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)