Bacterial Metabolism and
Biogeochemical Cycles
Redox Reactions
• All chemical reactions consist of transferring
electrons from a donor to an acceptor.
• Chemicals that donate electrons become
oxidized.
• Chemicals that accept electrons become
reduced.
Oxidation / Reduction Reactions
Chapter 5
Redox Reactions
• Energy is released during these electron
transfers.
• In order to capture that energy, bacteria need
to intercept the electrons during redox
reactions
Electron Carriers
Chapter 5
Metabolism
• The goal of metabolism is to conserve the
energy released during redox reactions by
making high energy compounds such as ATP.
• There are different strategies for conserving
this energy.
High Energy Compounds
Chapter 5
Metabolism
• Fermentation
– Transfer of electrons to organic substrate
• Respiration
– Transfer of electrons to inorganic acceptor
Glycolysis
• The initial stage of glucose metabolism is the
same in both fermentation and respiration.
• Glucose is partially oxidized to pyruvate and
energy is conserved through substrate-level
phosphorylation.
Glycolysis
Chapter 5
Fermentation
• In the absence of an external electron
acceptor, bacteria need to regenerate NAD+
from NADH.
• They do this by transferring the extra
electrons back onto the pyruvate.
Fermentation
Chapter 5
Respiration
• If an external electron acceptor is present,
bacteria can extract much more energy by
completely oxidizing the pyruvate.
• The series of chemical reactions that
accomplish complete oxidation is called the
Krebs Cycle.
Krebs Cycle
Chapter 5
Electron Transport Chain
• The Krebs cycle produces many more reduced
electron carriers than glycolysis.
• These carriers are regenerated by passing the
electrons and protons into the electron transport
chain (ETC).
• The ETC passes the electrons to a terminal electron
acceptor and pushes the protons outside of the cell.
• The amount of energy generated depends on the
terminal electron acceptor used.
Electron Transport
Proton Motive Force
• The accumulation of protons on the outside of
the cell membrane produces an electrical
charge gradient that can be used to do work.
• One of the most important uses of this proton
motive force (PMF) is to drive the synthesis of
ATP.
ATP Synthase
Biogeochemical Cycles
• Different nutrients undergo redox reactions as
electron donors and acceptors during
bacterial metabolism.
• These reactions help to cycle the nutrients
through different chemical forms.
• Three of the most important cycles are:
– Carbon
– Nitrogen
– Sulfur
Carbon Cycle
Anaerobic
Aerobic
Carbon Fixation
CO2
Respiration
And
Fermentation
H2
Carbon Fixation
Organic Matter
CH2O
CO2
Respiration
Methanogenesis
Methane Oxidation
CH4
Methanogenesis
Autotrophic
H2
CO2
H2
-CHO
H2
CH2OH
H2
CoM-CH3
CH4
CoEnzyme M
Acetoclastic
CoEnzyme M
CoM-CH3
CH4
H2O
CH3COOH
CH3CO
2H
CO
CO2
Nitrogen Cycle
Assimilitory
Nitrate
Reduction
NO3Nitrification
Denitrification
Organic N
N2 + N2O
NH3
Nitrogen Fixation
Ammonification
NO2-
Nitrification
NH4+
Denitrification
+5
2e-
NO3-
+3
NO2Nitrate
reductase
+2
1e-
NO
Nitrite
reductase
1e-
+1
1e-
N 2O
Nitrous oxide
reductase
0
N2
Sulfur Cycle
SO4-2
Sulfate Reduction
(Assimilitory)
Organic Sulfur
Sulfur Oxidation
Sulfate Reduction
(Dissimilitory)
Elemental Sulfur
Sulfur Reduction
Sulfur Oxidation
Mineralization
H2S
Sulfate Reduction
SO4-2
APS
SO3-2
S3O6
S2O3-2
2 ADP
2 ATP
ATP
Sulfur Reduction
S0 + H2
HS- + H+
Thiosulfate Disproportionation
S2O3-2 + H2O
SO4-2 + HS- + H+
Winogradsky Column
• Animation
REDOX Potentials
(electron tower)
CO2 / CO
2H+ / H2
SO3-2 / S-2
CH3OH / CH4
NO-3 / NO-2
Fe+3 / Fe+2
1/2 O2 / H2O
Metal Reduction
Fe+3
1 e-
Fe+2
MnO2
2 e-
Mn+2
2 e-
As+3
As+5
SeO4-2
CrO4-2
2 e-
SeO3-2
3 e-
4 e-
Cr+3
Se0
2 e-
HSe-