BIOGEOCHEMICAL REACTIONS
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BIOGEOCHEMICALREACTIONS
Used to harness energy for biosynthesis
Take advantage of chemical “potential” energy
Important consequences for element cycling
• Chemical potential energy implies a reaction yields net energy although may require activation/catalysis.
G = H - T S = Gibbs Free Energy
• = change in enthalpy - T *change in entropy– If negative, reaction will proceed– If positive requires energy input– For most biology can neglect 2nd term
• Many important biogeochemical reactions involve electron transfer (redox reactions)– Donor Donor + and e- (G = pos or neg)
– Acceptor+ and e- Acceptor (G = pos or neg)
D + A+ D+ + ASummed G must be negative for reaction to yield energy
Overall ∆G is negative
DONORD→D+ and e-
ACCEPTORA←A+ and e-
BIOTA
Enzymes (electron transport) are the “teeth” on the gears
electrons
electrons
PrimaryProduction (photosynthetic or chemosynthetic)
Decomposition
CH2OCO2
production
decomposition
organicinorganic
Fig. x. Weathers et al., Fundamentals of Ecosystem Science
Analogous for most biologically essential elements
CO2
CH2O
e-
CO2
CH2O
e-
EQUILIBRIA
• A + B C + D
• K = [C][D] / [A][B]– Equilibrium constant
G = G0 + rT ln CD/AB– Linked element cycles– Sources/sinks
EQUILIBRIA
• A + B C + D
• K = [C][D] / [A][B]– Equilibrium constant
G = G0 + rT ln CD/AB– Linked element cycles– Sources/sinks
SLOWERAdd C,DRemove A,B
FASTERRemove C,DAdd A,B
• Many important biogeochemical reactions involve electron transfer (redox reactions)
G = -nFE (E is voltage)+ voltage implies spontaneousn is # moles of electrons (equivalents)F is Faraday’s constant
• CH4 + 2 O2 CO2 + 2 H2O + heat
• CH2O + O2 CO2 + H2O + heat
• Both are redox reactions ie something gets oxidized (valence goes up); something gets reduced (valence goes down)
• CH4 + 2 O2 CO2 + 2 H2O + heat
C-4 C+4
O0 2O-2
G = -213 kcal
Two O2 per Carbon
H valence = +1O valence is -2 (when combined)
• CH2O + O2 CO2 + H2O + heat
C0 C+4
O0 2O-2
G = -29.8 kcal
One O2 per Carbon
Redox couples
• C0 H2O C+4 + 4 e-
E=0.47
• O02 + 4 e- 2O-2 E=0.81
= CH2O + O2 CO2 + H2O E = 1.28 v
CH2O is the electron donor O2 is the electron acceptor
Different electron acceptors (not O2)Org Matter is e- donor E=0.47
• NO3- + e- N2
N Val = +5 Val =0E = 0.75
• Fe+3 + e- Fe+2
E=0.77
• SO4-2 + e- HS-
S Val = +6 Val = -2E = -0.22
• CO2 + e- CH4C Val = +4 Val = -4E = -0.24
Other electron donors (not organic matter)
All have + E
• Mn +2 + O2 Mn +4 + H2O
• Fe +2 + O2 Fe +3 + H2O
• NH4+ + O2 NO3- (nitrification)
• H2 H+ e-
Fermentation(No “external” electron acceptor)
• Methanogenesis
CH3COOH CH4 + CO2– (C-3) (C+3) (C-4) (C+4)
• C3H6O3 CH3CH2OH + CO2
C0 C-3 , C-1 and C+4
• Humic acids
CARBON CYCLE Fenchel et alAcademic Press.
Fenchel et alAcademic Press.
N fixation
(reduction)
N2 Org N
(protein)
Anoxic;
Requires
Energy
Rhizobium
Cyanobct
Nitrification
(oxidation)
NH3 NO3 Oxic;
Yields energy
Chemoauto-trophic
Denitrification
(reduction)
NO3 N2 Accepts electrons
Widely distributed
Assimilation
(Same valence)
(reduction)
NH3 Org N
NO3 NH3
Intra-
cellular
Plants, fungi
bacteria
Process Reaction Conditions Who
Nitrogen Pathways (Burgin and Hamilton 2007)
REFERENCES
Fenchel et al. 1998 Bacterial Biogeochemistry Academic Press
Stumm and Morgan. Aquatic Chemistry Wiley
Maier et al. 2000 Environmental Microbiology Academic Press