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 ReactionsChapter 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 CarriersChapter 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 CompoundsChapter 5

Metabolism

• Fermentation– Transfer of electrons to organic substrate

• Respiration– Transfer of electrons to inorganic acceptor

Glycolysis

Fermentation

Respiration

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 CycleAnaerobic Aerobic

Carbon Fixation Carbon Fixation

CO2Respiration

AndFermentation

Organic MatterCH2O

Respiration

CO2

Methane Oxidation

MethanogenesisH2

CH4

MethanogenesisAutotrophic

Acetoclastic

CO2 -CHO CH2OH CoM-CH3 CH4

H2 H2 H2 H2

CoEnzyme M

CH3COOH CH3CO

CoM-CH3

CO CO2

CH4

H2O2H

CoEnzyme M

Nitrogen Cycle

AssimilitoryNitrate Reduction

Nitrification

NitrificationAmmonification

Nitrogen Fixation

Denitrification

Organic N NH3N2 + N2O

NH4+

NO2-

NO3-

NO3- NO2

- NO N2O N2

Denitrification

Nitratereductase

Nitritereductase

Nitrous oxidereductase

+5 +3 +2 +1 02e- 1e- 1e- 1e-

Sulfur CycleSO4

-2

Sulfate Reduction(Assimilitory)

Organic Sulfur

H2S

MineralizationSulfur Oxidation

Sulfate Reduction(Dissimilitory) Elemental Sulfur

Sulfur Oxidation

Sulfur Reduction

SO4-2

ATP

APS

2 ADP2 ATP

SO3-2

S3O6

S2O3-2

Sulfate Reduction

Sulfur Reduction

Thiosulfate Disproportionation

S2O3-2 + H2O SO4

-2 + HS- + H+

S0 + H2 HS- + H+

Winogradsky Column

• Animation

REDOX Potentials(electron tower)

1/2 O2 / H2OFe+3 / Fe+2

NO-3 / NO-2

CH3OH / CH4

SO3-2 / S-2

2H+ / H2

CO2 / CO

Metal Reduction

Fe+3 Fe+2

As+5 As+3

SeO4-2 Se0SeO3

-2 HSe-

MnO2 Mn+2

CrO4-2 Cr+3

1 e-

2 e-

2 e-

2 e- 4 e- 2 e-

3 e-