Microbial Metabolism: Anaerobic Respiration & Fermentation

• Microbial metabolism involves a multitude of chemical reactions that occur in microorganisms

Anaerobic Respiration

• The final electron acceptor in the electron transport chain is not O₂
• Anaerobic respiration yields less energy than aerobic respiration
• Nitrate (NO3-) as an electron acceptor produces nitrite (NO2-) or nitrogen gas (N2) and water
• Sulfate (SO4-) as an electron acceptor produces hydrogen sulfide (H2S) and water
• Carbonate (CO32-) as an electron acceptor produces methane (CH4) and water

Fermentation

• Releases energy from the oxidation of organic molecules
• Does not require oxygen
• Does not use the Krebs cycle or the electron transport chain (ETC)
• Uses an organic molecule as the final electron acceptor
• Produces only small amounts of ATP

Lactic Acid Fermentation

• Lactic acid fermentation produces lactic acid
• Homolactic fermentation produces lactic acid only
• Heterolactic fermentation produces lactic acid and other compounds
• Glucose is oxidized to pyruvic acid which is then reduced by NADH

Alcohol Fermentation

• Alcohol fermentation produces ethanol and CO2
• Glucose is oxidized to pyruvic acid which is then converted to acetaldehyde and CO2
• NADH reduces acetaldehyde to ethanol
• Streptococcus, Lactobacillus, and Bacillus perform lactic acid fermentation yielding lactic acid
• Saccharomyces (yeast) performs fermentation yielding ethanol and CO2
• Propionibacterium performs fermentation yielding propionic acid, acetic acid CO2, and H2
• Clostridium performs fermentation yielding butyric acid, butanol, acetone, isopropyl alcohol, and CO2
• Escherichia and Salmonella, perform fermentation yielding ethanol, lactic acid, succinic acid, acetic acid, CO2, and H2
• Enterobacter performs fermentation yielding ethanol, lactic acid, formic acid, butanediol, acetoin, CO2, and H2

Biochemical Tests

• Fermentation tests can identify bacteria that catabolize carbohydrates or protein, producing acid and changing the color of a pH indicator
• Oxidase tests identify bacteria with cytochrome oxidase, such as Pseudomonas

Energy Production

• Glycolysis produces ATP and reduces NAD+ to NADH while oxidizing glucose to pyruvic acid
• In respiration, pyruvic acid is converted to acetyl CoA, which is the first reactant in the Krebs cycle
• The Krebs cycle produces some ATP by substrate-level phosphorylation, reduces the electron carriers NAD+ and FAD, and gives off CO2
• Electron carriers from both glycolysis and the Krebs cycle donate electrons to the electron transport chain
• In the electron transport chain, the energy of the electrons is used to produce a great deal of ATP by oxidative phosphorylation
• In respiration, the final electron acceptor comes from outside the cell
• In fermentation, the final electron acceptor is a molecule made in the cell
• In fermentation, pyruvic acid and the electrons carried by NADH from glycolysis are incorporated into fermentation end-products

Catabolism

• Protein catabolism involves extracellular proteases that break down proteins into amino acids which can then be used in the Krebs cycle
• Lipid catabolism involves breaking down lipids (fats) into glycerol and fatty acids
• Glycerol is converted to dihydroxyacetone phosphate and then glyceraldehyde-3-phosphate, which enters glycolysis
• Fatty acids undergo beta-oxidation to form acetyl CoA, which enters the Krebs cycle

Photosynthesis

• Light-dependent (light) reactions: convert light energy into chemical energy (ATP and NADPH)
• Light-independent (dark) reactions: use ATP and NADPH to reduce CO2 to sugar (carbon fixation) via the Calvin-Benson cycle
• Oxygenic photosynthesis formula: 6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 H2O + 6 O2

Metabolic Diversity

• Phototrophs use light energy
• Photoautotrophs use energy in the Calvin-Benson cycle to fix CO2 to sugar
  • Oxygenic photoautotrophs produce O2
  • Anoxygenic photoautotrophs do not produce O2
• Photoheterotrophs use organic compounds as sources of carbon
• Chemoautotrophs use energy from inorganic chemicals and CO2 as a carbon source
• Chemoheterotrophs use energy and carbon from organic chemicals
• Cyanobacteria are photoautotrophs
• Green bacteria and purple nonsulfur bacteria are examples of photoheterotrophs
• Iron-oxidizing bacteria are examples of chemoautotrophs
• Fermentative bacteria, animals, protozoa, and fungi are examples of chemoheterotrophs

Biosynthesis

• Glycolysis is used to generate glucose for biosynthesis of Glycogen in bacteria via ADPG
• Glycolysis is used to generate glucose for biosynthesis of Glycogen in animals via UDPG
• Glycolysis is used to generate glucose for biosynthesis of Peptidoglycan in bacteria via UDPNAC
• Glycolysis is used to generate glucose for biosynthesis of simple lipids via glycerol, fatty acids, and acetyl CoA
• Pentose phosphate pathways are used for the biosynthesis of amino acids
• Entner-Doudoroff pathway is used for the biosynthesis of amino acids
• Biosynthesis of purine and pyrimidine nucleotides occurs via glycolysis, glutamate and aspartic acid

Integration of Metabolism

• Amphibolic pathways are metabolic pathways that function in both anabolism and catabolism
• Many pathways function simultaneously with common intermediates

Microbial metabolism includes anaerobic respiration and fermentation. In anaerobic respiration, the final electron acceptor in the electron transport chain is not O₂. Fermentation releases energy from the oxidation of organic molecules.