Nir Krakauer 2 04 the redox ladder halfreaction

Nir Krakauer 2/’ 04 the redox ladder half-reaction coupling: clockwise: spontaneous, can produce free energy (catabolic) ccw: requires free energy (anabolic) 1 O 2 - NO 3 NO 2 Mn+4 H 2 O 0. 5 NO 2 NH 4+ Mn+2 Fe. OOH 0 Fe+2 SO 4 -2 CO 2 H+ HCOO- HSCH 4 H 2 -0. 5 CH 2 O Eh (V)

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 O 2 • >2. 3 Gy BP: detrital UO 2, Fe. CO 3, Fe. S 2; photolytic? Mass-independent fractionation of S → O 2 at <~0. 01 PAL (Berkner and Marshall [1965]: photolysis of H 2 O generates <<10 -3 PAL) • 2. 3> Gy: red beds, Mn. O 2 fields → O 2 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 O 2 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 O 2 • The first eukaryotes likely didn’t have plastids and couldn’t produce O 2 • Aerobic respiration can occur quite well at ~0. 01 PAL O 2, the Pasteur point

so why aren’t there big eukaryotes much before the Cambrian? • Berkner and Marshall (1965): not enough oxygen for land 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