Terminal Electron Acceptor Processes I' Principles

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Terminal Electron Acceptor ProcessesI.
Principles theoryII. Manifestations in aquatic
systemsVolker Brüchert24 September 2007
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• Principles
• A. Reduction Oxidation Redox Chemistry

Oxidation reaction
Oxidized product
Reductant
Reduction reaction
Reduced product
Oxidant
Combined Redox reaction
3
An environmentally relevant example
2e- per O
Direction?
4

• Principles
• B. Thermodynamics of Redox Processes
• The Second Law of Thermodynamics
• ?Entropy (S) is always increasing, i.e. S is
  always positive
• For any given chemical reaction, we can derive a
  relationship between entropy, temperature, and
  enthalpy

GGibbs Free Energy the amount of energy
available or free to do work
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- reaction can proceed as written 0 reaction
is at equilibrium and will not proceed in either
direction reverse reaction can proceed
?G
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at Standard State

Pressure 1 atm Temperature 298.15 K
(25C) Concentration 1 mol dm-3 (1 M)
conditional (e.g., pH7)
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Obtaining ?G For any compound the Gibbs Free
Energy of Formation from the elements
?Gf can be
obtained (also true for enthalpy and entropy).
Elements at standard state ?Gf 0
Some examples ?Gf (kJ mol-1) C(graphite) 0 CO
2(g) -394.37 Ca2(aq) -553.54 H2
(g) 0 H(aq) 0 H2O(l) -237,18 O2(g) 0
O2(aq) 16,32
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Obtaining ?G for a given reaction
an example
?Grxn 327.87 -744.6 20 -
4-237.1840 120.5 kJ mol-1 where kJ
mol-1
Realistic?
pH0, dissolved H2S 1 M....
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Obtaining a realistic ?G for a given reaction
where R gas constant 8.314 J
mol-1K-1 TK ai activity of species
i?ici ciconcentration ?iactivity coefficient
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Obtaining a realistic ?G for a given reaction
for example, for the reaction aAbB cC
dD
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returning to our example
T 25 C activities of solids and liquids are by
definition 1 assume ? for all dissolved species
1 c(H2S)100 µM c(sulfate)28 mM pH 7.5
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Sulfur disproportionation will provide energy if
sulfide concentration is kept low.
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Redox Potentials and the Electrochemical Cell
where n number of moles of charge
tranferred F Faraday constant 96486 J
volt-1 Ecell potential (Volts)
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The Electron Tower
Electronegative Gives electrons
up Electropositve picks electrons up
e-
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The Electron Tower
Example H2 oxidation with O2 H2 ½O2 ?
H2O Electron donor H2 Electron acceptor
O2 Redox-jump, ?Eo -0,414 V ? 0,816 V
L
a
k
t
a
t
A
c
e
t
a
t

C
O
2
A
c
e
t
a
t

N
A
D
H
N
A
D
A
c
e
t
a
t
e
2
C
O
2
(
1
m
M
)
F
e
C
O
F
e
O
(
O
H
)
3
e-
-
-
N
O
N
O
2
3
2

O
M
n
2
2

3

(
F
e
F
e
)
(
b
e
i
p
H
0
)
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The Electron Tower
Example Fe(II) oxidation with Sulfate H2
½O2 ? H2O Electron donor FeCO3 Electron
acceptor Sulfate Redox-jump, ?Eo -0,13 V ?
-2.1 V Overall 0.08 V NOT ALLOWED
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• Principles
• C. Observations and theory

TEAP
TEAP
TEAP
TEAP
TEAP
TEAP
Hoehler et al., 1998
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• Principles
• C. Observations and theory

e--Acceptor Oxidation states Redox
Equivalents O2 ? H2O 0 ? -2 x2 -4 NO3- ?
½ N2 5 ? 0 -5 MnO2 ? Mn2 4 ? 2
-2 FeOOH ? Fe2 3 ? 2 -1 SO42- ? H2S 6 ?
-2 -8 Corg ? CO2 0 ? 4 4
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• Principles
• C. Observations and theory

CO2
100
SO42-
FeOOH
concentration
MnO2
NO3-
O2
time
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• Principles
• C. Observations and theory

CO2
100
SO42-
FeOOH
concentration
MnO2
NO3-
O2
time
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• Principles
• C. Observations and theory

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• Principles
• C. Observations and theory

Why this sequence?
A. Thermodynamic Energy Yield Model McKinney and
Conway , 1975 Richards, 1965 Claypool and Kaplan,
1974 Froehlich et al., 1979
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kJ molCH2O-1
Oxic Respiration CH2O O2 ? CO2 H2O
-479 Denitrification 5CH2O 4NO3- ? 2N2
4HCO3- CO2 3H2O -453 Mn(IV) Reduction CH2O
3CO2 H2O 2MnO2 ? 2Mn2 4HCO3- -349 Fe(III)
Reduction CH2O 7CO2 4Fe(OH)3 ? 4Fe2 8HCO3-
3H2O -114 Sulfate Reduction 2CH2O SO42- ?
H2S 2HCO3- -77 4H2 SO42- H ? HS-
4H2O -152 Methane Production 4H2 HCO3- H
? CH4 3H2O -136 CH3COO- H ? CH4
CO2 -28 Acetogenesis 4H2 2CO3- H ?
CH3COO- 4H2O -105 Fermentation CH3CH2OH H2O
? CH3COO- 2H2 H 10 CH3CH2COO- 3H2O ?
CH3COO- HCO3- 3H2 H 77
lt250 µM lt50 µM solid phase lt100 µmol/g solid
phase lt100 µmol/g 28 mM 3 50 mM 3 50 mM
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B. Hydrogen threshold model
H2 as key intermediate in the transformation of
organic matter
H2 ? 2H 2e-

• Reversibility
• Low redox potential allows oxidation with all
  common major natural electron acceptors

Hoehler et al., 1998
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The Electron Tower
Electronegative Gives electrons
up Electropositve picks electrons up
e-
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B. Hydrogen Threshold Model
H2 as key intermediate in the transformation of
organic matter
H2 ? 2H 2e-

• Reversibility
• Low redox potential allows oxidation with all
  common major natural electron acceptors
• High diffusivity easily transported across cell
  membranes

CO2/H2
Hoehler et al., 1998
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Microbial reactions involving hydrogen
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Syntrophy of two bacteriaInterspecies hydrogen
transfer
Example Degradation of benzoate to acetate and
hydrogen
At concentrations of 1 mol/l, this reaction is
thermodynamically unfavorable!
Solution reaction partner that consumes the
product
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?G as a function of H2 concentration for the
syntrophic fermentation of benzoate
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Reaction 1
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All compounds except hydrogen at 1 molar
activities, pH 7
0
Minimum free energy
DG (kJ/mol)
-50
-100
Reaction 2
-150
-200
-14
-12
-10
-8
-6
-4
-2
0
Log H2 (mol/l)
Hydrogen (mol/l)
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Hydrogen threshold concept (after Goodwin and
Lovley, 1988)
(Data from Hoehler et al. 1998)
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Hydrogen threshold concept (after Goodwin and
Lovley, 1988)
? G
-180
(Hoehler et al. 1998)
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? G ? G of next TEAP down the e Tower
-180
(Hoehler et al. 1998)
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Prokaryotes in marine systems operate close to
equilibrium with respect to energy
? G at given H2 conc. ? G of next TEAP down the e
tower
-180
(Hoehler et al. 1998)
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Hydrogen concentration as a function of Terminal
electron acceptor for Cape Lookout Bight
sediments (Hohler et al. 1998)
H2
nM
0
80
40
120
SO42- mM
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Hydrogen concentration as a function of
temperature for Cape Lookout Bight sediments
(Hohler et al. 1998)
H2
nM
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5
25
15
Temperature
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Take-home Concepts

• Basis for calculating free energies of reactions
  under environmental conditions
• Predictive power of thermodynamics in determining
  the sequence of terminal electron acceptors
• How and why TEAPs are thought to function in
  aquatic systems
• Role of hydrogen as an intermediate

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A. Consider the Reaction CH4(g) SO42- ?
H2S(aq) CO2 (aq) and given the following
information assume activity coefficient 1 and T
10C 1. Balance the reaction 2. Calculate
?G rxn 3. Calculate ?G rxn for the conditions
listed below 4. How would temperature possibly
influence this reaction? How about pH? 5. T.
Hohler has proposed that the reaction shown above
is the result of a syntrophic association of two
prokaryote groups. Describe how this might
function. C. Would you expect the sequence of
TEAPs to be different in near-shore as opposed to
deep-sea sediments? Why or why not?