biogeochemical reactions

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BIOGEOCHEMICAL REACTIONS Used to harness energy for biosynthesis Take advantage of chemical “potential” energy Important consequences for element cycling

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BIOGEOCHEMICAL REACTIONS. 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. - PowerPoint PPT Presentation

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Page 1: BIOGEOCHEMICAL REACTIONS

BIOGEOCHEMICALREACTIONS

Used to harness energy for biosynthesis

Take advantage of chemical “potential” energy

Important consequences for element cycling

Page 2: BIOGEOCHEMICAL REACTIONS

• 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

Page 3: BIOGEOCHEMICAL REACTIONS

• 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

Page 4: BIOGEOCHEMICAL REACTIONS

Overall ∆G is negative

DONORD→D+ and e-

ACCEPTORA←A+ and e-

BIOTA

Enzymes (electron transport) are the “teeth” on the gears

Page 5: BIOGEOCHEMICAL REACTIONS

electrons

electrons

PrimaryProduction (photosynthetic or chemosynthetic)

Decomposition

CH2OCO2

Page 6: BIOGEOCHEMICAL REACTIONS

production

decomposition

organicinorganic

Fig. x. Weathers et al., Fundamentals of Ecosystem Science

Analogous for most biologically essential elements

CO2

CH2O

e-

CO2

CH2O

e-

Page 7: BIOGEOCHEMICAL REACTIONS

EQUILIBRIA

• A + B C + D

• K = [C][D] / [A][B]– Equilibrium constant

G = G0 + rT ln CD/AB– Linked element cycles– Sources/sinks

Page 8: BIOGEOCHEMICAL REACTIONS

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

Page 9: BIOGEOCHEMICAL REACTIONS

• 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

Page 10: BIOGEOCHEMICAL REACTIONS

• 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)

Page 11: BIOGEOCHEMICAL REACTIONS

• 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)

Page 12: BIOGEOCHEMICAL REACTIONS

• CH2O + O2 CO2 + H2O + heat

C0 C+4

O0 2O-2

G = -29.8 kcal

One O2 per Carbon

Page 13: BIOGEOCHEMICAL REACTIONS

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

Page 14: BIOGEOCHEMICAL REACTIONS

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

Page 15: BIOGEOCHEMICAL REACTIONS

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-

Page 16: BIOGEOCHEMICAL REACTIONS

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

Page 17: BIOGEOCHEMICAL REACTIONS

CARBON CYCLE Fenchel et alAcademic Press.

Page 18: BIOGEOCHEMICAL REACTIONS

Fenchel et alAcademic Press.

Page 19: BIOGEOCHEMICAL REACTIONS

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

Page 20: BIOGEOCHEMICAL REACTIONS

Nitrogen Pathways (Burgin and Hamilton 2007)

Page 21: BIOGEOCHEMICAL REACTIONS

REFERENCES

Fenchel et al. 1998 Bacterial Biogeochemistry Academic Press

Stumm and Morgan. Aquatic Chemistry Wiley

Maier et al. 2000 Environmental Microbiology Academic Press