Bacterial Metabolism and Biogeochemical Cycles
Transcription
Bacterial Metabolism and Biogeochemical Cycles
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-