1. Nir Krakauer 2/’04 the redox ladder half-reaction coupling: 1 O2 H2O clockwise: spontaneous, can produce free energy (catabolic) 0.5 NO3- NO2- NO2- NH4+ ccw: requires free energy (anabolic) Mn+4 Mn+2 0 FeOOH Fe+2 SO4-2 HS- CH4 CO2 H2 H+ HCOO- CH2O -0.5 Eh (V)

2. 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

3. 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

4. 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):

5. 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 PALO2, the Pasteur point

6. 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