁴Hg²⁺ ¹⁹⁹Hg-FeS ¹⁹⁶Hg-humic nano-²⁰⁰HgS

Type of Hg added:

Mesohaline Slurry

Freshwater Slurry

Comparing the Hg-Methylating Microbial Communities

Difference because of abundance

• f hgcAB+ microbes?

Ndu et al., ES&T, 2018.

dissolved Hg(II)

Hg-DOM, Hg(HS)2

Bioavailable Hg

Inorganic Hg Speciation

in anaerobic settings

Anaerobic Microbiome

amorphous, hydrated nanostructured weakly sorbed well-crystalline macrostructured strongly sorbed

MeHg

Polymerization & Sorption: Sulfide, NOM ripening

• r aging

dissolution and desorption aggregation

Biogenic sulfide,

• rganic carbon,

redox gradients

h g c A B +

d-Proteobact.

h g c A B +

F i r m i c u t e

hgcAB+

methanogen

hgcAB+

microbial community composition

hgcAB+

Geochemical vs. Microbiome Controls on Mercury Methylation

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Mesohaline Slurry

Freshwater Slurry

Comparing the Hg-Methylating Microbial Communities

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20,000 40,000 60,000

2 4 6 hgcA copy number per ng DNA (Deltaproteobacteria)

Day

Mesohaline Freshwater hgcA+ Deltaproteobacteria

500 1000 1500 2000

2 4 6 hgcA copy number per ng DNA (Archaea)

Day

limit of quantification

hgcA+ methanogenic Archaea

qPCR hgcA genes

Diversity and abundance of methylators from DNA-based approaches

Ndu et al., ES&T, 2018.

Next Steps

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Can DGTs work in the real world?

Outdoor freshwater wetland mesocosms.

Added Hg:

dissolved 202Hg2+ dissolved 201Hg-humic

199Hg adsorbed to FeS

nano-200HgS

Next Steps

Can DGTs work in the real world?

1 2 3

5 10 15 20 25

MeHg in sediment (ng g-1)

Inorganic Hg flux into DGT

(ng Hg m-2 h-1) mesocosm 1 mesocosm 2 mesocosm 3

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Semi-Mechanistic Model

Model for Hg Methylation Potential

A possible simplification…..

Next Steps

Summary

• Quantifying MeHg potential in ecosystems:

1.Hg bioavailability (Hg uptake rate in DGTs) 2.Productivity of the methylating microbiome (hgcA gene expression?)

• Hg bioavailability for methylation:

Controlled by reactivity of Hg-S-NOM phases at microbial interfaces

Mercury: Strategies to Quantify Methylation Potential in the Environment

• Needs for site management &

remediation:

functional measures of MeHg production potential

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R01ES024344

Additional questions are welcome! Helen Hsu-Kim hsukim@duke.edu

References

Aiken, G.R.; Hsu-Kim, H.; Ryan, J.N. (2011). Influence of dissolved organic matter for the environmental fate of metals, nanoparticles, and

• colloids. Environ. Sci. & Technol. 45, 3196–3201.DOI: 10.1021/es103992s.

Engstrom, D.R., 2007. Fish respond when the mercury rises. Proceedings of the National Academy of Sciences, 104(42), pp.16394-16395. Gilmour, C.C., Podar, M., Bullock, A.L., Graham, A.M., Brown, S.D., Somenahally, A.C., Johs, A., Hurt Jr, R.A., Bailey, K.L. and Elias, D.A., 2013. Mercury methylation by novel microorganisms from new environments. Environmental science & technology, 47(20), pp.11810-11820. Hsu-Kim, H.; Eckley, C.S.; Achá, D.; Feng, X; Gilmour, C.C.; Jonsson, S.; Mitchell, C.P.J. (2018). Challenges and Opportunities for Managing Aquatic Mercury Pollution in Altered Landscapes. Ambio. 47(2). 141-169. DOI: 10.1007/s13280-017-1006-7 Hsu-Kim, H.; Kucharzyk, K.H.; Zhang, T.; Deshusses, M.A. (2013). Mechanisms regulating mercury bioavailability for methylating microorganisms in the aquatic environment: A critical review. Environ. Sci. & Technol. 47(6), 2441-2456. DOI: 10.1021/es304370g. Ndu, U.; Christensen, G.A.; Rivera, N.A.; Gionfriddo, C.M.; Deshusses, M.A.; Elias, D.A.; Hsu-Kim, H. (2018). Quantification of Mercury Bioavailability for Methylation Using Diffusive Gradient in Thin-Film Samplers. Environ. Sci. & Technol. 52, 8521-8529. DOI: 10.1021/acs.est.8b00647 Parks, J.M., Johs, A., Podar, M., Bridou, R., Hurt, R.A., Smith, S.D., Tomanicek, S.J., Qian, Y., Brown, S.D., Brandt, C.C. and Palumbo, A.V.,

• 2013. The genetic basis for bacterial mercury methylation. Science, 339(6125), pp.1332-1335.

Poulain, A.J. and Barkay, T., 2013. Cracking the mercury methylation code. Science, 339(6125), pp.1280-1281. Ticknor, J.L; Kucharzyk, K.H.; Porter, K.A.; Deshusses, M.A.; Hsu-Kim, H. (2015). Thiol-based selective extraction assay to comparatively assess bioavailable mercury in sediments. Environ. Engr. Sci. 32(7), 564-573. DOI: 10.1089/ees.2014.0526 Zhang, T.; Kim, B.; Levard, C.; Reinsch, B.C.; Lowry, G.V.; Deshusses, M.A.; Hsu-Kim, H. (2012). Methylation of mercury by bacteria exposed to dissolved, nanoparticulate, and microparticulate mercuric sulfides. Environ. Sci. & Technol. 46(13), 6950-6958. DOI: 10.1021/es203181m

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