All other things being equal, acetogens will out compete methanogens in an environment

Question

BlackTom AI · Accepted Answer

Methanogens have a lower hydrogen (H2) threshold and extract more free energy per mole H2 than acetogens, so they typically outcompete acetogens under equal conditions.

1. Relevant concepts
- Thermodynamics: Gibbs free energy change (ΔG) determines whether a pathway can support growth; more negative ΔG yields more energy for biomass synthesis.
- Substrate affinity and H2 threshold: microbes with lower minimum H2 partial pressure (threshold) can consume H2 at lower concentrations and thus outcompete others.
- Pathways: Hydrogenotrophic methanogenesis (CO2 + 4H2 → CH4 + 2H2O) vs reductive acetogenesis (2CO2 + 4H2 → CH3COO− + H+ + 2H2O) (Wood–Ljungdahl pathway).

2. Step-by-step reasoning
- Compare standard free energies and per-H2 yields.
- For methanogenesis: ΔG°' ≈ −131 kJ per mol CH4. Per mol H2: −131/4 = −32.8 kJ·mol−1 H2.
- For acetogenesis: ΔG°' ≈ −95 kJ per mol acetate. Per mol H2: −95/4 = −23.8 kJ·mol−1 H2.
- Because methanogenesis yields more energy per H2, it remains energetically favorable at lower H2 concentrations.

3. Calculations (intermediate steps shown)
- Methanogenesis per H2: −131 kJ mol−1 ÷ 4 = −32.75 kJ mol−1 H2.
- Acetogenesis per H2: −95 kJ mol−1 ÷ 4 = −23.75 kJ mol−1 H2.
- Lower ΔG required for growth → organisms with the more exergonic reaction (methanogens) succeed at lower H2 partial pressures.

4. Why the correct answer is correct
- Methanogens typically have hydrogenase systems and kinetics that allow growth at H2 partial pressures of ~10−5–10−4 atm, whereas acetogens require higher H2 (often 10−4–10−3 atm). Under “all other things being equal,” methanogens therefore outcompete acetogens.

5. Why the other option is incorrect
- The statement “acetogens will outcompete methanogens” (True) ignores energetic and kinetic realities: acetogenesis is less exergonic per H2 and acetogens have higher H2 thresholds, so they cannot displace methanogens when H2 is limiting.

6. Conclusion
- Given the thermodynamic and kinetic advantages of methanogens, the correct answer is False, which matches the provided answer.

All other things being equal, acetogens will out compete methanogens in an environment判断题

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