biogeochemical reactions

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