Q60. In legume nodules, nitrogen fixation requires low oxygen despite active respiration. Which mechanism maintains this balance?
[UP PGT Biology, 9 May 2026]
(a) Suppression of respiration lowers oxygen levels
(b) Leghemoglobin buffers oxygen, permitting respiration while protecting nitrogenase
(c) Direct diffusion of nitrogen into mitochondria produces ATP
(d) Storage-compound synthesis consumes all excess oxygen
Answer: (b) Leghemoglobin buffers oxygen, permitting respiration while protecting nitrogenase
Explanation: Leghemoglobin binds oxygen reversibly inside legume root nodules. It maintains a low concentration of free oxygen, protecting oxygen-sensitive nitrogenase, while still permitting oxygen delivery for energy-producing respiration. This coordination enables nitrogen-fixing bacteroids to function effectively.
Q59. Which of the following bacteria is incorrectly paired with its role?
[RRB Group D, 10 August 2026, Shift 3]
(a) Azotobacter — nitrogen fixation in soil
(b) Pseudomonas — nitrification
(c) Nitrobacter — nitrite-to-nitrate conversion
(d) Rhizobium — nitrogen fixation in legumes
Answer: (b) Pseudomonas — nitrification
Explanation: Pseudomonas species are often associated with denitrification rather than the conventional nitrification sequence. Nitrosomonas oxidizes ammonia to nitrite, while Nitrobacter oxidizes nitrite to nitrate. Azotobacter and Rhizobium fix atmospheric nitrogen in free-living and symbiotic settings respectively.
Q58. Which one of these is NOT a prokaryote–angiosperm symbiotic pair capable of fixing atmospheric nitrogen?
[CSIR-UGC NET Life Sciences, 18 December 2025]
(a) Nostoc and Gunnera
(b) Frankia and Casuarina
(c) Bradyrhizobium and Parasponia
(d) Azolla and rice
Answer: (d) Azolla and rice
Explanation: Azolla is an aquatic fern rather than a prokaryotic microorganism, and rice does not form the stated direct symbiosis with it. The nitrogen-fixing partnership involves Azolla and its cyanobacterial partner. The other pairs involve nitrogen-fixing microbes associated with higher plants.
Q57. Azotobacter is beneficial in agriculture because it ________.
[MP Excise Constable, 21 September 2025, Shift 2]
(a) decomposes organic matter
(b) acts as a pesticide
(c) increases phosphorus availability
(d) fixes atmospheric nitrogen
Answer: (d) fixes atmospheric nitrogen
Explanation: Azotobacter is a free-living aerobic soil bacterium capable of biological nitrogen fixation. By transforming atmospheric nitrogen into more accessible nitrogenous compounds, it helps improve soil fertility and can supplement nitrogen nutrition of non-leguminous crops without requiring root nodules.
Q56. Which of the following organisms is NOT involved in nitrogen fixation?
[MP Excise Constable, 11 September 2025, Shift 2]
(a) Azotobacter
(b) Frankia
(c) Nitrosomonas
(d) Rhizobium
Answer: (c) Nitrosomonas
Explanation: Nitrosomonas is a nitrifying bacterium: it oxidizes ammonia to nitrite rather than fixing atmospheric nitrogen. Azotobacter fixes nitrogen independently, Rhizobium does so in legume nodules, and Frankia fixes nitrogen in symbiosis with several non-leguminous woody plants.
Q55. Which of the following is a free-living nitrogen-fixing anaerobic bacterium?
[DSSSB TGT Natural Science, 7 September 2025, Shift 1]
(a) Anabaena
(b) Nostoc
(c) Clostridium
(d) Azotobacter
Answer: (c) Clostridium
Explanation: Certain Clostridium species, notably Clostridium pasteurianum, fix atmospheric nitrogen in oxygen-free conditions. Azotobacter is free-living but aerobic; Anabaena and Nostoc are photosynthetic cyanobacteria. The question specifically requires an anaerobic free-living bacterium rather than simply any nitrogen fixer.
Q54. Which biofertilizer fixes nitrogen in symbiotic association with leguminous plants?
[UPSSSC Agriculture Technical Assistant Mains, 13 July 2025]
(a) Rhizobium
(b) Azolla
(c) Phosphobacteria
(d) Mycorrhiza
(e) None of the above
Answer: (a) Rhizobium
Explanation: Rhizobium enters a mutually beneficial relationship with legume roots and establishes nodules where nitrogen fixation occurs. The plant supplies carbohydrates and shelter, while the bacteria supply fixed nitrogen. Azolla uses cyanobacterial partners; mycorrhizae primarily improve phosphorus and water uptake.
Q53. Under the Fertiliser (Control) Order, what is the minimum viable cell count per millilitre for liquid Rhizobium biofertilizer?
[UPSSSC Agriculture Technical Assistant Mains, 13 July 2025]
(a) 1 × 10⁷ CFU/ml
(b) 1 × 10⁶ CFU/ml
(c) 1 × 10⁹ CFU/ml
(d) 1 × 10⁸ CFU/ml
(e) None of the above
Answer: (d) 1 × 10⁸ CFU/ml
Explanation: The specified minimum for the liquid Rhizobium formulation in this examination is 1 × 10⁸ colony-forming units per millilitre. Viable cell counts help assess inoculant quality because sufficient living microorganisms must survive for effective root colonization and nitrogen fixation.
Q52. Which of these organisms cannot fix atmospheric nitrogen?
A. Azotobacter;
B. Oscillatoria;
C. Anabaena;
D. Volvox;
E. Nostoc.
[NEET UG, 4 May 2025]
(a) A only
(b) D only
(c) B only
(d) E only
Answer: (b) D only
Explanation: Volvox is a colonial green alga and lacks biological nitrogen-fixing machinery. Azotobacter is a nitrogen-fixing bacterium, and cyanobacterial groups such as Anabaena and Nostoc include nitrogen-fixing members. The examination answer identifies Volvox as the non-nitrogen-fixing organism.
Q51. Which characteristic is NOT true for all cyanobacteria?
[HTET PGT Biology 2024 examination, conducted 2025]
(a) They can fix atmospheric nitrogen
(b) They lack a membrane-bound nucleus
(c) They possess chlorophyll a
(d) Their cell walls are not made of cellulose
Answer: (a) They can fix atmospheric nitrogen
Explanation: Cyanobacteria are prokaryotic and contain chlorophyll a, but nitrogen fixation is not universal among them. Only particular taxa possess the appropriate nitrogen-fixing machinery. Genera such as Anabaena and Nostoc include well-known nitrogen-fixing species used in agricultural biofertilization.
Q50. Microbes like Rhizobium, Nitrosomonas and Nitrobacter are involved in which cycle?
[SSC CGL Tier I, 12 September 2024, Shift 1]
(a) Nitrogen cycle
(b) Carbon cycle
(c) Water cycle
(d) Sulphur cycle
Answer: (a) Nitrogen cycle
Explanation: Rhizobium fixes atmospheric nitrogen in legume nodules. Nitrosomonas oxidizes ammonia into nitrite, and Nitrobacter converts nitrite to nitrate. These activities form different stages of the nitrogen cycle that determine which nitrogen compounds become available to plants.
Q49. Mycorrhiza is a symbiotic association between roots of higher plants and ________.
[RSMSSB Agriculture Supervisor, 4 February 2024]
(a) Fungi
(b) Bacteria
(c) Nematodes
(d) Viruses
(e) Question not attempted
Answer: (a) Fungi
Explanation: Mycorrhizae consist of fungi associated with plant root systems. Fungal hyphae extend the effective absorption area, especially for phosphorus and water. In return, the plant supplies photosynthates. This beneficial interaction is the basis of several fungal biofertilizer applications.
Q48. Which of these cyanobacterial genera can fix atmospheric nitrogen: Anabaena, Nostoc and Oscillatoria?
[Bihar TRE 3.0 PGT Botany, 22 July 2024]
(a) Anabaena
(b) Nostoc
(c) Oscillatoria
(d) More than one of the above
(e) None of the above
Answer: (d) More than one of the above
Explanation: Anabaena and Nostoc are established examples of nitrogen-fixing cyanobacteria. Their nitrogenase activity allows them to convert atmospheric nitrogen into usable compounds. The official multiple-answer format therefore selects more than one; nitrogen fixation among Oscillatoria species is not universally established.
Q47. Which organism is NOT helpful in fixing atmospheric nitrogen into a plant-usable form?
[CUET PG B.Ed., 15 March 2024, Shift 2]
(a) Rhizobium
(b) Azospirillum
(c) Mycorrhiza
(d) Azotobacter
Answer: (c) Mycorrhiza
Explanation: Mycorrhizal fungi mainly help plant roots obtain phosphorus, water and other minerals. They do not themselves perform atmospheric nitrogen fixation. Rhizobium, Azospirillum and Azotobacter, in contrast, include bacteria able to fix atmospheric nitrogen and contribute to plant nutrition.
Q46. Which aquatic plant is commonly used as a biofertilizer in waterlogged rice fields?
[UPPSC PCS Prelims, 14 May 2023]
(a) Wolffia
(b) Azolla
(c) Trapa
(d) Lemna
Answer: (b) Azolla
Explanation: Azolla is a floating aquatic fern that hosts a nitrogen-fixing cyanobacterium in specialized leaf cavities. This association contributes biologically fixed nitrogen to flooded rice systems. Farmers can incorporate Azolla biomass into fields, reducing reliance on some chemical nitrogen inputs.
Q45. Which alga serves as a biofertilizer in rice fields?
[BPSC PGT Botany, 26 August 2023, Shift 2]
(a) Chlamydomonas
(b) Cladophora
(c) Nostoc
(d) More than one of the above
(e) None of the above
Answer: (c) Nostoc
Explanation: Nostoc is a nitrogen-fixing cyanobacterial genus commonly called blue-green algae. Its ability to enrich flooded soils with fixed nitrogen makes it valuable in rice cultivation. Chlamydomonas and Cladophora are green algae and are not the standard nitrogen-fixing rice biofertilizers.
Q44. Which biofertilizer is used for soybean cultivation?
[MP Police Constable, 19 August 2023, memory-based paper]
(a) Nostoc
(b) Azospirillum
(c) Rhizobium
(d) Azotobacter
Answer: (c) Rhizobium
Explanation: Soybean is a leguminous crop that develops nitrogen-fixing root nodules with compatible rhizobia. Appropriate Rhizobium or Bradyrhizobium inoculation improves nodulation and nitrogen nutrition. Free-living Azotobacter and Azospirillum are beneficial microbes but not the conventional soybean symbiotic inoculant.
Q43. Biofertilizers convert atmospheric nitrogen into which compound?
[MP Patwari, 24 March 2023, memory-based paper]
(a) Ammonia
(b) Nitrate
(c) Nitride
(d) Uric acid
Answer: (a) Ammonia
Explanation: Biological nitrogen fixation uses nitrogenase to reduce molecular nitrogen into ammonia. Ammonia subsequently forms ammonium ions in soil and may be converted into nitrite and nitrate through nitrification. Fixation and nitrification are separate steps of the soil nitrogen cycle.
Q42. Which pigment is essential for effective nitrogen fixation in the root nodules of legumes?
[Bihar TRE 2.0, 9 December 2023]
(a) Anthocyanin
(b) Leghemoglobin
(c) Phycocyanin
(d) More than one of the above
(e) None of the above
Answer: (b) Leghemoglobin
Explanation: Leghemoglobin is the oxygen-binding red pigment found in functional legume root nodules. It regulates free oxygen concentrations, enabling respiration while protecting oxygen-sensitive nitrogenase. Anthocyanins color plant tissues, whereas phycocyanin is a photosynthetic pigment of cyanobacteria.
Q41. What term denotes fungi living symbiotically with roots of higher plants?
[MP Forest Guard, 7 June 2023, Shift 3]
(a) Mycorrhiza
(b) Phycorrhiza
(c) Lichens
(d) Corals
Answer: (a) Mycorrhiza
Explanation: Mycorrhiza describes the association of fungal hyphae with plant roots. The fungi increase effective access to phosphorus and soil moisture, while the plant supplies carbohydrates. These associations improve nutrient capture and are used in sustainable agriculture and ecological restoration.
Q40. The blue-green alga Anabaena lives in Azolla leaves. What relationship do they form?
[SSC MTS, 12 September 2023, Shift 1]
(a) Autotrophic
(b) Symbiotic
(c) Parasitic
(d) Saprophytic
Answer: (b) Symbiotic
Explanation: Anabaena supplies biologically fixed nitrogen to the aquatic fern Azolla, while the fern provides a protected habitat and nutrition to its cyanobacterial partner. Both benefit, making the association symbiotic or mutualistic. This partnership underpins Azolla use in rice cultivation.
Q39. Match:
A. Blue-green algae as biofertilizers;
B. Fungal biofertilizers;
C. Free-living nitrogen-fixers;
D. Phosphate-solubilizing bacteria.
With
i. Ectomycorrhiza,
ii. Thiobacillus,
iii. Anabaena,
iv. Clostridium.
[KVS PGT Biology, 20 February 2023, Shift 1]
(a) A–iii, B–i, C–iv, D–ii
(b) A–iv, B–i, C–ii, D–iii
(c) A–iv, B–iii, C–ii, D–i
(d) A–iv, B–i, C–iii, D–ii
Answer: (a) A–iii, B–i, C–iv, D–ii
Explanation: Anabaena is a cyanobacterial nitrogen fixer; ectomycorrhiza represents a fungal root association; Clostridium includes free-living anaerobic nitrogen-fixing bacteria. The examination associates Thiobacillus with phosphorus mobilization, completing the supplied correspondence among the four categories.
Q38. What is Azolla?
[SSC Stenographer, 17 November 2022, Shift 2]
(a) A fern
(b) An alga
(c) A fungus
(d) A bryophyte
Answer: (a) A fern
Explanation: Azolla is a tiny free-floating aquatic fern, not an alga. Its leaf cavities host nitrogen-fixing cyanobacteria, making the association useful in rice farming. Azolla is also used as green manure and sometimes as a protein-rich supplementary livestock feed.
Q37. Which fertilizer contains the nitrogen-fixing cyanobacterium Anabaena?
[WB TET Paper 1, 2022]
(a) Azolla biofertilizer
(b) Dung manure
(c) Diammonium phosphate fertilizer
(d) NPK chemical fertilizer
Answer: (a) Azolla biofertilizer
Explanation: The aquatic fern Azolla contains a nitrogen-fixing cyanobacterial symbiont conventionally called Anabaena azollae. The organism supplies fixed nitrogen through its association with the fern. DAP and NPK are mineral fertilizers, and ordinary dung manure does not contain this specialized partnership.
Q36. What are preparations containing microorganisms that enhance availability of plant nutrients in soil called?
[NABARD Grade A, 7 September 2022, Shift 1, memory-based]
(a) Biofertilizers
(b) Azolla
(c) Ammonia-oxidizing bacteria
(d) NPK fertilizers
(e) None of the above
Answer: (a) Biofertilizers
Explanation: Biofertilizers contain beneficial living microorganisms applied to soil, seeds or plant surfaces. By fixing atmospheric nitrogen, making insoluble nutrients available or supporting root absorption, they increase plant nutrient supply. The term identifies the broader category, not one particular organism.
Q35. Which of the following is used as a biofertilizer?
[UTET Paper 1, 2022]
(a) Neem
(b) Azolla
(c) Urea
(d) Potassium
Answer: (b) Azolla
Explanation: Azolla is a fern whose cyanobacterial partner fixes atmospheric nitrogen, making it useful as biofertilizer in wet rice cultivation. Neem is mainly used for botanical pest management, while urea and potassium are chemical nutrient sources rather than living microbial inoculants.
Q34. Select the correctly matched microorganism and description.
[SSC JE Electrical, 14 November 2022, Shift 3]
(a) Rhizobium — pathogenic bacteria
(b) Rhizobium — symbiotic bacteria
(c) E. coli — bioleaching
(d) Clostridium — symbiotic bacteria
Answer: (b) Rhizobium — symbiotic bacteria
Explanation: Rhizobium establishes a beneficial symbiotic association with leguminous plant roots. Bacteria occupy root nodules and fix atmospheric nitrogen, while host plants provide carbohydrates. This relationship is the basis of Rhizobium inoculants used as biofertilizers for pulse crops.
Q33. Which bacterium is associated with formation of root nodules in leguminous plants?
[RRB NTPC CBT I, 24 July 2021, Shift 2]
(a) Clostridium
(b) Escherichia
(c) Rhizobium
(d) Salmonella
Answer: (c) Rhizobium
Explanation: Rhizobium species interact with legumes and form root nodules that house nitrogen-fixing bacteria. The bacteria obtain energy from plant carbohydrates and convert atmospheric nitrogen to usable nitrogen compounds. The resulting symbiosis helps reduce dependence on synthetic nitrogen fertilizers.
Q32. Which is a free-living, aerobic nitrogen-fixing bacterium?
[SSC GD, 14 December 2021, Shift 2]
(a) Clostridium
(b) Azotobacter
(c) Azospirillum
(d) Rhizobium
Answer: (b) Azotobacter
Explanation: Azotobacter is a free-living aerobic soil bacterium that fixes atmospheric nitrogen independently of root-nodule symbiosis. Clostridium includes anaerobic nitrogen fixers; Rhizobium forms legume nodules; Azospirillum is commonly described as a root-associated nitrogen-fixing plant-growth promoter.
Q31. A biofertilizer used in paddy fields is:
[HTET TGT Science, 2021]
(a) Azolla pinnata
(b) Bacillus thuringiensis
(c) Rhizobium meliloti
(d) Thermus aquaticus
Answer: (a) Azolla pinnata
Explanation: Azolla pinnata is an aquatic fern grown in flooded rice systems. Its symbiotic cyanobacteria fix atmospheric nitrogen, enriching the crop environment. Bacillus thuringiensis is mainly a biological insecticide, while Thermus aquaticus is a thermophilic bacterium best known for heat-stable enzymes.
Q30. Mycorrhiza is an example of which ecological interaction?
[UP Junior Aided Teacher, 2021]
(a) Fungistasis
(b) Mutualism
(c) Amensalism
(d) Antibiosis
Answer: (b) Mutualism
Explanation: In a mycorrhizal relationship, the fungus receives carbohydrates made by the host plant while helping roots absorb phosphorus, water and other nutrients. Because both partners benefit under typical conditions, this interaction is classified as mutualism rather than antibiosis or amensalism.
Q29. Which biofertilizer is added to rice fields?
[MPSC Subordinate Services Prelims 2020, held 4 September 2021]
(a) Red algae
(b) Brown algae
(c) Rhizobium
(d) Blue-green algae
Answer: (d) Blue-green algae
Explanation: Nitrogen-fixing blue-green algae, or cyanobacteria, thrive in flooded rice fields and enrich them with biologically fixed nitrogen. Common examples include Nostoc and Anabaena. Rhizobium instead forms symbiotic nodules primarily on leguminous plants and is not the standard answer here.
Q28. Azotobacter is a/an ________.
[SSC JE Civil, 29 October 2020, morning]
(a) Nitrogen-fixing bacterium
(b) Antibiotic
(c) Sulphur-fixing bacterium
(d) Virus
Answer: (a) Nitrogen-fixing bacterium
Explanation: Azotobacter is a genus of free-living nitrogen-fixing bacteria common in soil. Its capacity to reduce atmospheric nitrogen helps improve fertility and supports plant growth. It is neither a virus nor an antibiotic, and its principal agricultural role concerns nitrogen rather than sulphur fixation.
Q27. Which is NOT a nitrogen-fixing biofertilizer?
[SSC JE Civil, 30 October 2020, evening]
(a) Azotobacter
(b) Rhizobium
(c) Pseudomonas
(d) Azolla
Answer: (c) Pseudomonas
Explanation: In the classification used for this question, Pseudomonas is not grouped with conventional nitrogen-fixing inoculants. Many Pseudomonas strains instead solubilize phosphorus or support plant growth. Azotobacter, Rhizobium and the Azolla–cyanobacterium partnership are familiar nitrogen-fixing systems.
Q26. Which nitrogen-fixing actinomycete forms associations with higher plants?
[UGC NET Environmental Science, 29 September 2020, Shift 2]
(a) Azospirillum
(b) Clostridium
(c) Frankia
(d) Anabaena
Answer: (c) Frankia
Explanation: Frankia is a filamentous actinobacterium that forms nitrogen-fixing nodules in actinorhizal plants such as Alnus and Casuarina. Unlike typical Rhizobium–legume partnerships, Frankia associates with several non-leguminous woody plants and supplies biologically fixed nitrogen to these hosts.
Q25. Match the biofertilizers to crops:
(a) Rhizobium,
(b) Azospirillum,
(c) Acetobacter,
(d) Azolla;
(i) non-legume crops,
(ii) sugarcane,
(iii) legume crops,
(iv) lowland paddy.
[MPSC State Services Mains 2020, General Studies IV]
(a) (a)–(iii), (b)–(ii), (c)–(iv), (d)–(i)
(b) (a)–(iii), (b)–(ii), (c)–(i), (d)–(iv)
(c) (a)–(i), (b)–(iii), (c)–(iv), (d)–(ii)
(d) (a)–(iii), (b)–(i), (c)–(ii), (d)–(iv)
Answer: (d) (a)–(iii), (b)–(i), (c)–(ii), (d)–(iv)
Explanation: Rhizobium is used with legumes; Azospirillum is associated with roots of non-legume crops; Gluconacetobacter diazotrophicus is notable in sugarcane; and Azolla supports nitrogen nutrition in flooded rice cultivation. This is the correspondence specified in the MPSC paper.
Q24. Which of the following is used as a biofertilizer?
[CGPSC Civil Services Prelims, 2019]
(a) Rhizobium
(b) Blue-green algae
(c) Azolla
(d) All of these
Answer: (d) All of these
Explanation: Rhizobium fixes nitrogen in legume nodules, nitrogen-fixing cyanobacteria enrich soil, and Azolla houses cyanobacteria capable of nitrogen fixation. All three are used in biofertilization, although they differ in biology and farming applications. Therefore the inclusive choice is correct.
Q23. Nitrogen-fixing bacteria form a close association with root cells of which crops?
[UPPSC PCS Prelims, 2019]
(a) Pulses
(b) Rice
(c) Wheat
(d) Sugarcane
Answer: (a) Pulses
Explanation: Pulse crops are legumes and commonly form root nodules containing rhizobia. These bacteria fix atmospheric nitrogen and share the fixed compounds with the host, while receiving plant-derived carbon. Rice, wheat and sugarcane generally do not form this conventional Rhizobium–legume nodule symbiosis.
Q22. Which group is associated with biological nitrogen fixation?
[UPPSC PCS Prelims, 2019]
(a) Red algae
(b) Brown algae
(c) Green algae
(d) Blue-green algae
Answer: (d) Blue-green algae
Explanation: Certain blue-green algae, now classified as cyanobacteria, fix nitrogen using nitrogenase. Nostoc and Anabaena are familiar examples. Their biological nitrogen inputs are particularly valuable in submerged paddy soils and are a foundation of cyanobacterial biofertilizer technology.
Q21. Which of the following is a phosphatic biofertilizer?
[UPPSC Block Education Officer Prelims, 2019]
(a) Rhizobium
(b) Blue-green algae
(c) Azolla
(d) Vesicular-arbuscular mycorrhiza
Answer: (d) Vesicular-arbuscular mycorrhiza
Explanation: Vesicular-arbuscular mycorrhizal fungi, now usually called arbuscular mycorrhizal fungi, extend hyphae beyond the root depletion zone. They increase uptake of poorly mobile phosphorus and can improve water acquisition. The other options are associated primarily with nitrogen fixation.
Q20. What is the function of leghemoglobin in root nodules of leguminous plants?
[Chhattisgarh Teacher SETE, 11 August 2019]
(a) Nodule differentiation
(b) Expression of nif genes
(c) Removal or buffering of free oxygen
(d) Inhibition of nitrogenase activity
Answer: (c) Removal or buffering of free oxygen
Explanation: Leghemoglobin controls free oxygen concentration inside active root nodules. Oxygen is required for bacterial respiration but can inactivate nitrogenase when freely abundant. By binding oxygen, leghemoglobin sustains metabolism while protecting the nitrogen-fixing enzyme system.
Q19. Which of the following can fix atmospheric nitrogen?
1. Azolla;
2. E. coli;
3. Cyanobacteria;
4. Azospirillum.
[UP LT Grade Teacher Biology, 2018]
(a) 1, 2 and 4
(b) 2 and 3
(c) 1, 3 and 4
(d) 1, 2, 3 and 4
Answer: (c) 1, 3 and 4
Explanation: Azolla harbours nitrogen-fixing cyanobacteria; many cyanobacteria and Azospirillum bacteria also fix atmospheric nitrogen. E. coli is not generally nitrogen-fixing. Because the first, third and fourth entries qualify in this examination, the correct combination is 1, 3 and 4.
Q18. Which of the following does NOT fix nitrogen?
[UP LT Grade Teacher Biology, 2018]
(a) Mycorrhiza
(b) Frankia
(c) Rhizobia
(d) Cyanobacteria
Answer: (a) Mycorrhiza
Explanation: Mycorrhizal fungi chiefly facilitate phosphorus, water and mineral absorption rather than atmospheric nitrogen fixation. Frankia and rhizobia contain nitrogen-fixing bacteria in plant associations; certain cyanobacteria also use nitrogenase to fix nitrogen. Hence mycorrhiza is the exception.
Q17. Which nitrogen-fixing agent and crop pair is NOT correctly matched?
[UPPSC PCS Prelims, 2017]
(a) Blue-green algae — paddy
(b) Rhizobium leguminosarum — pea
(c) Azotobacter — wheat
(d) Azolla — maize
Answer: (d) Azolla — maize
Explanation: Azolla is strongly associated with flooded rice cultivation through its symbiotic nitrogen-fixing cyanobacteria, rather than maize biofertilization. Blue-green algae support paddy, Rhizobium leguminosarum associates with peas, and free-living Azotobacter can be applied to wheat.
Q16. Which is NOT a nitrogen-fixing organism?
[UPPSC PCS Mains, 2017]
(a) Nostoc
(b) Anabaena
(c) Azotobacter
(d) Mycorrhiza
Answer: (d) Mycorrhiza
Explanation: Nostoc and Anabaena include nitrogen-fixing cyanobacteria, while Azotobacter is a nitrogen-fixing soil bacterium. Mycorrhiza is the association of fungi with plant roots; its best-known nutrient function is improved phosphorus uptake, not atmospheric nitrogen fixation.
Q15. Which free-living bacterium can fix atmospheric nitrogen?
[MP Lab Assistant, 15 April 2017, Shift 1]
(a) Azospirillum
(b) Spirulina
(c) Rhizobium
(d) Nitrosomonas
Answer: (a) Azospirillum
Explanation: Azospirillum includes nitrogen-fixing bacteria frequently living in the rhizosphere or close association with cereal roots. Rhizobium is best known for symbiotic legume nodules, while Nitrosomonas oxidizes ammonia. The paper selects Azospirillum among the given alternatives.
Q14. Which biofertilizer is also used as supplementary feed for milking cattle?
[RPSC RAS/RTS Prelims, 2016]
(a) Azospirillum
(b) Azolla
(c) Azotobacter
(d) Rhizobium
Answer: (b) Azolla
Explanation: Azolla is a rapidly growing aquatic fern used as a biological nitrogen source in rice fields. Its protein-rich biomass can also supplement livestock feed. Azospirillum, Azotobacter and Rhizobium are microbial inoculants, not conventional leafy cattle-feed supplements.
Q13. In which plant are root nodules NOT formed by Rhizobium?
[UP TGT Biology, 2016]
(a) Cajanus cajan
(b) Cicer arietinum
(c) Glycine max
(d) Casuarina glauca
Answer: (d) Casuarina glauca
Explanation: Casuarina is not a leguminous host of Rhizobium; its nitrogen-fixing root nodules are associated with Frankia actinobacteria. Pigeon pea, chickpea and soybean are legumes and establish rhizobial symbioses. The question distinguishes two major kinds of nitrogen-fixing plant partnerships.
Q12. Which aquatic fern is used as a biofertilizer?
[UP Review Officer/Assistant Review Officer Mains, 2014]
(a) Salvinia
(b) Azolla
(c) Marsilea
(d) Pteridium
Answer: (b) Azolla
Explanation: Azolla is a floating water fern that contains nitrogen-fixing cyanobacteria within specialized leaf structures. The symbiosis contributes nitrogen to flooded rice systems and allows Azolla biomass to serve as green manure. Other listed ferns are not the conventional answer.
Q11. Consider: 1. Agaricus; 2. Nostoc; 3. Spirogyra. Which are used as biofertilizers?
[UPSC Civil Services Prelims, 2013]
(a) 1 and 2
(b) 2 only
(c) 2 and 3
(d) 3 only
Answer: (b) 2 only
Explanation: Nostoc is a nitrogen-fixing cyanobacterium used to increase soil nitrogen availability. Agaricus is a mushroom genus, while Spirogyra is a green alga that is not conventionally used as a nitrogen-fixing inoculant. Therefore only the second organism qualifies.
Q10. Which of the following microorganisms are used as biofertilizers?
[UPPSC PCS Mains, 2012]
(a) Virus
(b) Cyanobacteria
(c) Protozoa
(d) None of the above
Answer: (b) Cyanobacteria
Explanation: Nitrogen-fixing cyanobacteria such as Nostoc and Anabaena are commonly used to enrich rice-growing soils. Although certain beneficial fungi also serve as biofertilizers, the examination identifies cyanobacteria as the specific expected category among the listed choices.
Q9. Azolla is often used as a biofertilizer because it is associated with which organism?
[UPPSC PCS Mains, 2011]
(a) Fungus
(b) Blue-green algae
(c) Red algae
(d) Moss
Answer: (b) Blue-green algae
Explanation: Azolla hosts nitrogen-fixing cyanobacteria traditionally called blue-green algae. The symbiont supplies usable nitrogen, while the fern supplies housing and nutrients. This partnership explains why Azolla biomass is incorporated into submerged rice paddies as a biological nitrogen source.
Q8. Blue-green algae are used to supply nitrogen to which crop?
[UPPSC PCS Mains, 2010]
(a) Paddy
(b) Bajra
(c) Maize
(d) Wheat
Answer: (a) Paddy
Explanation: Cyanobacteria flourish in many flooded rice fields, where nitrogen-fixing species can convert atmospheric nitrogen into usable nitrogen compounds. This contribution supports paddy cultivation. Dryland cereals do not normally offer the same continuously flooded environment needed for typical paddy inoculation.
Q7. Which of the following is a source of biofertilizer?
[UPPSC PCS Mains, 2009]
(a) Yeast
(b) Chlorella
(c) Azolla
(d) Mould
Answer: (c) Azolla
Explanation: Azolla is a nitrogen-enriching aquatic fern that contains nitrogen-fixing cyanobacteria. It is widely recognized for its use as a biofertilizer in wetland rice systems. The other listed organisms are not the conventional biofertilizer source intended by this question.
Q6. Azolla provides a useful biofertilizer when associated with which of the following?
[Uttarakhand PCS Prelims, 2007]
(a) Blue-green algae
(b) Bone meal
(c) Cow dung
(d) Urea
Answer: (a) Blue-green algae
Explanation: Azolla harbours blue-green algae, especially the Anabaena-associated nitrogen-fixing symbiont, in leaf cavities. This partnership adds biologically fixed nitrogen to the aquatic fern and ultimately to cultivated soil. Bone meal, dung and urea do not describe this symbiotic partner.
Q5. Blue-green algae are chiefly used as biofertilizers for which crop?
[UPPSC PCS Prelims, 2006]
(a) Wheat
(b) Gram
(c) Paddy
(d) Mustard
Answer: (c) Paddy
Explanation: Certain cyanobacteria such as Nostoc and Anabaena fix atmospheric nitrogen and thrive in the flooded conditions of paddy farming. They help improve nitrogen availability and can reduce the requirement for mineral nitrogen fertilizers. Thus rice is their classic agricultural application.
Q4. Which bacterium is associated with nitrogen fixation in soybean?
[UPPSC PCS Prelims, 2006]
(a) Rhizobium leguminosarum
(b) Rhizobium japonicum
(c) Rhizobium phaseoli
(d) Rhizobium trifolii
Answer: (b) Rhizobium japonicum
Explanation: Rhizobium japonicum is the older taxonomic name for the soybean-associated nitrogen fixer now generally classified as Bradyrhizobium japonicum. In root nodules, the symbiont converts atmospheric nitrogen to compounds supporting soybean growth. The other names are linked with different legume hosts.
Q3. Where is leghemoglobin found?
[UPPSC PCS Mains, 2005]
(a) Human blood
(b) Rabbit blood
(c) Root nodules of legumes
(d) Chicken blood
Answer: (c) Root nodules of legumes
Explanation: Leghemoglobin is an oxygen-binding heme protein located in active root nodules of legumes. Its red pigment buffers oxygen so that bacteria can respire while the nitrogenase enzyme remains protected. It is distinct from the haemoglobin circulating in animal blood.
Q2. Which of the following pairs is incorrectly matched?
[UP Lower Subordinate Special Prelims, 2003]
(a) Blue-green algae — biofertilizer
(b) Chrysopa — biological control of aphids
(c) BIO 902 — mustard variety
(d) Azotobacter — bioinsecticide
Answer: (d) Azotobacter — bioinsecticide
Explanation: Azotobacter is a free-living nitrogen-fixing bacterium used as a biofertilizer, not a bioinsecticide. Chrysopa lacewing larvae prey on insect pests, while nitrogen-fixing blue-green algae improve soil fertility. The distinction separates nutrient-enriching inoculants from pest-control agents.
Q1. Which of the following is a biofertilizer?
[Uttarakhand PCS Mains, 2002]
(a) Urea
(b) 2,4-D
(c) Streptocycline
(d) Azolla
Answer: (d) Azolla
Explanation: Azolla is an aquatic fern associated with nitrogen-fixing cyanobacteria and used to enrich rice-growing soils. Urea is a mineral nitrogen fertilizer, 2,4-D is a herbicide, and streptocycline is an antibiotic preparation. Only Azolla functions as the specified biofertilizer.