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

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

electrons Primary Production (photosynthetic or chemosynthetic) Decomposition CH2O CO2

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

EQUILIBRIA A + B  C + D K = [C][D] / [A][B] –Equilibrium constant  G =  G 0 + rT ln CD/AB –Linked element cycles –Sources/sinks

EQUILIBRIA A + B  C + D K = [C][D] / [A][B] –Equilibrium constant  G =  G 0 + rT ln CD/AB –Linked element cycles –Sources/sinks SLOWER Add C,D Remove A,B FASTER Remove C,D Add A,B

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

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 O 0  2O -2  G = -213 kcal Two O2 per Carbon H valence = +1 O valence is -2 (when combined)

CH2O + O2  CO2 + H2O + heat C 0  C +4 O 0  2O -2  G = kcal One O2 per Carbon

Redox couples C 0 H2O  C e - E=0.47 O 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 O 2 ) Org Matter is e - donor E=0.47 NO e -  N2 N Val = +5 Val =0 E = 0.75 Fe +3 + e -  Fe +2 E=0.77 SO e -  HS - S Val = +6 Val = -2 E = CO2 + e -  CH4 C Val = +4 Val = -4 E = -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 C 0  C -3, C -1 and C +4 Humic acids

CARBON CYCLE Fenchel et al Academic Press.

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