Metabolic Diversity in Microorganisms — Phototrophy, Chemolithotrophy, and Fermentation Explained | Chapter 14 from Brock Biology of Microorganisms

Microorganisms exhibit an extraordinary range of metabolic strategies that allow them to thrive in virtually every environment on Earth. Chapter 14 of Brock Biology of Microorganisms explores this rich metabolic diversity, explaining how microbes conserve energy, maintain redox balance, and cycle elements critical to global biogeochemistry. This chapter offers a comprehensive look at microbial phototrophy, chemolithotrophy, anaerobic respiration, fermentation, and hydrocarbon metabolism.

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Metabolism and Redox Principles in Microbes

Microbial metabolism is fundamentally driven by redox reactions. Microorganisms utilize a wide array of electron donors and acceptors with varying reduction potentials (E₀'). ATP is synthesized through substrate-level phosphorylation, oxidative phosphorylation, or photophosphorylation. Key carriers like NADH and ferredoxin provide reducing power for biosynthesis. Additionally, flavin-based electron bifurcation allows coupling of exergonic and endergonic redox reactions, increasing energy efficiency.

CO₂ Fixation and Autotrophic Pathways

Autotrophs fix carbon dioxide into organic molecules using specialized pathways:

• Calvin Cycle: Uses RuBisCO; common among phototrophs and chemolithotrophs.
• Reverse Citric Acid Cycle: Operates in green sulfur bacteria.

Many autotrophs compartmentalize CO₂ fixation enzymes within carboxysomes, enhancing metabolic efficiency.

Phototrophy and Light-Driven Energy Generation

Phototrophic microbes harvest light energy through photophosphorylation. Chlorophylls and bacteriochlorophylls are central pigments, organized into reaction centers and antenna complexes. Structures like chloroplasts, chromatophores, lamellae, and chlorosomes support this function.

Photosynthetic Pigments

• Carotenoids: Absorb blue light (400–500 nm), offer photoprotection.
• Phycobilins: Light-harvesting pigments in cyanobacteria, organized into phycobilisomes.

Types of Photosynthesis

• Anoxygenic: Uses donors like H₂S; does not produce O₂.
• Oxygenic: Uses H₂O, producing O₂; involves PSI and PSII in a Z-scheme.

Chemolithotrophy and Inorganic Electron Donors

Chemolithotrophs oxidize inorganic compounds such as H₂, H₂S, Fe²⁺, and NH₄⁺ to conserve energy. Many require reverse electron flow to generate reducing power for biosynthesis.

Sulfur and Iron Oxidation

• Sox system: Direct oxidation of H₂S and S⁰ to sulfate (SO₄²⁻).
• Iron oxidation: Fe²⁺ → Fe³⁺ by organisms like Acidithiobacillus ferrooxidans.

Nitrogen Oxidation and Anammox

• Nitrification: Ammonia → nitrite → nitrate.
• Anammox: Anaerobic ammonia oxidation to N₂, performed in anammoxosomes by Planctomycetes.

Anaerobic Respiration Pathways

Microbes utilize alternative electron acceptors when oxygen is absent:

• Denitrification: NO₃⁻ → N₂ via intermediates.
• DNRA: NO₃⁻ reduced to NH₄⁺.
• Sulfate reduction: SO₄²⁻ → H₂S.
• Metal reduction: Fe³⁺ and Mn⁴⁺ reduced by bacteria like Geobacter using conductive nanowires.

CO₂ and C1 Metabolism

• Acetogenesis: CO₂ → acetate via the Wood–Ljungdahl pathway.
• Methanogenesis: CO₂ or acetate → CH₄ by Archaea, using coenzymes like F420, CoM, and CoB.
• Methanotrophy: CH₄ oxidized to CO₂ aerobically or anaerobically.

Fermentation: Redox Without External Acceptors

Fermentation generates ATP via substrate-level phosphorylation without using external electron acceptors. Redox balance is maintained by producing lactate, ethanol, or hydrogen. Hydrogenases and electron bifurcation increase metabolic efficiency.

Diversity of Fermentative Pathways

• Lactic acid bacteria: Homofermentative vs. heterofermentative.
• Enterobacteriaceae: Mixed-acid and butanediol fermentation.
• Clostridia: Ferment amino acids via the Stickland reaction.
• Pyrococcus furiosus: Uses ion pumps during fermentation.

Ion-Coupled and Syntrophic Metabolism

Some microbes generate motive force with ion-coupled reactions (e.g., Na⁺ decarboxylases). Syntrophy involves cooperative metabolism—like hydrogen transfer or direct interspecies electron transfer (DIET)—to degrade complex substrates in anaerobic conditions.

Hydrocarbon Metabolism

• Aerobic: Hydrocarbons oxidized via monooxygenases and dioxygenases; fatty acids undergo β-oxidation.
• Anaerobic: Hydrocarbons activated by fumarate addition or carboxylation, then metabolized via the benzoyl-CoA pathway.

These pathways are critical in natural and contaminated ecosystems.

Glossary of Key Concepts

• Electron bifurcation: Energy-efficient coupling of redox reactions
• Calvin cycle / RuBisCO: Primary CO₂ fixation pathway
• Chlorosomes: Specialized light-harvesting structures in green bacteria
• Anammoxosome: Organelle for anaerobic ammonia oxidation
• Hydrogenase: Catalyzes H₂ oxidation/reduction
• Sox system: Sulfur oxidation enzyme complex
• Denitrification / DNRA: Nitrate respiration strategies

Conclusion: Microbial Metabolism and Environmental Impact

Chapter 14 highlights the remarkable metabolic ingenuity of microbes—from light harvesting and anaerobic respiration to interspecies electron transfer. These diverse pathways not only support microbial life in extreme and changing environments but also drive elemental cycles and shape ecosystems.

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