Q.66 The terminal acceptor of electron during anaerobic respiration in Methanococcus
is ______________.
(A) Nitrate ion
(B) Sulfate ion
(C) Carbon dioxide
(D) Oxygen

The terminal electron acceptor in anaerobic respiration by Methanococcus is carbon dioxide (C).

Methanococcus, a hydrogenotrophic methanogen, performs methanogenesis—a specialized anaerobic process where CO₂ serves as the terminal electron acceptor, reduced to methane (CH₄) using H₂ as the electron donor. This distinguishes it from typical anaerobic respirations using inorganic ions like nitrate or sulfate.​

Option Analysis

• (A) Nitrate ion: Incorrect. Nitrate serves as a terminal acceptor in denitrifying bacteria (e.g., Pseudomonas), reducing NO₃⁻ to N₂, but Methanococcus lacks this pathway and is strictly methanogenic.​

• (B) Sulfate ion: Incorrect. Sulfate reduction occurs in sulfate-reducing bacteria (e.g., Desulfovibrio), producing H₂S, but Methanococcus cannot respire sulfate and competes poorly with sulfate reducers for substrates.​

• (C) Carbon dioxide: Correct. In Methanococcus (e.g., M. maripaludis), CO₂ acts as the electron acceptor in the Wolfe cycle, coupled to heterodisulfide (CoM-S-S-CoB) reduction in the respiratory chain for energy conservation.​

• (D) Oxygen: Incorrect. Oxygen is the aerobic acceptor; methanogens like Methanococcus are obligate anaerobes, inhibited by O₂.​

Methanococcus exemplifies anaerobic respiration through methanogenesis, where the terminal electron acceptor anaerobic respiration Methanococcus relies on is carbon dioxide (CO₂). This process powers these archaea in oxygen-free environments like wetlands and sediments.​

Methanococcus Biology

Methanococcus species, such as M. maripaludis, are hyperthermophilic or mesophilic methanogens using H₂ and CO₂ for growth. CO₂ functions dually as electron acceptor and carbon source, reduced stepwise via the Wood-Ljungdahl pathway to CH₄.​

Anaerobic Respiration Mechanism

Electrons from H₂ flow through hydrogenases and membrane complexes (e.g., Ech, Rnf) to reduce CO₂ to formyl-methanofuran, generating a proton motive force. The heterodisulfide (CoM-S-S-CoB) acts as the immediate terminal acceptor in the electron transport chain, enabling ATP synthesis.​

Why Not Other Acceptors?

Nitrate and sulfate support respiration in specialized bacteria but inhibit or outcompete methanogens. Oxygen is toxic, blocking key enzymes like methyl-coenzyme M reductase.​