Q.72 Which one of the following DO NOT use water as an electron source during photosynthesis?
(A) Sulfate reducing bacteria
(B) Methanogenic bacteria
(C) Green and purple bacteria
(D) Nitrifying bacteria
Methanogenic bacteria do not use water as an electron source during photosynthesis, unlike the other options which involve photosynthetic processes.
The correct answer is (B).
Photosynthesis Electron Donor Analysis
Photosynthesis requires electron donors to replace those excited in reaction centers. Oxygenic photosynthesis (plants, cyanobacteria) uses H₂O → O₂ + 4H⁺ + 4e⁻. Anoxygenic photosynthesis (certain bacteria) uses H₂S, organic acids, or H₂ instead. None of the options are oxygenic photosynthesizers.
Option Breakdown
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(A) Sulfate reducing bacteria: Use H₂ or organic compounds to reduce SO₄²⁻ → H₂S (anaerobic respiration), not photosynthesis. Some related phototrophs (e.g., purple sulfur) use H₂S, not water.
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(B) Methanogenic bacteria: Archaea performing methanogenesis (CO₂/H₂ → CH₄), chemolithoautotrophic respiration—no photosynthesis or light reactions, thus no electron source like water.
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(C) Green and purple bacteria: Anoxygenic phototrophs—green sulfur (H₂S → S⁰), purple sulfur/nonsulfur (H₂S/organics). Explicitly avoid water due to lack of PSII oxygen-evolving complex.
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(D) Nitrifying bacteria: Chemolithoautotrophs oxidizing NH₄⁺ → NO₂⁻ (Nitrosomonas) or NO₂⁻ → NO₃⁻ (Nitrobacter)—respiration, not photosynthesis.
Introduction to Use Water Electron Source Photosynthesis
Use water electron source photosynthesis distinguishes oxygenic (H₂O → O₂) from anoxygenic types—key GATE Life Sciences microbiology. Methanogens lack photosynthesis entirely; green/purple bacteria use H₂S/organics instead.
Electron Donor Comparison
| Group | Process | Electron Donor | Uses Water? |
|---|---|---|---|
| Sulfate reducers (A) | Respiration | H₂/organics → H₂S | No |
| Methanogens (B) | Methanogenesis | H₂ → CH₄ | No (correct) |
| Green/purple bacteria (C) | Anoxygenic PS | H₂S/organics | No |
| Nitrifying bacteria (D) | Nitrification | NH₄⁺/NO₂⁻ | No |
Why Methanogens Stand Out
No photosynthetic apparatus—CO₂ reduction via ferredoxin, not light-driven electron transport. Others have phototrophy or could indirectly relate.
GATE Strategy
Use water electron source photosynthesis PYQs test bacterial metabolism. Key: methanogens = chemolithotrophs only = (B). Memorize anoxygenic donors (H₂S, not H₂O).
