(A) Pseudomonas and Azotobacter
(B) Pseudomonas and Desulfovibrio
(C) Azotobacter and Thiobacillus
(D) Nitrosomonas and Nitrobacter
Nitrate- and sulfate-rich, oxygen-depleted soils favor denitrifying and sulfate-reducing bacteria. This article solves Q.74 identifying dominant inhabitants with explanations for microbiology exams.
Correct Answer
The correct answer is (B) Pseudomonas and Desulfovibrio.
Pseudomonas species perform denitrification, sequentially reducing NO₃⁻ → NO₂⁻ → NO → N₂O → N₂ using nitrate as terminal electron acceptor in anoxic conditions. Desulfovibrio thrives on sulfate reduction (SO₄²⁻ → H₂S) when nitrate depletes, dominating strictly anaerobic niches.
Option Breakdowns
(A) Pseudomonas and Azotobacter
Pseudomonas fits denitrification, but Azotobacter requires oxygen for nitrogen fixation and cannot survive anaerobic conditions.
Invalid for oxygen-depleted soil.
(B) Pseudomonas and Desulfovibrio
Perfect match: Pseudomonas uses nitrate respiration; Desulfovibrio follows with sulfate reduction in stratified anoxic zones.
Ecological succession in waterlogged, nutrient-rich soils.
(C) Azotobacter and Thiobacillus
Azotobacter needs O₂ (aerobic N-fixer); Thiobacillus oxidizes reduced sulfur compounds, requiring oxygen as electron acceptor.
Both fail in anoxic environments.
(D) Nitrosomonas and Nitrobacter
Both perform nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻), strictly aerobic processes inhibited by anoxia.
Produce nitrate but cannot inhabit oxygen-depleted soils.
| Option | Pseudomonas | Anaerobic Metabolism | Matches Anoxic Soil? |
|---|---|---|---|
| (A) + Azotobacter | Denitrification | Aerobic N-fixation | No |
| (B) + Desulfovibrio | Denitrification | Sulfate reduction | Yes |
| (C) + Thiobacillus | Denitrification | Sulfur oxidation (aerobic) | No |
| (D) Nitrosomonas/Nitrobacter | – | Nitrification (aerobic) | No |
