Q.67 Which one of the following converts sulfate to hydrogen sulfide?
(A) Beggiatoa
(B) Desulfovibrio
(C) Thiobacillus
(D) Thiothrix
Desulfovibrio converts sulfate to hydrogen sulfide in microbial sulfur metabolism. This process, known as dissimilatory sulfate reduction, is a key anaerobic respiration pathway relevant for CSIR NET Life Sciences exams.
Correct Answer
(B) Desulfovibrio performs dissimilatory sulfate reduction, using sulfate (SO₄²⁻) as a terminal electron acceptor and reducing it to hydrogen sulfide (H₂S). This anaerobic process couples organic matter oxidation (like lactate or acetate) to sulfate reduction via enzymes such as ATP sulfurylase, APS reductase, and dissimilatory sulfite reductase (Dsr).
Option Analysis
(A) Beggiatoa
Filamentous, chemolithoautotrophic bacteria that oxidize hydrogen sulfide (H₂S) or elemental sulfur (S⁰) to sulfate using oxygen or nitrate, storing sulfur intracellularly. They do not reduce sulfate.
(B) Desulfovibrio
Correct. Gram-negative, sulfate-reducing bacteria (SRB) that thrive in anaerobic environments like sediments and sewage, reducing sulfate to H₂S, often causing blackening from iron sulfide precipitation.
(C) Thiobacillus
Chemolithotrophic sulfur-oxidizing bacteria (SOB) that oxidize reduced sulfur compounds (e.g., H₂S, thiosulfate, elemental sulfur) to sulfate under aerobic or denitrifying conditions, acidifying environments.
(D) Thiothrix
Filamentous sulfur bacteria common in wastewater and sulfur springs that oxidize H₂S to elemental sulfur, accumulating it intracellularly. They perform lithotrophic growth on reduced sulfur but not sulfate reduction.
Which Bacterium Converts Sulfate to Hydrogen Sulfide? Desulfovibrio Explained for CSIR NET
Introduction to Sulfur Bacteria Metabolism
In microbial ecology, the bacterium that converts sulfate to hydrogen sulfide plays a crucial role in anaerobic environments, driving biogeochemical sulfur cycling. Desulfovibrio, a sulfate-reducing bacterium (SRB), performs dissimilatory sulfate reduction, impacting ecosystems like sediments, oil fields, and wastewater. This process is essential for CSIR NET aspirants studying microbial metabolism and environmental biotechnology.
Desulfovibrio: Mechanism of Sulfate Reduction
Desulfovibrio species activate sulfate to adenosine phosphosulfate (APS), reduce it to sulfite, then to H₂S using ferredoxin-dependent enzymes. The reaction is: SO₄²⁻ + 2 lactate → 2 pyruvate + HS⁻ + H⁺ + 2 HCO₃⁻. This generates energy via electron transport, producing the “rotten egg” odor in anoxic zones and corrosion in industries.
Why Not Other Options?
Sulfur-oxidizing bacteria dominate the alternatives:
- Beggiatoa oxidizes sulfide aerobically or with nitrate.
- Thiobacillus oxidizes thiosulfate/sulfur to sulfate.
- Thiothrix stores sulfur from H₂S oxidation.
Sulfur Bacteria Metabolism Comparison
| Bacterium | Metabolism Type | Key Reaction Pathway |
|---|---|---|
| Desulfovibrio | Reduction (anaerobic) | SO₄²⁻ → H₂S |
| Beggiatoa | Oxidation (aerobic/NO₃) | H₂S/S⁰ → SO₄²⁻ |
| Thiobacillus | Oxidation (aerobic) | S₂O₃²⁻/S⁰ → SO₄²⁻ |
| Thiothrix | Oxidation (microaerobic) | H₂S → S⁰ |
