Biogeochemical Framework to
Evaluate Mercury Methylation
Potential During in-situ Remediation
of Contaminated Sediments
Heileen Hsu-Kim, Marc Deshusses
Duke University
Dwayne Elias
Oak Ridge National Lab
NIEHS R01ES024344
2014-2018
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:
• Many sources to biosphere
• Long range transport
• Food web accumulation
• Mechanisms of MeHg
production
Selin, 2009, Annu. Rev.
Environ. Resour.
Objective:
To establish biogeochemical indicators
for methylmercury production potential
Factors contributing towards mercury
methylation potential
Productivity of
methylating
microorganisms
Bioavailability
of Hg
?
?
Framework to predict methylation potential
Geochemical Forms of Mercury in Sediments
HgHxS2x-2
Hg-thiol
Hg-DOM
Dissolved Hg(II)
complexes
High methylation
potential
DOM-capped
polynuclear
HgS clusters
Amorphous or
crystalline HgS
nanoparticles
Aggregated or
micro-crystalline
HgS(s)
Low methylation
potential
Microorganisms that Methylate Mercury
Gilmour et al., ES&T, 2013
Firmicutes, Clostridia
• Obligate anaerobes
• Phylogenically
diverse
• hgcAB: two gene
cluster
Deltaproteobacteria
ARCHAEA
Euryarchaeota
Biomethylation in Sediment Slurry Microcosms
Saline water
Ambient Hg: 2 nmol g-1
Spike Hg: 2 nmol g-1
Freshwater
[SO42-]0 = 15 mM
[SO42-]0 < 0.07 mM
dissolved Hg+sulfide
dissolved Hg+sulfide
nano-HgS
nano-HgS
microcrystalline HgS
microcrystalline HgS
Bioavailability-limited
methylation
Zhang et al., ES&T, 2014
Productivity-limited
methylation
Bioavailability vs. Productivity
Mixed microbe community enriched from sediments
Dissolved Hg added
Nano HgS added
C-substrate
for growth
Threshold?
Kucharzyk et al., ICMGP, 2013
Project Objectives:
• To establish biogeochemical indicators for methylmercury
production potential
• To test the effectiveness of in-situ remediation
Sediment-water microcosms with
samples from Superfund site
Measurements of Methylation
Potential
Site Characterization and
Remediation
Aim 1: Activity of methylating
microbes
• hgcAB gene abundance/expression
• Microbial diversity
Aim 3: Delineation of the controls
on methylation potential
• Limited by microbial activity
• Limited by bioavailability
Aim 2: Hg bioavailability
• Size fractionation, solid-water
partitioning of Hg
• Thiol-extraction potential
• Passive sampler with thiolated resin
Aim 4: Effects of sediment
amendments
• Activated carbon
• Clay mineral particles
• Ferrous iron (FeCl2)
Activity of Methylating Microbes
Amino acid sequence alignments for hgcAB for all predicted
Hg-methylating organisms
hgcA qPCR & RT-qPCR
Design schematic for
PCR, qPCR and RTqPCR primers.
hgcAB PCR
Parks et al., Science, 2013
Gilmour et al., ES&T, 2013
Quantification of Hg bioavailability
Thiol-based selective extraction
microbial culture: D. priopionicus 1pr3
nano-HgS
bulk-scale HgS
Thiol-extractable Hg, % of total Hg
dissolved Hg+sulfide
dissolved
Hg+S
60%
40%
20%
nanoparticulate
HgS
microcystalline HgS
0%
0
25
50
75
pM of MeHg
Zhang et al., ES&T, 2012
100
Evaluation of Hg methylation potential in sediments
Anaerobic sedimentwater microcosms
• Sediment origin
• Type of Hg added
(dissolved, nanoHgS,
Hg-FeS) differentiated
by Hg isotope
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)
Measurements of
methylation potential:
• MeHg concentration or
net production rate
• Gene abundance &
expression
• Hg bioavailability (thiol
selective extraction)
MeHg production rate
Data Analysis:
Calibration of
measurement methods
Major experimental
variables:
hgcAB abundance
or expression
Assess the
effectiveness of
remediation