the redox ladder1

0.5

0

-0.5Eh (V)

O2 H2O

NO3- NO2

-

NO2

- NH4+

Mn+4 Mn+2

FeOOH Fe+2

SO4-2 HS-

CO2 CH4

H+ H2HCOO-

CH2O

clockwise:spontaneous,can produce free energy(catabolic)

ccw:requires free energy(anabolic)

half-reactioncoupling:

Nir Krakauer2/’04

Oxygen units

• In air (sea level): 0.21 atm = 160 Torr = present atmospheric level (PAL)

• In water at equilibrium with PAL: 9 ml/l at 0 °C, 5 ml/l at 25 °C

Geochemical evidence for atmospheric O2

• >2.3 Gy BP: detrital UO2, FeCO3, FeS2; photolytic? Mass-independent fractionation of S

→ O2 at <~0.01 PAL (Berkner and Marshall [1965]: photolysis of H2O generates <<10-3 PAL)

• 2.3> Gy: red beds, MnO2 fields

→ O2 at >0.01 PAL

Oxygen in the Proterozoic• Canfield and Teske

(1996) argue based on sedimentary S isotopes for around 0.1 PAL in the Late Proterozoic, so that there would be just enough O2 to oxidize sulfide on shelf bottoms

• Anbar and Knoll (2002):

Eukaryotes evolved in an oxic world • Eukaryote anaerobic respiration uses

organic electron acceptors like pyruvate, so that it is inefficient

• Sterols, eukaryotic cell membrane constituents, are always made with O2

• The first eukaryotes likely didn’t have plastids and couldn’t produce O2

• Aerobic respiration can occur quite well at ~0.01 PAL O2, the Pasteur point

so why aren’t there big eukaryotes much before the Cambrian?

• Berkner and Marshall (1965): not enough oxygen for land and sea surface UV shielding

• Towe (1969): making collagen demands a lot of oxygen

• Rhodes and Morse (1971): products of anaerobic metabolism inhibit calcification

• Runnegar (1981): oxygen levels not high enough to diffuse into complex organisms

• Anbar and Knoll (2002): metal and N limitation