Nir Krakauer
2/’04
the redox ladder
half-reaction
coupling:
clockwise:
spontaneous,
can produce
free energy
(catabolic)
ccw:
requires
free energy
(anabolic)
1
O2
-
NO3
NO2
Mn+
4
H 2O
0.5
NO2NH4+
Mn+
2
FeOOH
0 Fe+2
SO42
CO2
H+
HCOO-
HSCH4
H2
-0.5CH2O
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 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