Q.57 Which one of the following is the terminal electron acceptor in the given metabolic reaction catalyzed
by methanogens?
𝟒𝐇𝟐 + 𝐂𝐎𝟐 ⟶ 𝐂𝐇𝟒 + 𝟐𝐇𝟐𝐎
(A) H2
(B) CO2
(C) CH4
(D) H2O

Terminal Electron Acceptor in Methanogenesis: CO2 in 4H2 + CO2 → CH4 + 2H2O Reaction

Methanogens use CO2 as the terminal electron acceptor in the reaction 4H₂ + CO₂ → CH₄ + 2H₂O, making option (B) correct. This process, known as hydrogenotrophic methanogenesis, occurs in anaerobic environments like wetlands and ruminant guts.​

Correct Answer

Answer: (B) CO₂
CO₂ accepts electrons from H₂ during methanogenesis, getting reduced to CH₄ while H₂ acts as the electron donor. This archaeal metabolism plays a key role in the global carbon cycle by converting CO₂ and H₂ into methane.​

Reaction Overview

Methanogens, strictly anaerobic archaea, catalyze 4H₂ + CO₂ → CH₄ + 2H₂O using specialized enzymes and cofactors like F420 and methanofuran. Electrons from four H₂ molecules reduce CO₂ stepwise to methane, releasing two H₂O molecules. This differs from typical respiration where O₂ serves as the acceptor.​

Option Analysis

• (A) H₂: Incorrect; H₂ donates electrons as the reducing agent, not accepts them.​

• (B) CO₂: Correct; CO₂ is reduced to CH₄, functioning as the terminal electron acceptor in this anaerobic pathway.​

• (C) CH₄: Incorrect; CH₄ is the reduced end product, not an acceptor.​

• (D) H₂O: Incorrect; H₂O forms as a byproduct of the reduction, without accepting electrons.​

Biological Significance

Hydrogenotrophic methanogenesis sustains methanogens in oxygen-free niches and contributes to ~30% of Earth’s methane emissions. Understanding this aids biogas production and climate studies on greenhouse gases.​