Bacterial Metabolism and Biogeochemical Cycles

1. Bacterial Metabolism and Biogeochemical Cycles

2. 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

3. 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

4. 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

5. Metabolism • Fermentation • Transfer of electrons to organic substrate • Respiration • Transfer of electrons to inorganic acceptor

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. Carbon Cycle Anaerobic Aerobic Carbon Fixation Carbon Fixation CO2 Organic Matter CH2O Respiration And Fermentation CO2 Respiration Methanogenesis H2 Methane Oxidation CH4

13. Methanogenesis Autotrophic H2 H2 H2 H2 CO2 -CHO CH2OH CoM-CH3 CH4 CoEnzyme M Acetoclastic CoM-CH3 CH4 CoEnzyme M H2O CH3COOH CH3CO 2H CO CO2

14. Nitrogen Cycle Assimilitory Nitrate Reduction NO3- Nitrification Denitrification NO2- N2 + N2O Organic N NH3 Nitrogen Fixation Ammonification Nitrification NH4+

15. Denitrification +5 +3 +2 +1 0 2e- 1e- 1e- 1e- NO3- NO2- NO N2O N2 Nitrite reductase Nitrate reductase Nitrous oxide reductase

16. Sulfur Cycle SO4-2 Sulfate Reduction (Assimilitory) Sulfur Oxidation Sulfate Reduction (Dissimilitory) Elemental Sulfur Organic Sulfur Sulfur Reduction Sulfur Oxidation Mineralization H2S

17. Sulfate Reduction SO4-2 ATP APS SO3-2 S3O6 S2O3-2 2 ADP 2 ATP

18. Sulfur Reduction HS- + H+ S0 + H2 Thiosulfate Disproportionation S2O3-2 + H2O SO4-2 + HS- + H+

19. Winogradsky Column • Animation

20. 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

21. Metal Reduction 1 e- Fe+3 Fe+2 2 e- MnO2 Mn+2 2 e- As+3 As+5 2 e- 4 e- 2 e- SeO4-2 SeO3-2 Se0 HSe- 3 e- CrO4-2 Cr+3