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Microbes in the Biosphere
From Whitman et al. 1998 PNAS 95:6578-6583:
• 4 x 1030 prokaryotic cells on Earth
–
–
–
–
–
Subsurface
Aquatic
Soils
Animals (termites)
Air
~3.8 x 1030
~1 x 1029
~2.5 x 1029
~5 x 1024
~ 5 x 1019
• If laid end to end, would span Earth-Sun
distance one trillion (1012) times.
• 350-550 Pg C = 60-100% of C in plants
• 90% of organic N, P
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Microbes R Us
• 70 x 1012 prokaryotic cells per person
– Mostly in gut: colon has 300 x 109/g
– Gut microbiome > 100 x human genome
• Human microbiome project
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“Tree of Life”
Billion years ago
• All organisms we
know of on Earth
today are
descended from a
common ancestor
that lived about 4
billion years ago.
Bacteria
Eukarya Archaea
0
4 Symbiosis of
chloroplast ancestor
with ancestor of green
plants
1
3 Symbiosis of
mitochondrial
ancestor with
ancestor of
eukaryotes
4
2
3
2 Possible fusion of
bacterium and
archaean, yielding
ancestor of
eukaryotic cells
2
3
1
Origin of life
4
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Campbell & Reece, Fig. 25.18
1 Last common
ancestor of all living
things
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Evolutionary Time
• Life originated
almost 4 billion
years ago.
• Living organisms
have
fundamentally
altered Earth.
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Campbell & Reece, Fig. 26.10
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History of life on Earth
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Microfossils
2.5-2.7 Ga microfossils
(Schopf, 2006. Phil. Trans. R. Soc. B 361: 869-885)
Cyanobacteria (Nostocales) from the Bitter Springs Chert, Central Oz, 850 Ma
(J.W. Schopf, UCLA http://www.cushmanfoundation.orgt/slides/stromato.html)
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Stromatolites
• Stromatolite fossils are
structurally
indistinguishable from
living examples
Campbell & Reece, Fig. 26.11
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Microbes are planetary engineers
• Invented all metabolism
– Catabolism
– Anabolism
• Depleted ocean of dissolved iron (Fe2+)
– Anoxygenic photosynthesis
• 4 Fe2+ + CO2 + 4 H+  4 Fe3+ + CH2O + H2O
– Oxygenic photosynthesis
• H2O + CO2 +  CH2O + O2
• 4 Fe2+ + O2 + 4 H+  4 Fe3+ + 2 H2O
• And injected oxygen into atmosphere!
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Banded Iron Formations
(Hayes, 2002, Nature 417: 127-128)
(Image courtesy of Dr. Pamela Gore,
Georgia Perimeter College)
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How did bacteria and archaea get
energy before oxygen?
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Respiration = oxidation/reduction
• Higher-energy molecules are oxidized
(lose electrons)
• Lower-energy molecules are reduced
(gain electrons)
• G = -nFE (kJ/mol)
– n = # e- transferred
– F = Faraday constant
– E = redox potential difference
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Respiration: electrons from NADH
charge a membrane pH gradient
H+
Plasma membrane
NADH
H+ electrochemical gradient
Electron transport chain
Terminal electron acceptors
O2, NO3-, SO42-, Mn4+, Fe3+,
CO2, etc.
NAD+
See also:
http://www.microbelibrary.org/images/Tterry/anim/
ETSbact.html
Electron donors {[CH2O], H2, H2S, CH4, Fe2+, etc.}
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NAD+/NADH is the cell’s main electron
(hydrogen) carrier
NAD = nicotinamide adenine dinucleotide.
For NADH + H+ +1/2 O2 ↔ NAD+ + H2O, ΔGo = -52.4 kcal/mol.
Georgia Tech School of Biology
Terminal Electron Acceptors
• Different e- acceptors are used sequentially in
microbial ecosystems, reflecting the energy yields of
different pathways.
– O2 ∆G = -479 kJ mol-1
– NO3- ∆G = -453 kJ mol-1
– Mn4+ ∆G = -349 kJ mol-1
– Fe3+
∆G = -114 kJ mol-1
– SO42- ∆G = -77 kJ mol-1
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Redox Stratification in Marine Sediments
(Jorgensen 2000, Fig. 5.11)
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Proton gradient across the plasma membrane
drives chemiosmotic ATP synthesis and active
transport
Fenchel, Origin & Early Evolution
of Life, Oxford U Press 2002, Fig 6.2
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Periplasmic space
Oxidative phosphorylation
F1 ATPase video
H+
Stator
Rotor
http://www.youtube.com/watch?v=PjdPTY1wHdQ
F0 portion in membrane
-resembles flagellar motor
F1 portion (ATP synthase)
-resembles DNA helicase
Internal
rod
Catalytic
knob
See also:
http://www.microbelibrary.org/images/Tterry/ani
m/ATPsynthbact.html
ADP
+
P
i
Cytoplasm
ATP
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Q: If the proton concentration outside the cell
is low, then
A. The rate of ATP synthesis will decrease
B. The rate of ATP synthesis will increase
C. ATP synthase will hydrolyze ATP and pump
protons out of the cell
D. ATP synthase will hydrolyze ATP and pump
protons into the cell
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Extraction of electrons from carbohydrates
to reduce NAD+
H+ electrochemical gradient
ETC
ADP
ATP NADH
NADH
Pyruvate
oxidation
Glycolysis
NAD+ CO2
Glucose, NAD+, ADP
NADH + FADH2
ATP
Citric acid cycle
NAD+
FAD
CO2
ADP
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A soil-based microbial fuel cell
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