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Biogeochemical Framework to Evaluate Mercury Methylation
Potential During in-situ Remediation of Contaminated Sediments
NIEHS R01ES024344
2014-2018
Heileen Hsu-Kim, Marc DeshussesDuke University
Dwayne Elias Oak Ridge National Lab
Project team
Duke University
Helen Hsu-Kim (PI) – Aquatic Geochemistry
Marc Deshusses (Co-PI) – Bioremediation
Oak Ridge National Lab
Dwayne Elias (Co-PI) – Microbial Ecology
Other Collaborators
Steven Brown,Ph.D., Dow Chemical, Berry’s Creek Study Group
Challenges of the mercury problem:
Selin, 2009, Annu. Rev. Environ. Resour.
• Many sources to biosphere
• Long range transport
• Food web accumulation
• Mechanisms of MeHg production
Factors contributing towards mercury methylation potential
Framework to predict methylation potential
Productivity of methylating microorganisms
Bioavailability of Hg
??
High methylation potential
Low methylation potential
HgHxS2x-2
Hg-thiolHg-DOM
Amorphous or crystalline HgS nanoparticles
Aggregated or micro-crystalline HgS(s)
DOM-capped polynuclear HgS clusters
Dissolved Hg(II) complexes
Geochemical Forms of Mercury in Sediments
Microorganisms that Methylate Mercury
Deltaproteobacteria
Gilmour et al., ES&T, 2013Firmicutes, Clostridia
ARCHAEAEuryarchaeota
•Obligate anaerobes•Phylogenically diverse
•hgcAB: two gene cluster
FreshwaterSaline water Ambient Hg: 2 nmol g-1
Spike Hg: 2 nmol g-1
[SO42-]0 < 0.07 mM[SO4
2-]0 = 15 mM
Biomethylation in Sediment Slurry Microcosms
Bioavailability-limited methylation
Productivity-limited methylation
Zhang et al., ES&T, 2014
dissolved Hg+sulfide
nano-HgS
microcrystalline HgS
dissolved Hg+sulfide
nano-HgS
microcrystalline HgS
Bioavailability vs. Productivity
Threshold?Kucharzyk et al., ICMGP, 2013
Mixed microbe community enriched from sediments
Dissolved Hg added Nano HgS added
C-substrate for growth
Project Objectives:
Sediment-water microcosms with samples from Superfund site
Aim 1: Activity of methylating microbes •hgcAB gene abundance/expression•Microbial diversity
Aim 2: Hg bioavailability •Size fractionation, solid-water partitioning of Hg•Thiol-extraction potential•Passive sampler with thiolated resin
Aim 3: Delineation of the controls on methylation potential •Limited by microbial activity•Limited by bioavailability
Aim 4: Effects of sediment amendments•Activated carbon•Clay mineral particles•Ferrous iron (FeCl2)
Measurements of Methylation Potential
Site Characterization and Remediation
• To establish biogeochemical indicators for methylmercury production potential
• To test the effectiveness of in-situ remediation
hgcAB PCR
hgcA qPCR & RT-qPCR
Design schematic for PCR, qPCR and RT-qPCR primers.
Activity of Methylating Microbes
Parks et al., Science, 2013Gilmour et al., ES&T, 2013
Amino acid sequence alignments for hgcAB for all predicted Hg-methylating organisms
Quantification of Hg bioavailability
Thiol-based selective extractionmicrobial culture: D. priopionicus 1pr3
bulk-scale HgS
dissolved Hg+sulfide
nano-HgS
Zhang et al., ES&T, 2012
Evaluation of Hg methylation potential in sediments
Anaerobic sediment-water microcosms
Major experimental variables:•Sediment origin•Type of Hg added (dissolved, nanoHgS, Hg-FeS) differentiated by Hg isotope
Measurements of methylation potential:•MeHg concentration or net production rate•Gene abundance & expression•Hg bioavailability (thiol selective extraction)
MeH
g pr
oduc
tion
rate
hgcAB abundance or expression
bioavailable [Hg]
Data Analysis: Calibration of measurement methods
Assess the effectiveness of remediation
Field samples of benthic sediments
(Oak Ridge, TN; Berry’s Creek Study Area; other sites?)
Range of characteristics:•Salinity•Organic Carbon•MeHg concentration (relative to total Hg)