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 AnaerobicAerobic Carbon Fixation CO 2 Respiration And Fermentation Organic Matter CH 2 O Respiration CO 2 Methane Oxidation Methanogenesis H2H2 CH 4

Methanogenesis Autotrophic Acetoclastic CO 2 -CHOCH 2 OHCoM-CH 3 CH 4 H2H2 H2H2 H2H2 H2H2 CoEnzyme M CH 3 COOHCH 3 CO CoM-CH 3 COCO 2 CH 4 H2OH2O 2H CoEnzyme M

Nitrogen Cycle Assimilitory Nitrate Reduction Nitrification Ammonification Nitrogen Fixation Denitrification Organic NNH 3 N 2 + N 2 O NH 4 + NO 2 - NO 3 -

NO 2 - NON2ON2O N2N2 Denitrification Nitrate reductase Nitrite reductase Nitrous oxide reductase e - 1e -

Sulfur Cycle SO 4 -2 Sulfate Reduction (Assimilitory) Organic Sulfur H2SH2S Mineralization Sulfur Oxidation Sulfate Reduction (Dissimilitory) Elemental Sulfur Sulfur Oxidation Sulfur Reduction

SO 4 -2 ATP APS 2 ADP 2 ATP SO 3 -2 S3O6S3O6 S 2 O 3 -2 Sulfate Reduction

Sulfur Reduction Thiosulfate Disproportionation S 2 O H 2 OSO HS - + H + S 0 + H 2 HS - + H +

Winogradsky Column Animation

REDOX Potentials (electron tower) 1/2 O 2 / H 2 O Fe +3 / Fe +2 NO -3 / NO -2 CH 3 OH / CH 4 SO 3 -2 / S -2 2H + / H 2 CO 2 / CO

Metal Reduction Fe +3 Fe +2 As +5 As +3 SeO 4 -2 Se 0 SeO 3 -2 HSe - MnO 2 Mn +2 CrO 4 -2 Cr +3 1 e - 2 e - 4 e - 2 e - 3 e -