Q.56 Which one of the following is the end product of dissimilatory sulfate reduction by sulfate reducing
bacteria?
(A) Hydrogen sulfide
(B) Sulfur dioxide
(C) Sulfur
(D) Thiosulfate
The end product of dissimilatory sulfate reduction by sulfate-reducing bacteria (SRB) is hydrogen sulfide (H₂S), making option (A) the correct choice.
Correct Answer
Hydrogen sulfide (A). Sulfate-reducing bacteria use sulfate (SO₄²⁻) as a terminal electron acceptor in anaerobic respiration, reducing it stepwise through sulfite (SO₃²⁻) to sulfide (HS⁻ or H₂S). This dissimilatory process generates energy via ATP production and releases H₂S as the primary excreted product, distinguishing it from assimilatory reduction that incorporates sulfur into biomass like cysteine.
Option Explanations
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(A) Hydrogen sulfide: Correct. The final step involves sulfite reductase converting sulfite to sulfide, which combines with protons to form H₂S, the hallmark toxic byproduct of SRB metabolism in environments like sediments and guts.
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(B) Sulfur dioxide: Incorrect. SO₂ is a gas from oxidation processes, not reduction; SRB produce it only under aerobic conditions or via sulfide oxidation, unrelated to dissimilatory sulfate reduction.
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(C) Sulfur: Incorrect. Elemental sulfur (S⁰) or zero-valent sulfur arises as a minor byproduct in some SRB under specific conditions like high salinity, but it is not the primary end product—typically only 6-13% of reduced sulfate.
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(D) Thiosulfate: Incorrect. Thiosulfate (S₂O₃²⁻) forms from partial sulfide oxidation by enzymes like sulfide:quinone oxidoreductase in certain bacteria, but SRB dissimilation directly yields H₂S, not thiosulfate.
