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Question #46585
The Question
When a leguminous crop is cultivated in a heavily waterlogged paddy field (a methanogenic hotspot), its symbiotic nitrogen-fixing efficiency is drastically reduced despite the presence of intact, pink root nodules. Based on microbial resource allocation and metabolic demands, which of the following best explains this physiological reduction?
Options
- A.
The strictly anaerobic conditions favor methanogenic archaea, which act as a competitive sink for host-derived carbon photosynthates, depriving bacteroids of the energy required to generate the 16 ATP needed per N2 reduced.
- B.
The accumulation of methane in the rhizosphere competitively inhibits the Mo-Fe protein of the nitrogenase complex, preventing the reduction of N2 to NH3.
- C.
Under severe anaerobiosis, Rhizobium reverts to its free-living aerobic state, causing the premature senescence of the root nodules.
- D.
Methanogens utilize the leghemoglobin in the root nodules as a terminal electron acceptor, exposing the nitrogenase enzyme to oxygen toxicity.
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Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Comparative_Analysis
Sharpen your ability to evaluate multiple scenarios and distinguish between competing potential answers.
Concepts Behind This Question
Rhizobium|Methanogens|Nitrogenase|ATP
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often memorize that the nitrogenase enzyme requires an anaerobic environment to function (protected by leghemoglobin).
Therefore, a common misconception is assuming that severe waterlogging (which creates strict anaerobiosis) would enhance or at least support nitrogen fixation.
This question forces the student to synthesize the ecological concept of resource competition (methanogens vs. bacteroids) with the quantitative physiological fact from NCERT that nitrogen fixation has a massive energy demand (16 ATP per N2), which requires abundant carbon substrates.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Microbes in Human Welfare - Class 12 |
| Concept | Methanogens Metabolism |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |