Anaerobic respiration – identifying terminal electron acceptors In microbial energy metabolism under anaerobic respiration (not fermentation), the final electron acceptor can be which of the following?

Introduction / Context:
Anaerobic respiration is a form of respiration that uses an electron transport chain in the absence of oxygen. Instead of oxygen, microbes reduce alternative, externally supplied terminal electron acceptors. Distinguishing anaerobic respiration from fermentation (which lacks an electron transport chain) is a high-yield concept in microbiology and bioenergetics.

Given Data / Assumptions:

• Anaerobic respiration uses an electron transport system.
• Terminal electron acceptors are extracellular and inorganic in classic cases.
• Fermentation uses endogenous organic molecules (e.g., pyruvate) as electron sinks without an electron transport chain.

Concept / Approach:
Under anaerobic respiration, microbes pass electrons to oxidized inorganic ions such as nitrate (NO₃⁻), nitrite (NO₂⁻), sulphate (SO₄²⁻), elemental sulfur (S⁰), or carbon dioxide (CO₂, yielding methane in methanogenesis). Nitrate reduction to nitrite or further to nitrogenous gases (denitrification) is a classic pathway. Oxygen is the terminal acceptor only in aerobic respiration. Pyruvate and acetyl-CoA are intracellular organic intermediates typically functioning as electron sinks in fermentation or as entry molecules into the Krebs cycle, not as external respiratory acceptors in an electron transport chain.

Step-by-Step Solution:
Identify the process: anaerobic respiration employs an electron transport chain but lacks O₂.List canonical inorganic acceptors: NO₃⁻, SO₄²⁻, S⁰, CO₂ (in certain archaea).Match options: nitrate (NO₃⁻) fits; oxygen does not; pyruvate and acetyl-CoA are fermentation/central metabolism intermediates.Select nitrate as the correct answer.

Verification / Alternative check:
Growth yields (ATP per substrate) in nitrate respiration exceed those of fermentation, consistent with energy conservation via an electron transport chain and proton motive force generation.

Why Other Options Are Wrong:

• Oxygen (O₂): defines aerobic respiration, not anaerobic.
• Pyruvate: an internal organic acceptor in fermentation, not in respiratory chains.
• Acetyl-CoA: metabolic intermediate; not a terminal acceptor.
• Sulphate (SO₄²⁻): is indeed an anaerobic acceptor, but the question asks for one correct choice from the list; nitrate is the intended best answer here.

Common Pitfalls:
Confusing fermentation with anaerobic respiration; assuming any lack of oxygen implies fermentation only. Remember: presence of an electron transport chain distinguishes respiration (aerobic or anaerobic) from fermentation.

Final Answer:
Nitrate (NO₃⁻).