Chapter 14: Metabolic Diversity of Microorganisms

All microbial metabolisms rely on oxidation reduction reactions to conserve energy, generate reducing power, and maintain redox balance. Energy conservation occurs through mechanisms such as substrate level phosphorylation, oxidative phosphorylation, and photophosphorylation, while reducing power for biosynthesis is commonly carried by electron carriers such as NADH, NADPH, or reduced ferredoxin. Microbial metabolic diversity is supported by the modular structure of metabolic pathways, allowing new metabolic strategies to evolve through enzyme modification and horizontal gene transfer. Autotrophic microbes fix carbon dioxide into organic molecules through pathways such as the Calvin cycle, which uses the enzyme RuBisCO, or the reverse citric acid cycle used by certain anaerobic and chemolithotrophic organisms. Phototrophic metabolism captures light energy using pigments such as chlorophylls and bacteriochlorophylls organized within reaction centers and antenna complexes, allowing both oxygenic photosynthesis that produces oxygen from water and anoxygenic photosynthesis that uses alternative electron donors such as hydrogen sulfide. Many microorganisms obtain energy through chemolithotrophic oxidation of inorganic compounds including sulfur, ferrous iron, and nitrogen compounds, supporting key biogeochemical cycles such as nitrification and sulfur oxidation. Other microbes perform anaerobic respiration using alternative electron acceptors such as nitrate, sulfate, metals, or carbon dioxide, enabling processes like denitrification, sulfate reduction, and metal reduction in anoxic environments. Specialized metabolic pathways also enable microorganisms to transform single carbon compounds such as carbon dioxide, methane, and methanol through processes including acetogenesis, methanogenesis, and methanotrophy, which play major roles in global carbon cycling. Fermentation pathways allow cells to generate energy in the absence of external electron acceptors by producing reduced metabolic end products that maintain redox balance. Additional metabolic diversity arises through syntrophic interactions in which multiple microbial species cooperate metabolically through interspecies electron transfer. Finally, microbes are capable of degrading hydrocarbons both aerobically and anaerobically through oxygenase reactions or alternative activation pathways, allowing them to metabolize complex organic compounds such as aliphatic and aromatic hydrocarbons and contributing to environmental carbon turnover.