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Nir Krakauer 2/’04. the redox ladder. half-reaction coupling:. 1. O 2. H 2 O. clockwise: spontaneous, can produce free energy (catabolic). 0.5. NO 3 -. NO 2 -. NO 2 -. NH 4 +. ccw: requires free energy (anabolic). Mn +4. Mn +2. 0. FeOOH. Fe +2. SO 4 -2. HS -. CH 4.

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the redox ladder

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)

oxygen units
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
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
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
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
so why aren t there big eukaryotes much before the cambrian
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