Q.84
How many electrons are accepted when sulfate acts as the terminal electron acceptor in bacteria such as Desulfovibrio?
Sulfate (SO₄²⁻) is reduced to sulfide (S²⁻) in sulfate-reducing bacteria like Desulfovibrio, accepting 8 electrons in the overall reaction: SO₄²⁻ + 8e⁻ + 10H⁺ → H₂S + 4H₂O. The correct answer is (A) 8.
Correct Answer
(A) 8
Sulfate Reduction Electron Balance
Sulfate respiration follows a stepwise reduction requiring 8 electrons total:
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SO₄²⁻ + 2e⁻ + 2H⁺ → SO₃²⁺ + H₂O (ATP-sulfurylase + APS reductase)
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SO₃²⁺ + 6e⁻ + 6H⁺ → S²⁻ + 3H₂O (dissimilatory sulfite reductase, Dsr)
Net: SO₄²⁻ + 8e⁻ + 8H⁺ → S²⁻ + 4H₂O. Desulfovibrio uses menaquinone/menaquinol cycling and Qmo/Tmr complexes for electron transfer to sulfate adenylyltransferase (Sat) and dissimilatory sulfite reductase.
Option Analysis
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(A) 8: Correct. Matches oxidation state change: S(+6) in SO₄²⁻ → S(-2) in H₂S (8e⁻ gain).
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(B) 6: Wrong. Partial reduction to sulfite (SO₃²⁻); incomplete respiration.
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(C) 4: Wrong. Might confuse with nitrite (NO₂⁻ → N₂) or partial sulfur reduction.
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(D) 2: Wrong. Single 2e⁻ step (e.g., fumarate → succinate); insufficient for sulfate.
Introduction to Sulfate Electron Acceptance
Sulfate terminal electron acceptor Desulfovibrio electrons calculations dominate GATE Life Sciences microbial metabolism Q.84. Desulfovibrio reduces SO₄²⁻ to H₂S accepting exactly 8 electrons via dissimilatory sulfite reductase—distinguishing from nitrate (6e⁻) or fumarate (2e⁻).
Complete 8-Electron Pathway
SO₄²⁻ ─ATP→ APS ─2e⁻→ PAPS ─2e⁻→ SO₃²⁻ ─6e⁻→ S²⁻
↓
Total: 8 electrons
Oxidation states: S⁶⁺ → S²⁻ = 8e⁻ change. Qmo/Tmr complexes deliver electrons.
Q.84 Electron Balance Table
| Acceptor | Reduction Product | Electrons | Bacteria Example |
|---|---|---|---|
| Sulfate | H₂S | 8 | Desulfovibrio |
| Nitrate | N₂ | 6 (per NO₃⁻→½N₂) | Pseudomonas |
| Sulfite | H₂S | 6 | Thiobacillus |
| Fumarate | Succinate | 2 | E. coli |
Answer: (A) 8—matches S(+6→-2).
Why 8 Electrons for Sulfate?
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ATP investment: 2 ATP → APS activation.
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High energy: SO₄²⁻ poor acceptor (E₀’ = -220 mV).
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Anaerobic niche: Outcompetes methanogens in marine sediments.
GATE Microbial Metabolism Tips
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Mnemonic: “Sulfate Swallows 8” (SO₄²⁻ → S²⁻).
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Compare: Nitrate → nitrite (2e⁻), nitrite → NH₄⁺ (6e⁻), nitrate → N₂ (8e⁻ total).
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High-yield: Desulfovibrio = classic sulfate-reducer (oilfield corrosion).
Master sulfate terminal electron acceptor Desulfovibrio electrons for perfect anaerobic respiration scores.
