Geobiology Week 3 How do microbes garner energy and carbon? Review of redox couples, reaction potential and free energy yields Hydrogen as an energy currency for subsurface microbes. Acknowledgements: Tori Hoehler Redox structure of modern microbial ecosystems Deep biosphere as an analogue of Early Earth Ecosystems ‡O 2 as a driver of biological innovation Readings : Brock Biology of Microorganisms. Hoehler et al., 1998.Thermodynamic control on hydrogen concentration in anoxic sediments Geochim. Cosmochim. Acta 62: 1745-1756. Hoehler TM, et al., 2002. Comparative ecology of H2 cycling in sedimentary and phototrophic ecosystems Antonie von Leeuwenhoek 81: 575- 582. Hoehler et al., 2001. Apparent minimum free energy requirements for methanogenic Archaea and Sulfate reducing bacteria in an anoxic marine sediment. FEMS Microbial Ecol. 38; 33-41.
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Geobiology Week 3
How do microbes garner energy and carbon?
Review of redox couples, reaction potential and free energy
yields
Hydrogen as an energy currency for subsurface microbes.
Acknowledgements: Tori Hoehler
Redox structure of modern microbial ecosystems
Deep biosphere as an analogue of Early Earth Ecosystems
‡O2 as a driver of biological innovation
Readings : Brock Biology of Microorganisms. Hoehler et al., 1998.Thermodynamic
control on hydrogen concentration in anoxic sediments Geochim. Cosmochim.
Acta 62: 1745-1756. Hoehler TM, et al., 2002. Comparative ecology of H2 cycling
in sedimentary and phototrophic ecosystems Antonie von Leeuwenhoek 81: 575-
582. Hoehler et al., 2001. Apparent minimum free energy requirements for
methanogenic Archaea and Sulfate reducing bacteria in an anoxic marine
sediment. FEMS Microbial Ecol. 38; 33-41.
A staggering number of organism-organism and organism-environment interactions underlie global biogeochemistry
These can be studied at vastly different spatial and time scales
PRESS RELEASEDate Released: Thursday, February 21, 2002
Texas A&M UniversityRock-eating microbes survive in deep ocean off Peru
Rock-eating microbes survive in deep ocean off Peru Way
down deep in the ocean off the coast of Peru, in the rocks
that form the sea floor, live bacteria that don't need sunlight,
don't need carbon dioxide, don't need oxygen. These
microbes subsist by eating the very rocks they call home.
Researchers from the Ocean Drilling Program (ODP) have
embarked aboard the world's largest scientific drillship on a
voyage to understand the abundance and diversity of these
microbes and the environments in which they live.
Biogeochemical Redox CouplesWhat is the energy currency of metabolicreactions in cells ??
How do cells make it ?
What powers those reactions?
How do we measure the energy outputs orrequirements of metabolism?
How can we use this kind of information in anecological and biogeochemical sense?
Biogeochemical Redox CouplesD
CO2 + H2O ‡ CH2 O + O2 oxygenic photosynthesis
Interdependency?
CH2 O + O2 ‡ CO2 + H2O (+D) aerobic respiration
oxidative methanotrophy
D CH4 + 2O2 ‡ CO2 + 2H2O (+D)
anoxygenicCO2 + HS- + H2O ‡ biomass + SO4
2-photosynthesis
O6 ‡ 2CO2 + 2C2H6O (+D) fermentationC6H12
4H2 + SO42-‡ S2- + 4H2O (+D)
sulfate reduction
methanogenesisCO2 + 2H2 ‡ CH4 + 2H2O (+D)
pe(W)
–10
0
+10
–10
0
+10
E (V)oDGP680* P680+
OXIDATION
NO3
NO3
-0.5
& Energy Yields H
+H2 H
+ H2
NH4+ N2 NH4
+N2 The electron tower…….. CH4 CO2 CO2 CH4
100
+0.5
Fe3+ ‡ Fe2+ +0.76 V
Redox Potentials kJ/mol e-
CO2 CH2OCH2O CO2
S H2SH2S S Strongest reductants, or e donors, 2–SO4
2–SO4 H2SH2S on top LHS
Fe2+ Fe2+Fe(OH)3 Fe(OH)3 0 Electrons ‘fall’ until they are
‘caught’ by available acceptors
REDUCTION The further they fall before being
NH4+ 50 caught, the greater the differenceNO2–
32–+NH4
in reduction potential and energy2–NO3
2–NO2 NO2 released by the coupled reactionsMnO2 Mn2+Mn2+ MnO2
CO CO2 CO2 CO
2–N NO3 2–NO3 N22
0H2O O2 H2OO2 (Last Common Ances
P680+ P680 +1.0
pe(W)
OXIDATION
0
+10
0
+10
–10 –10
E (V)oDGP680+
NO3
NO3
P680* kJ/mol e- -0.5 Redox Potentials CO2 CH2O
100
+0.5
Fe3+ ‡ Fe2+ +0.76 V
CH2O CO2
& Energy Yields + +HH2 H H2 +
NH4 N2 NH4+
N2 CH4 CO2 CO2 CH4 Reaction must be exergonic (-ve DG)
S H2SH2S S 2– SO2–
4SO4H2S H2S The energetically most favored
reaction proceeds first ieFe2+ Fe2+Fe(OH)3 Fe(OH)3 0
CH2O first degraded with O2 -CH2O degraded with NO3 nextREDUCTION
CH2O degraded with Mn4+ next2– + 50NO3
2–
2–
+NH4
NO2
NH4 followed by SO42-, 2–NO3 NO2
Mn2+Mn2+ MnO2MnO2 and CO2 last (methanogenesis)CO CO2 CO2 CO
2–N NO3 2–NO3 N22
0H2O O2 H2O (Last Common Ancestor) O2
P680+ P680 +1.0
Energy Calculations
aA +bB ‡ cC + cD
DG = Gf°’ (aA + bB) – Gf °’ (cC + dD)
Where Gfo’ is the free energy of formation of 1 mole
1ATP ‡ 7kcal/mole so 1 molecule glucose ‡ 266 kcal
Glucose oxidation with O2 DG = 688kcal Therefore aerobic respiration ca. 39% efficient
In contrast, glucose fermentation ‡ lactate = 29 kcal/mol ca. 50% efficient
Reactions of the TCA Cycle
Pyruvate
The TCA cycle showing enzymes, substrates and products. The abbreviated enzymes are: IDH = isocitrate dehydrogenase and a-KGDH = a-ketoglutarate dehydrogenase. The
GTP generated during the succinate thiokinase (succinyl-CoA synthetase) reaction is
equivalent to a mole of ATP by virtue of the presence of nucleoside diphosphokinase.
The 3 moles of NADH and 1 mole of FADH2 generated during each round of the cycle feed
into the oxidative phosphorylation pathway. Each mole of NADH leads to 3 moles of ATP
and each mole of FADH2 leads to 2 moles of ATP. Therefore, for each mole of pyruvate
which enters the TCA cycle, 12 moles of ATP can be generated