The GATE Grind

GATE 2026 ES – Question 43

Environmental Microbiology · Microbial Metabolism · 2 marks · Multiple select

Which statements about electron donors (ED) and acceptors (EA) in wastewater reactions are correct? Select all that apply.

  1. In anaerobic heterotrophic acid fermentation, organic compounds are both ED and EA.
  2. In anaerobic heterotrophic methanogenesis, volatile fatty acids and CO₂ are ED and EA, respectively.
  3. In denitrification, organic compounds and CO₂ are ED and EA, respectively.
  4. In aerobic autotrophic iron oxidation, Fe(II) and O₂ are ED and EA, respectively.

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Show answer and explanation

Correct answer: (A) In anaerobic heterotrophic acid fermentation, organic compounds are both ED and EA.; (B) In anaerobic heterotrophic methanogenesis, volatile fatty acids and CO₂ are ED and EA, respectively.; (D) In aerobic autotrophic iron oxidation, Fe(II) and O₂ are ED and EA, respectively.

Explanation

Fermentation balances oxidation and reduction within organic substrates, so A is correct. D is correct because Fe(II) supplies electrons and oxygen accepts them. Denitrification uses nitrate/nitrite as the terminal acceptor, not CO₂,so C is false. The official key also accepts B, using a simplified consortium-level description. Individual methanogenic pathways require care: hydrogenotrophic methanogenesis uses H₂ to reduce CO₂,while acetoclastic methanogenesis splits acetate through internal redox. Preserve official **A, B, D** without treating B as a universal pathway equation.

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