geochemical and molecular mechanisms controlling ... · geochemical modeling * critical...
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Geochemical and Molecular MechanismsControlling Contaminant Transformation
in the Environment
ORNL ERSP Science Focus Area (SFA)
ERSP 3rd Annual PIMeeting, April 7 – 9, 2008
Managed by UT-Battellefor the Department of Energy
Strategy for understanding contaminanttransformation and environmental behavior
Microbial andgenetic controls
Fundamental rates and mechanisms
Transformation in field Speciation & mechanisms Molecular dynamics
Reaction mechanisms and kinetics at groundwater–surface water interface
Sediment-water interface
Species/ abundance
Microbial communities
Coupled microbial
and geochemical
reactions
Molecular level understanding
of contaminant association
and reaction
Molecular structureand simulations
catalytic
domain
NmerA
MerA core
N-terminus
C-terminus
CYS 11
CYS 14
CYS 136
CYS 141
CYS 558
CYS 559
Fieldbiogeochemistry
(Luther et al. 1999)
Thiol-like binding
Carboxyl bindingCarboxyl bindingCarboxyl binding
Thiol-like binding
Carboxyl bindingCarboxyl bindingCarboxyl binding
Hg2+ + 2e- = Hg(0)
1000 1200 1400 1600 1800
Inte
ns
ity
(a
.u.)
Haitzer et al. (2003)
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Research focus and goals
• Mercury – the net balance of methylationand demethylation
Geochemical/biological controls on Hg speciation andtransformation, and how and what Hg precursors areproduced, transported and methylated
Enzymatic mechanisms of transformation between majorHg species and methyl mercury.
• Uranium – stability in subsurface
Microbial oxidizers – Rates and mechanisms in theoxidation of U(IV) minerals
Structure and function of key heme proteins required fordirect electron transfer, microbial-mineral interface models
Y-12
ORNL
ETTP
Hg in water, sediment and biota in streams (in red)
300,000 kg of Hg lost to soil
and groundwater at Y-12
Impact UEFPC
Industrial use areasHg contaminated soil, buildings, storm
drain network, sediments, ground and
surface water
Mercury concerns at Oak Ridge Reservation
Source Areas
High mercury concentrations in biota
• High concentrations of elemental andHg(II) complexes in shallow soils nearindustrial infrastructure
• Oak Ridge environment: stronggroundwater/surface water interactions(>50” annual rainfall)
• Methyl mercury is readily accumulatedand can increase up the food chain
• Hg exceeds regulatory limits—newstandards could significantly impact Y-12operations and costs
• TN TDEC developing an East Fork PoplarCreek TMDL; focus on loading/flux, notconcentration
• TN recently lowered Hg level that triggers anadvisory
• EPA concern for ecological risks
• Modernization of facilities could result inincreased transport of Hg to streams
EFPC downstream of Y-12
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0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
Hg
, g
/g
(fi
sh
),
g/
L (
wate
r)
Water
Fish
• Mercury bioaccumulation in fish is not proportional toconcentrations of waterborne Hg
Examples of water/ fish disconnect (see poster):Oak Ridge Sites Hg in water Hg in fish
(ng/L) (mg/kg)
White Oak Creek 60 0.5
EF Poplar Cr (upper) 400 0.8
EF Poplar Cr (lower) 100 0.8
Bear Cr 1–3 0.7
Rogers Quarry 1 1.1
Reference site 1–3 0.2
• Hg in fish correlates with methyl Hg in water, but not withtotal Hg in water. So, at contaminated sites, there is nomodel relating methyl Hg and total Hg in water
• Not possible to eliminate inorganic Hg inputs; alternativestrategies to reduce methylation may be only means to reachfish concentration targets
Basic research needs: elucidate Hg methylationprocesses at sediment-water interface and thecontrols on methyl Hg production
Source control has not lowered Hg in fish
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The mercury challenge
National:
• Global pollutant readily transportedand re-emitted
• Highly toxic to human and ecologicalreceptors
Methylmercury (MeHg) is a potenthuman neurotoxin, highlybioaccumulative
• Hg found at all DOE sites; waste andenvironmental issues at many (e.g.,Savannah River, Paducah…)
• Complex chemistry /speciation/methylation–demethylationprocesses
• Hg at industrial contaminated sites
Hg methylation at sediment-water interface
Need to elucidate:
Oxidation, reduction, andspecies transformation
Dominant chemical speciesand bioavailability
Abiotic /biotic methylationand demethylation
Biochemical pathways formethylation anddemethylation
Coupled biogeochemicalreactions – sorption,complexation, precipitation,stabilization, fate andtransport
Surface catalyzed andphotochemical reactions
MeHg Hg(II ) Hg(0)Reduction
OxidationHgS(s )
HgS(HS )-
Hg(HS)2
Hg-NOM
Hg-clay/oxide
MeHg Hg(II ) Hg(0)Reduction
Oxidation
HgS(s)
Hg(0)
Hg-particle
Hg(OH)2
Part
icu
late
DOM
Hg(0)MeHgRedox /DOM photo/redox
Dis
solv
ed
Hg-DOM
HgCln(n -2)-
HgCl(OH )
HgS(HS )-
cell
cell
Biological demethylation
Catalyzed and photochemical demethylation
Water
Sediment
9 Managed by UT-Battellefor the Department of Energy
Research team
Advisory Panel
T Barkay (Microbiologist)
S Lindberg (Hg cycling)
R Mason (Hg geochemist)
A Summers (Biochemist)
R Wildung (Geochemistry)
E Phillips (DOE-ORO)
Microbial and
genetic controls
(Hg and U)
A Palumbo (ORNL)
C Gilmour
(Smithsonian)
T Phelps (ORNL)
S Brown (ORNL)
J Wall (U Missouri)
Haakrho Yi (ORISE)
Molecular scale &
simulations
(Hg and U)
L Liang (ORNL)
J Smith (UTK)
L Shi (PNNL)
D Myles (ORNL-
CSD)
New hire (TBD)
A Johs (ORISE)
Fundamental
Mechanisms
(Hg)
B Gu (ORNL)
K Kemner (ANL)
K Littrell (ORNL-
SNS)
K Nagy (UIC)
H Zhang (TN Tech)
C Miller (ORISE)
Site
Biogeochemistry
(Hg)
S Brooks (ORNL)
G Southworth
(ORNL)
G Luther (U
Delaware)
Postdoc (TBN)
BER ERSD
Program
Managers
ORNL
Liyuan Liang
(Research Manager)
Diverse expertise
Strong partnership
10 Managed by UT-Battellefor the Department of Energy
Integrated research approach
New science: fundamental understanding
of cont. transformation
Path forward: addressing cont. remediation
challenges
MeHg Hg(II)?
I. Field investigation & geochemical modeling*
• Site geochemistry,
species, flux, microbial
community
• Coupled reactions on
Hg speciation
II. Fundamental mechanisms*
• Speciation, DOM/POM on
methylation & demethyl.
and catalyzed reactions
Hg2+ + 2e- = Hg(0)
• MeHg bioavailable Hg species; NOM and
particulate surfaces methyl. & demethyl.
III. Microbial and genetic controls
• Community, genes, and
geochemical controls
• SRB methylation;
others demethylation?
• U oxidation
VI. Molecular structure and simulations
• Proteins complexes in electron transfers (U
and Hg), quantum/ molecular simulation
• Mercury resistance genes,
biomolecular mechanisms
* These tasks Hg only
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I. Site biogeochemistryI. Site biogeochemistry
Field studies (UEFPC)
• Measurement of Hg flux
• Quantify geochemical gradients
• Benthic flux chambers
• EXAFS and XANES analysis
Microcosm Studies
• Stable Hg isotopes to facilitate analysis
• Transformation pathways
Microbial Community Structure at the
sediment-water interface
• Functional gene arrays
• Principal microbial communities
Geochemical Modeling
* Critical understanding of Hg flux,biogeochemical controls, andmicrobial determinants
(Menheer, 2004)
Flux chambers
(Luther et al., 1999)
Microelectrodes
Field measurementsField measurements
Complementary
laboratory
microcosms
Functional gene
arraysGeochemical
modeling
-20
-15
-10
-5
-5.5 -4.5 -3.5 -2.5 -1.5
log {Cl-}
log
{H
S-}
MeHgS-
(MeHg)2S
MeHgOH0 MeHgCl0
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II. Fundamental mechanisms and transformations
2 4 6 8 10
0.0
1.0x10-9
2.0x10 -9
3.0x10-9
4.0x10-9
5.0x10 -9
Hg-DOM
Hg
(II)
sp
ecie
s (
M)
pH
HgCl2
Hg(OH)2
Hg(OH)CO-
3
Thiol - like binding
Carboxyl bindingCarboxyl bindingCarboxyl binding
Thiol - like binding
Carboxyl bindingCarboxyl bindingCarboxyl binding
Haitzer et al. (2003)
Hg(II ) speciation: H +–Cl-–CO3
2-–DOM
MeHg nitrate
Barradell et al. (1993)* Critical understanding of dominant Hg
species, its bioavailability, andbiogeochemical controls on rates andmechanisms of Hg methylation anddemethylation
Determine speciation and abiotic controls
• Rates and mechanisms, oxidation/reduction
• Single reactant to multi-component systems
• Real-time spectroscopic analysis coupled with CVAA orCVAFS analysis
Establish roles of DOM and POM in Hg
methylation, demethylation, complexation,
and stabilization of particulate Hg species
• Specific moieties and functional groups
• Labeled isotope studies; EXAFS and XANES analysis,speciation and coordination chemistry
• Species, models, and effects on bioavailability
Surface catalyzed and photochemical reactions
• Roles in methyl. and demethylation
• Sorbed species and reactions
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III. Microbial and genetic controlson mercury methylation
Genome wide PCA of COGs
Hg methylators
Non-Hg
methylato
rs
Desulfovibrio cell pellet
grown with and without Hg
Elucidate the genetic determinants of methyl Hg
production and regulation
• Comparative gene expression, mutagenesis, andcomplementation
Determine the effect of geochemical factors on
gene regulatory networks for mercury methylation
• Use whole genome microarrays to examine both bioticand abiotic effects on the methylating and nonmethylatingDesulfovibrio transcriptomic profiles
Examine relationships among community
structure, geochemical conditions, and methyl Hg
production in sediments collected from Hg-
contaminated sites
• Functional gene arrays
• 16S rRNA gene clone library analysis
Desulfovibrio africanus(SEM by Dwayne Elias, U of Missouri)
*Critical understanding of the geneticbasis of the methylation and demethylationprocesses and the geochemical controlson microbial transformation.
14 Managed by UT-Battellefor the Department of Energy
Establish biochemical pathways in
bacterial demethylation
Obtain structure of protein/protein
and protein/DNA complexes
• Apply small angle neutron scattering toreveal structure-function relationships
Reveal enzymatic mechanisms to
understand the processes of
demethylation and reduction
• Use quantum mechanical/molecularmechanical simulations
Barkay, T., S.M. Miller, and A.O. Summers: FEMS Microbiol Rev,
2003. 27(2-3): p. 355-84.
* Critical understanding ofbiomolecular mechanisms in Hgtransformation (demethylationand methylation) by investigationof structure-functionrelationships
IV. Molecular structure and simulations
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Small Angle Neutron Scattering to elucidateSmall Angle Neutron Scattering to elucidatestructure and function of molecular machinesstructure and function of molecular machines
SAXS/SANS for characterization of
proteins and protein complexes
Define protein shapes and compare
solution and crystal structures
SAXS/SANS is used to elucidate
dynamic protein functional
relationships
SANS with contrast variation provides
a method to reveal the orientation and
location of specific components in
complex biomolecular systems
Figure adapted from: Brown, N. L, et al.: The MerR family of transcriptional regulators. FEMS microbiology reviews 27:145-163
(2003)
Conformational change in ternary
MerR-DNA-RNAP complex induced
by Hg2+ binding initiates transcription
+
RNAP
MerR-DNA
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Uranium focus on microbial U oxidation andsubcellular electron transfer processes
U(VI)
Fe(III)
Microbial oxidation
U(IV), U(0)
Fe(II)
Microbial oxidation• Rates and mechanisms
• Controlling factors
• Microbial communities
Additional biogeochemical
processes addressed by
other ERSP PIs and IFCs
Zachara & Fredrickson, 2004; ERSP PI
meeting
Reguera, G., et al.: Extracellular electron
transfer via microbial nanowires. Nature
435:1098-1101(2005).
U(IV)
Fe(II)
Microbial reduction
Microbial reduction
Function of cytochromes required for
dissimilatory metal reduction
• Protein purification,
crystallization
• Structure analyses by neutrons, x-ray
diffraction /scattering
• Biomimetic membrane systems
• Homology modeling
e- donor
Oxidized
e-
17 Managed by UT-Battellefor the Department of Energy
Roole of metal oxidizing bacteriaaffecting U speciation
Study microbial oxidation of U using site materials from Oak
Ridge IFC
• Use microcosms
Use Acidithiobacillus ferrooxidans as a model organism to
study genetic responses
• Construct whole genome microarray
• Conduct single and multi-factor experiments to investigate genetic responses tovarious geochemical conditions
Ongoing work (see Phelps Poster)
• Have obtained cultures of Acidithiobacillus ferrooxidans & testing pH tolerance inlab media
• Designing the whole genome microarray and will print and test it this FY
*Critical understanding of role ofspecific metal oxidizing bacteriaaffecting U oxidation state andthus mobility in the subsurface
Acidothiobacillus ferrooxidans growingon culture plates. The cells themselvesare colorless, the rust coloring associatedwith growing colonies results from the
microbial production of Fe(III)
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Cytochrome protein structure indissimilatory metal reduction
* Critical understanding offunction of cytochromesrequired for dissimilatorymetal reduction
Elucidate structure ofcytochrome complexes andstructure functionrelationships
• Use of small angle neutronscattering (SANS) to understandmechanisms of electrontransport to minerals
Investigate membraneinsertion properties andinteraction with mineralsurfaces
• development of a biomimeticbacterial-mineral interface modelsystem for neutron reflectometrystudies
Shi et al., Journal of Bacteriology, 188:4705-4714, 2006
Weber et al., Nature Reviews Microbiology, 4(10), 752-764, 2006
Ross et al., Applied and Environmental Microbiology, 73:5797-5808, 2007
MtrAMtrA
MtrBMtrB
MtrCMtrC
OmcAOmcA
Inner Membrane
Outer Membrane
Periplasm
LPS/EPS
Menaquinones
NADH
Dehydrogenase
NADH NAD+NADH NAD+
Cytoplasm
Environment
e-
OmcAOmcA
CymACymA
StcStc e-
N
C
?
1
2
3
N
C
?
1
2
3
Hcc-like
domain
19 Managed by UT-Battellefor the Department of Energy
MeHg+
Time / s
0 500 1000 1500
De
flec
tion
, nm
-35
-30
-25
-20
-15
-10
-5
0
5
10
15
Injection
Concentration, M
1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 1e-10 1e-9 1e-8 1e-7 1e-6 1e-5 1e-4
De
flec
tion
, nm
0
5
10
15
20
25
Bending of the cantilever to 1x10-6 M
of CH3Hg+ in water (three experiments)
Bending of this cantilever as
a function of the concentration of CH3Hg+
Au
S
SH
H HS
SH
S
SH
HHS
SH
S
SH
HHS
SH
= HgMe+
Directed research (short term)
High throughput methyl mercury detection using 1,6-
Hexanedithiol monolayers modified cantilevers
20 Managed by UT-Battellefor the Department of Energy
Expected deliverables
Improved understanding
of contaminant behavior
and biogeochemical
mechanisms and rates
Align with ERSD mission
I. Field investigations
• Critical understanding of Hg
flux, biogeochemical
controls, and microbial
determinants
Hg2+ + 2e- = Hg(0)
MeHg Hg(II)?
• Dominant Hg species, its
bioavailability, and
biogeochemical controls on
rates and mechanisms of Hg
methylation and
demethylation
III. Microbial and genetic control
• The genetic basis of the
methylation and demethylation
processes and the
geochemical controls on
microbial transformation
• Role of metal oxidizing
bacteria
IV. Molecular dynamics and simulation• Biomolecular mechanisms in Hg transformation
(demethylation and methylation) by investigation of
structure-function relationships
• Function of cytochromes
required for dissimilatory metal
reduction
II. Fundamental mechanisms
21 Managed by UT-Battellefor the Department of Energy
Partnerships and CollaborationKey collaborations
• ORNL task leaders and staff
Field geochemistry, Brooks, Southworth; Aqueous chemistry, Gu, Miller;Microbiology, Palumbo, Brown, Phelps; Environmental surface chemistry,Liang; Biophysics, Johs; other existing staff as needed, new hires (TBD)
• University connections-- External Science collaborators
C. Gilmour (Microbiology, Smithsonian), H. Guo, J Smith (MolecularDynamics and enzyme simulation, UTK), G. Luther (Sediment sulfidechemistry), S. Miller (Hg molecular biology, UCSF), K. Nagy (coordinationchemistry, UIC), L. Shi (protein biochemistry, PNNL), A. Summers (Hgbiochemistry, UGA), J. Wall (Microbiology, UMC), H. Zhang (Hg chemistry,TTU)
• Nat Lab and User FacilitiesT. Droubay (Materials physicist,EMSL), Ken Kemner (EXAFS,APS), K. Littrell (Neutronscattering, ORNL SNS/HFIR),D. Myles (Deuterium labeling,CSMB)
Together with
Advisory panel
Oak Ridge DOE
EM applied science programSpallation Neutron Source at ORNL
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Scientific Impact and DOE Benefits
Understand key Hg precursors for microbial methylation
– Geochemical manipulation
– Role of sulfide, thiosulfate, NOM etc influencing Hg speciation
– Catalyzed and photochemical transformation of Hg
Reduce net methylation
– Change biochemistry, microbialprocesses; ecology
– Stimulate demethylation in microbialcommunity
– Use of genomics sequence data,microarray technology and advancedanalytical methods
Other products or contributions
• EM-22 Hg workshop
• Communicate to EM
MeHg Hg(II ) Hg(0)Reduction
OxidationHgS(s )
HgS(HS )-
Hg(HS)2
Hg-NOM
Hg-clay/oxide
MeHg Hg(II ) Hg(0)Reduction
Oxidation
HgS(s)
Hg(0)
Hg-particle
Hg(OH)2
Part
icu
late
DOM
Hg(0)MeHgRedox /DOM photo/redox
Dis
solv
ed
Hg-DOM
HgCln(n -2)-
HgCl(OH )
HgS(HS )-
cell
cell
Biological demethylation
Catalyzed and photochemical demethylation
Water
Sediment
24 Managed by UT-Battellefor the Department of Energy
LDRD and internal investment Supporting theERSP SFA
• Tracing Nanoparticle Transport in Porous Mediaby Neutron Radiography and SANS (LDRD Seedmoney fund)
• ESD –subsurface laboratory renovation
• Probing molecular interactions betweenmicrobial-cell proteins and mineral surfaces withneutrons (neutron sciences initiative)
• (Seed Money Fund)
25 Managed by UT-Battellefor the Department of Energy Presentation_name
Site Investigation
• Relationship between groundwater-surface waterinteraction and Hg concentrations at sediment-waterinterface
• Sediment-to-water column flux of Hg & MeHg in relationto water chemistry, biogeochemical gradients, andenvironmental variables (e.g., photoinduced effects)
• Biogeochemical controls on the transformations thatsustain methyl Hg concentrations in water
• Relationships among microbes, community structure,geochemistry, and Hg transformations
26 Managed by UT-Battellefor the Department of Energy
Hg(0), Hg(II), Hg(I)
Methylmercury
(MeHg)
mic
rob
es
Meth
yla
tio
n
?
Bioavailable
Hg species or
Precursors?
SITE BIOGEOCHEMICAL PROCESSES and MICROCOSM STUDIES
THg, MeHg flux,
geochemistry, microbial
community structure
Biogeochemical
controls and redox
couples
Relationship between
geochemistry, microbial
communities, Hg
transformations
Diffusive –vs– Advective Flux
Small Advective Flux >>
Diffusive
Relationship to SnCl2reactive Hg
Demethylation
Surface catalyzed rxns
Photochemical rxns
*Free radicals
*Reactive species
*Surface functional groups
and characterization
Demethylation
MeHg Hg(0) or Hg(II)
Microbial processes
*Sorbed and reactive species
(both chemical & biological
origin)
Ab
ioti
c m
eth
yla
tio
n –
ra
tes
an
d m
ec
ha
nis
ms
?
*NO
M,
*cata
lyzed
reacti
on
s,
*meth
yl
fun
cti
on
al
gro
up
s
Geochemistry – Microbial Community – Hg Transformations
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Hg(0), Hg(II), Hg(I)
Methylmercury
(MeHg)
mic
rob
es
Meth
yla
tio
n
?
?
Bioavailable
Hg species or
Precursors?
Fundamental mechanisms and transformations
Species Identification and
Controlling Processes
Species Identification and
Controlling Processes
Geochemical controls
and redox couples*rates and mechanisms
Roles of natural organic
matter (DOM and POM) *complexation, species
*dissolution, stabilization
*methylation/demethylation
Oxidation and reduction
Hg(0) Hg2+
Precipitation, sorption,
and particulates
e.g., Hg2+ + S= HgS
Speciation and Geochemical Controls
Demethylation
Surface catalyzed rxns
Photochemical rxns
*Free radicals
*Reactive species
*Surface functional groups
and characterization
Demethylation
MeHg Hg(0) or Hg(II)
Microbial processes
*Sorbed and reactive species
(both chemical & biological
origin)
Ab
ioti
c m
eth
yla
tio
n –
ra
tes
an
d m
ec
ha
nis
ms
?
*NO
M,
*cata
lyzed
reacti
on
s,
*meth
yl
fun
cti
on
al
gro
up
s
28 Managed by UT-Battellefor the Department of Energy
mic
rob
es
Hg(0), Hg(II), Hg(I)
Methylmercury
(MeHg)
mic
rob
es
species
rates
Bioavailable
Hg species or
Precursors?
Microbial MeHg Production and Genetic Determinants
Demethylation
Demethylation
Demethylation –
microbial processes
• Reduction by dissimilatory
metal reduction
• Ubiquitous mercury
resistance genes (mer):
Expression of enzymes
specific for demethylation
and reduction
?
• Microbial community
structure
• Methylation strains
• Demethylation strains
• Hg Bioavailability
• Geochemical controls
• Sequence and genetic
controls
• Protein over expression
29 Managed by UT-Battellefor the Department of Energy
Co
ntr
ollin
g p
rocesses
Demethylation
Surface catalyzed rxns
Photochemical rxns• Free radicals
• Reactive species
• Surface functional groups
and characterization
• Sorbed and reactive species
(both chem. & biol. origin)
DemethylationMeHg Hg(0) or Hg(II)
Microbial processes
Hg(0), Hg(II), Hg(I)
Methylmercury
MeHg
Meth
yla
tio
n
?
Bioavailable
Hg species or
Precursors?
Integrated tasks
Species Identification and
Controlling Processes
Geochemical controls
& redox couples*rates and mechanisms
Roels of Natural organic
matter (DOM and POM)*complexation and species
*dissolution and stabilization
*methylation/demethylation
Oxidation and reduction
Hg(0) Hg2+
Molecular dynamics and
simulation
Volatilization by
dissmilatory metal
reduction
Hg2+ + e- Hg0
Is a
bio
tic m
eth
yla
tio
n s
ign
ific
an
t?
Na
tura
l o
rga
nic
ma
tte
r a
nd
ca
taly
ze
d r
ea
cti
on
sN
atu
ral
org
an
ic m
att
er
an
d c
ata
lyze
d r
ea
cti
on
s
Is a
bio
tic m
eth
yla
tio
n s
ign
ific
an
t?
Na
tura
l o
rga
nic
ma
tte
r a
nd
ca
taly
ze
d r
ea
cti
on
sN
atu
ral
org
an
ic m
att
er
an
d c
ata
lyze
d r
ea
cti
on
s
Is a
bio
tic m
eth
yla
tio
n s
ign
ific
an
t?
Na
tura
l o
rga
nic
ma
tte
r a
nd
ca
taly
ze
d r
ea
cti
on
s
• Reduction by dissimilatory
metal reduction
• Ubiquitous mercury
resistance genes (mer):
Expression of enzymes
specific for demethylation
and reduction
Dem
eth
yla
tio
n
microbes
Precipitation, sorption,
particulates
e.g., Hg2+ + S= HgS
Demethylation and
reduction – mer operon
MerA: Hg2+ Hg0
MerB: HgR Hg2+ + R
MerR, MerD, MerOP, RNAP:
Transcriptional regulation
MerT, MerC – Hg2+
transport
NADPH
NADP+
Methylation
Hg2+ + ? MeHg
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Hg(0), Hg(II), Hg(I)
Methylmercury
(MeHg)
Meth
yla
tio
n
microbes
Hg
Precursors
Focus on key Hg biogeochemical processes
Task 1: Hg, MeHg flux,
geochemistry, microbial
community structure
Task 2: Species identification
and controlling processes
Dem
eth
yla
tio
nTask 3: Microbial processes,
genes sequence and genetic
controls
Task 4: Structure, dynamics,
and function of relevant
enzymes
HypothesesEnzymatic mechanisms
of transformation between
major Hg species and
methyl mercury
Oxidation, reduction,
and species transformation
Dominant chemical species
and bioavailability
Biological and abiotic
methylation and
demethylation
Coupled biogeochemical
reactions – sorption,
complexation, precipitation,
stabilization, fate and transport
Surface catalyzed and
photochemical reactions
31 Managed by UT-Battellefor the Department of Energy
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
Hg
, g
/g
(fi
sh
),
g/
L (
wate
r)
Water
Fish
So far, mercury bioaccumulation not proportional to the
concentration of waterborne Hg
ORR Examples of water/fish disconnect:
Site Hg in water Hg in fish
(ng/L) (mg/kg)
White Oak Creek 60 0.5
EF Poplar Cr (upper) 400 0.8
EF Poplar Cr (lower) 100 0.8
Bear Cr 1–3 0.7
Rogers Quarry 1 1.1
Reference site 1–3 0.2
Mercury Concentrations in FishRemain Elevated
Elimination of inorganic Hg inputs not possible; alternative
strategies that reduce methylation in-situ may be the only
way to reach fish concentration targets
Basic research needs on mercury
methylation at sediment-water
interface and particularly what limits
methyl mercury production
32 Managed by UT-Battellefor the Department of Energy
• Ab initio shape reconstruction by DAMMIN.(D. Svergun, Biophys J. 76: 2879-2886, 1999)
Low resolution shape reconstruction fromSANS/SAXS data
33 Managed by UT-Battellefor the Department of Energy
Data flow and integration
Coupled processes
Microcosm
Studies
Integrated Data Analysis,
Modeling and Assessment
Field Investigation and
Improved Understanding
Geochemistry Microbiology
Fundamental Rates
and Mechanisms
Microbial Community
and Genetic Controls
Molecular Structure,
Dynamics
Species SequenceDominant microbes
ProteinElectron transfer
Determination of site
geochemistry, dominant chemical
species and microbial community
provides critical information for
controlled laboratory mechanistic
studies
Fundamental understanding of Hg
species transformation,
geochemical controls, and genetic
determinants essential for
methylation and demethylation
processes
Critical understanding of
biomolecular mechanisms in Hg
transformation (demethylation and
methylation) using structure-
functional relationships and genetic
regulation
Geochemical modeling, molecular
simulation, and data integration for
improved understanding of field
processes and remedial controls
34 Managed by UT-Battellefor the Department of Energy
Microbial MeHg Production
• Elucidate the genetic determinants of MeHgproduction and regulation.
Comparative gene expression, mutagenesis, andcomplementation.
• Determine the effect of geochemical factors on generegulatory networks for mercury methylation.
Use whole genome microarrays to examine both bioticand abiotic effects on the methylating andnonmethylating Desulfovibrio transcriptomic profiles.
Examine relationships among community structure,geochemical conditions, and MeHg production insediments collected from Hg-contaminated sites.
Functional gene arrays
16S rRNA gene clone library analysis
Scientific Issues Addressed
EFPC downstream of Y-12
Desulfovibrio africanus(SEM by Dwayne Elias, U of
Missouri)