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.
