Managing Aquatic Mercury Pollution: Strategies to Quantify Mercury Biomethylation Potential in Sediments Helen Hsu-Kim, D UKE U NIVERSITY hsukim@duke.edu Udonna Ndu, Natalia Neal-Walthall, Marc Deshusses, D UKE U NIVERSITY Dwayne Elias, Geoff Christensen, Caitlin Gionfriddo, O AK R IDGE N ATIONAL L ABORATORY R01ES024344 1
Methylmercury: the driver of risk at Hg-contaminated sites • Mercury biomagnifies in aquatic food webs as monomethylmercury (MeHg) • MeHg is produced by anaerobic microorganisms Engstrom, 2007, PNAS 2
Management of Mercury-Contaminated sites Onondaga Lake (NY) cleanup estimate: ~$500 million East Fork Poplar Creek (Tennessee) cleanup estimate : ~$3 billion Penobscot River estuary (Maine) cleanup estimate : >$130 million 3
Management of Mercury-Contaminated sites Benchmarks for Site Assessment Challenges: • Total Hg content is a poor predictor 1000 10 3 of risk Sediment MeHg (ng g -1 ) 100 10 2 • Current water quality standard: MeHg in fish 10 1 10 10 0 1 Urban Needs: 10 -1 0.1 Industrial Areas • More functional shorter-term Rice/Agriculture 0.01 10 -2 Mining target for watershed management Reservoirs & remediation 0.001 10 -3 10 -3 10 -2 10 -1 10 0 10 1 10 2 0.001 0.01 0.1 1 10 100 1000 10 3 10000 10 4 (e.g., Biomethylation potential of Hg) Sediment Total Hg (µg g -1 ) 4 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? Total Hg Methylation Risk Profile high high %MeHg ( d[MeHg]/dt ) methylating microbes HgS contaminated Productivity of Hg- mesohaline tidal marsh marsh moderate %MeHg ( d[MeHg]/dt ) Cinnabar mine drainage Hg(0) discharge in low order stream low low %MeHg (d[MeHg]/dt) low high Hg Bioavailability
Why do we need a model to predict Hg methylation potential? Impacts of Remediation Activities Total Hg Methylation Risk Profile and Other Perturbations high high high %MeHg activated carbon methylating microbes ( d[MeHg]/dt ) methylating microbes amendment HgS contaminated Productivity of Hg- Productivity of Hg- mesohaline tidal marsh ? marsh moderate %MeHg ( d[MeHg]/dt ) Water wetland creation, column flooding, sea level aeration rise, sulfate deposition Cinnabar mine drainage Site A (original status) Hg(0) discharge in e.g. upland, unsat’d soil low low order stream low low %MeHg (d[MeHg]/dt) low high low high Hg Bioavailability Hg Bioavailability
Methods for Quantifying Mercury Biomethylation Potential The conventional approach: Equilibrium speciation Bioavailable Dissolved Hg-methylating microbes: Particulate weakly complexed Mineral phase Hg(OH) 2 , HgCl 2 , MeHg HgS (s) HgCl 3- K sp or K d photo credit : Poulain and Barkay, 2013, Science Sorbent •All have the HgcA and HgcB proteins strongly •Ubiquitous in anaerobic niches Organic matter complexed (sulfate reducers, Fe reducers, methanogens) FeS (s) Hg(HS) x , Hg-DOM Parks et al . 2013 Science Hg 2+ + x HS - ↔ Hg(HS) x2-x Gilmour et al. 2013 ES&T K Hg(HS)x 7 Hg 2+ + DOM ↔ Hg-DOM K HgDOM
Hg speciation in benthic settings Hg T = 700 ng/g Site 2 (lake center) 3000 Sediment porewater of [Hg] in supernatant (ng/L) a freshwater lake 2000 1000 0 3000 g 6,700 g 370,000 g <0.2μm 20 min 5 min 2 hr filter Most of the mercury in porewater is bound to particles 8
Bioavailability of Mercury for Methylation: An Alternative Approach Inorganic Hg is primarily DOM-capped polynuclear associated with particles Hg-sulfide clusters precipitation with DOM cluster formation in Heterogeneous presence of DOM amorphous HgS dissociation nanoparticles dissolution bioavailable “free” ion, weak or ripening or sorption of labile complexes aging DOM dissolved Hg(II) complexes ligand-promoted Stable colloid chelation by or inhibited DOM and nanoparticle not dissolution suspension aggregation bioavailable Rates of these reactions at microbial Aggregated colloids interfaces determine bioavailability 9 and nanoparticles Aiken et al ., ES&T 2011
Methods to Quantify Hg Bioavailability bioavailable HgSR dissolved Hg(II) Thiol-based selective extraction Glutathione (GSH) Extraction Add GSH (1 mM) Quantify Hg in <0.2 µ m fraction end-over-end mix anaerobic for 30 min Slurry sample 10 Ticknor et al., Env Engr Sci (2015)
Methods to Quantify Hg Bioavailability Diffusive Gradient in Thin-film (DGT) samplers polypropylene cover Conventional approach: derive a Area A ‘truly dissolved’ concentration 0.45- µ m membrane filter D g agarose diffusion layer thiolated silica resin layer polypropylene support base Hg concentration Dissolved Hg in Our approach: bulk solution, C b Sediment or depth Mass of Hg reactive Hg uptake into DGT: surface water uptake m Hg fraction D g C agarose Thiolated resin interface Soluble Hg concentration in 11 DGT agarose diffusion layer
Testing Methods of Quantifying Hg Methylation Potential Diffusive Gradient in Thin-Films Glutathione (GSH) Selective Extraction (DGT) passive samplers polypropylene cover Add GSH (1 mM) Slurry Area A Quantify Hg in 0.45- µ m membrane filter sample D g end-over-end mix <0.2 µ m fraction agarose diffusion layer thiolated silica resin layer anaerobic for 30 min polypropylene support base Method testing: sediment microcosms Quantify over time: Added Hg: (100-200 ng g -1 dw per species) • MeHg from each isotopic dissolved 204 Hg-nitrate endmember • Hg on DGTs dissolved 196 Hg-humic • GSH-extractable Hg fraction 199 Hg adsorbed to FeS • hgcA gene copy number and humic-coated nano- 200 HgS microbial community composition sediment slurry with DGT 12 ( sample origin: tidal marsh, freshwater lake )
Testing Methods of Quantifying Hg Methylation Potential Methylation of Hg added to slurries Tidal marsh (mesohaline) 1 sediment slurry MeHg (% of total Hg) 0.8 0.6 0.4 Uptake of total Hg in DGTs 0.2 Hg on DGT(% of Tot Hg) 0.3 0 0 2 4 6 Time (days) 0.2 Type of Hg added: 0.1 ²⁰⁴Hg² ⁺ ¹⁹⁹Hg-FeS ¹⁹⁶Hg-humic nano-²⁰⁰HgS 0 0 2 4 6 13 Time (days) Ndu et al. , ES&T , 2018.
Hg uptake in DGTs correlates with MeHg production DGT Sampler Filter-passing fraction GSH-extractable fraction Net MeHg production: • correlated with uptake on the DGT sampler • did not correlate with the <0.45 µ m or the GSH-extractable fraction 14 Ndu et al. , ES&T , 2018.
Hg uptake in DGTs correlates with MeHg production DGT Sampler Freshwater Lake Sediment Slurry with 1 mM pyruvate Type of Hg added: ²⁰⁴Hg² ⁺ ¹⁹⁹Hg-FeS Filter-passing ¹⁹⁶Hg-humic nano-²⁰⁰HgS fraction 20 MeHg (% of total Hg) 15 10 GSH-extractable fraction 5 0 0 2 4 6 Time (days)
Comparing the Hg-Methylating Microbial Communities Mesohaline Slurry Difference because of abundance of hgcAB+ microbes? Freshwater Slurry Ndu et al. , ES&T , 2018.
Geochemical vs. Microbiome Controls on Mercury Methylation Inorganic Hg Anaerobic Speciation Microbiome in anaerobic settings amorphous, hydrated microbial community composition ripening nanostructured or aging weakly sorbed hgcAB + + Bioavailable Hg B A Polymerization c e t MeHg g u c h & Sorption: i m r i F Sulfide, NOM dissolved dissolution Hg(II) and Hg-DOM, Hg(HS) 2 desorption hgcAB + hgcAB + methanogen h g c A B + well-crystalline d -Proteobact. macrostructured aggregation strongly sorbed Biogenic sulfide, organic carbon, redox gradients 17
Comparing the Hg-Methylating Microbial Communities Mesohaline Slurry Diversity and abundance of methylators from DNA-based approaches hgcA+ Deltaproteobacteria Mesohaline hgcA copy number per ng DNA 60,000 Freshwater (Deltaproteobacteria) 40,000 20,000 qPCR hgcA genes 0 0 2 4 6 Day Freshwater Slurry 2000 hgcA+ methanogenic Archaea hgcA copy number per ng DNA 1500 (Archaea) 1000 500 limit of quantification 0 18 0 2 4 6 Day Ndu et al. , ES&T , 2018.
Next Steps Can DGTs work in the real world? 19
Next Steps Can DGTs work in the real world? 3 mesocosm 1 MeHg in sediment (ng g -1 ) mesocosm 2 mesocosm 3 2 Outdoor freshwater Added Hg: 1 dissolved 202 Hg 2+ wetland mesocosms. dissolved 201 Hg-humic 199 Hg adsorbed to FeS nano- 200 HgS 0 0 5 10 15 20 25 Inorganic Hg flux into DGT (ng Hg m -2 h -1 )
Next Steps Model for Hg Methylation Potential A possible simplification….. Semi-Mechanistic Model 21
Summary Mercury: Strategies to Quantify Methylation Potential in the Environment • Needs for site management & remediation: functional measures of MeHg production potential • Hg bioavailability for methylation: Controlled by reactivity of Hg-S-NOM phases at microbial interfaces • Quantifying MeHg potential in ecosystems: Additional questions are welcome! Helen Hsu-Kim 1.Hg bioavailability (Hg uptake rate in DGTs) hsukim@duke.edu 2.Productivity of the methylating microbiome ( hgcA gene expression?) 22 R01ES024344
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