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BIOGEOCHEMICAL REACTIONS

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.

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BIOGEOCHEMICAL REACTIONS

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  1. BIOGEOCHEMICALREACTIONS Used to harness energy for biosynthesis Take advantage of chemical “potential” energy Important consequences for element cycling

  2. 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

  3. 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+ + A Summed G must be negative for reaction to yield energy

  4. DONOR D→D+ and e- BIOTA Overall ∆G is negative Enzymes (electron transport) are the “teeth” on the gears ACCEPTOR A←A+ and e-

  5. electrons Primary Production (photosynthetic or chemosynthetic) CH2O CO2 electrons Decomposition

  6. Analogous for most biologically essential elements e- CO2 CH2O production inorganic organic decomposition CH2O CO2 e- Fig. x. Weathers et al., Fundamentals of Ecosystem Science

  7. EQUILIBRIA • A + B  C + D • K = [C][D] / [A][B] • Equilibrium constant • G = G0 + rT ln CD/AB • Linked element cycles • Sources/sinks

  8. EQUILIBRIA • A + B  C + D • K = [C][D] / [A][B] • Equilibrium constant • G = G0 + rT ln CD/AB • Linked element cycles • Sources/sinks SLOWER Add C,D Remove A,B FASTER Remove C,D Add A,B

  9. Many important biogeochemical reactions involve electron transfer (redox reactions) • G = -nFE (E is voltage) + voltage implies spontaneous n is # moles of electrons (equivalents) F is Faraday’s constant

  10. 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)

  11. CH4 + 2 O2  CO2 + 2 H2O + heat C-4 C+4 O0 2O-2 G = -213 kcal Two O2 per Carbon H valence = +1 O valence is -2 (when combined)

  12. CH2O + O2  CO2 + H2O + heat C0  C+4 O0 2O-2 G = -29.8 kcal One O2 per Carbon

  13. 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

  14. Different electron acceptors (not O2)Org Matter is e- donor E=0.47 • NO3- + e- N2 N Val = +5 Val =0 E = 0.75 • Fe+3 + e-  Fe+2 E=0.77 • SO4-2 + e- HS- S Val = +6 Val = -2 E = -0.22 • CO2 + e- CH4 C Val = +4 Val = -4 E = -0.24

  15. 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-

  16. 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

  17. Fenchel et al Academic Press. CARBON CYCLE

  18. Fenchel et al Academic Press.

  19. Process Reaction Conditions Who

  20. Nitrogen Pathways (Burgin and Hamilton 2007)

  21. REFERENCES Fenchel et al. 1998 Bacterial Biogeochemistry Academic Press Stumm and Morgan. Aquatic Chemistry Wiley Maier et al. 2000 Environmental Microbiology Academic Press

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