Microbes like Rhizobium, Nitrosomonas and Nitrobacter are used for:
- (a)carbon cycling
- (b)nitrogen cycling
- (c)water cycling
- (d)sulphur cycling
Answer
Why
Correct — B. All three genera work on nitrogen, at three consecutive steps of one cycle.
Rhizobium lives in the root nodules of legumes and fixes atmospheric N₂ into ammonia.
Nitrosomonas oxidises ammonia to nitrite.
Nitrobacter oxidises that nitrite to nitrate, the form roots absorb.
Fixation followed by two-stage nitrification is the nitrogen cycle → option (b).
Why the others are wrong
- (a)carbon cycling — The carbon cycle turns on photosynthesis, respiration and decomposition, and its microbial specialists are decomposers and methanogens. None of the three genera named belongs to it.
- (c)water cycling — The water cycle is physical — evaporation, condensation, precipitation, runoff. No bacterium drives it, so a stem naming three bacterial genera cannot be pointing here.
- (d)sulphur cycling — Sulphur is worked by Thiobacillus, which oxidises sulphur compounds, and by Desulfovibrio, which reduces sulphate. The nitro- in two of the three names is the giveaway.
Concept
The nitrogen cycle moves nitrogen from the air into living tissue and back.
Fixation converts inert atmospheric N₂ into ammonia — by Rhizobium inside legume root nodules, and by free-living fixers such as Azotobacter.
Nitrification then runs in two steps: ammonia to nitrite by Nitrosomonas, nitrite to nitrate by Nitrobacter.
Denitrification, by bacteria such as Pseudomonas, returns nitrate to atmospheric N₂ and closes the loop.
The stem's phrase is 'are used for', which points at agriculture. Rhizobium is the basis of legume biofertilisers, and it is the reason a legume is rotated into a field to restore its nitrogen.
Key facts
- Rhizobium fixes atmospheric nitrogen symbiotically in the root nodules of leguminous plants.
- Nitrosomonas converts ammonia to nitrite.
- Nitrobacter converts nitrite to nitrate, the form plants absorb.
- Denitrifying bacteria such as Pseudomonas convert nitrate back to atmospheric nitrogen.
Study next
Common traps
- Treating Nitrosomonas and Nitrobacter as doing the same step — one makes nitrite, the other nitrate.
- Calling Rhizobium a free-living fixer when it fixes nitrogen only in symbiosis inside a nodule.
SSC's microbiology in General Awareness is set as organism-to-process: a genus is named and you supply the step or the cycle it belongs to.
Learn each genus glued to the exact step it performs. The option set here is four cycles, so the organisms alone decide it.
Related PYQs
No directly related past PYQ was found.