Biofertilizers MCQs with Explanation

Biofertilizers MCQs with Answer and Explanation

Biofertilizers
General account of the microbes used as biofertilizers for various crop plants and their advantages over chemical fertilizers.
Symbiotic N₂ fixers: Biological pathway, Rhizobium - Isolation, characteristics, types, inoculum production and field application, legume/pulses plants
Frankia - Isolation, characteristics, Alder, Casurina plants, non-leguminous crop symbiosis. Cyanobacteria, General characteristic, Heterocyst and its function, Azolla - Isolation, characterization, mass multiplication, Role in rice cultivation, Crop response, field application.
Indole acetic acid (IAA) producer isolation, biological pathway, characteristics, inoculam production and field application

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This page provides a comprehensive collection of 50 practice MCQs for B.Sc. Microbiology students, meticulously created based on previous year questions (PYQs) to cover key topics like Rhizobium, Frankia, Azolla, and IAA production. Each question includes a detailed explanation to ensure a clear understanding of microbial roles and their practical applications in sustainable agriculture.

1. Which of the following is considered a primary advantage of biofertilizers over chemical fertilizers?

A) They provide immediate high-concentration nitrogen

B) They improve soil health and long-term fertility

C) They are synthetic and fast-acting

D) They are more expensive but highly potent

View Answer & Explanation

Correct Answer: B)

Explanation: Biofertilizers are eco-friendly, replenish soil nutrients, and improve soil structure, whereas chemical fertilizers can degrade soil health over time.

2. Rhizobium forms a symbiotic relationship with which plant family?

A) Poaceae

B) Fabaceae

C) Solanaceae

D) Brassicaceae

View Answer & Explanation

Correct Answer: B)

Explanation: Rhizobium species are well-known for their symbiotic nitrogen-fixing association with the roots of leguminous plants, which belong to the family Fabaceae.

3. What is the specific function of the heterocyst in Cyanobacteria like Anabaena?

A) Photosynthesis

B) Reproduction

C) Nitrogen fixation

D) Storage of carbohydrates

View Answer & Explanation

Correct Answer: C)

Explanation: Heterocysts are specialized, thick-walled cells in filamentous cyanobacteria where nitrogen fixation occurs, as the enzyme nitrogenase is sensitive to oxygen produced during photosynthesis.

4. Which organism is known as the "biofertilizer of the rice field"?

A) Rhizobium

B) Azolla

C) Frankia

D) Aspergillus

View Answer & Explanation

Correct Answer: B)

Explanation: Azolla is an aquatic fern that harbors the nitrogen-fixing cyanobacterium Anabaena azollae, significantly boosting nitrogen levels in rice paddies.

5. Frankia typically forms a symbiotic association with which type of plants?

A) Leguminous plants

B) Non-leguminous woody plants

C) Aquatic ferns

D) Cereal crops

View Answer & Explanation

Correct Answer: B)

Explanation: Frankia is an actinobacterium that forms symbiotic root nodules in many non-leguminous plants, such as Alnus (Alder) and Casuarina.

6. Indole acetic acid (IAA) produced by certain microbes acts as a:

A) Fungicide

B) Fertilizer

C) Plant growth promoter

D) Herbicide

View Answer & Explanation

Correct Answer: C)

Explanation: IAA is a common auxin, a type of plant hormone produced by various plant-growth-promoting rhizobacteria (PGPR) that stimulates root elongation and plant growth.

7. The enzyme responsible for nitrogen fixation in Rhizobium is:

A) Nitrogenase

B) Nitrate reductase

C) Nitrogen oxidase

D) Urease

View Answer & Explanation

Correct Answer: A)

Explanation: The Nitrogenase complex is the essential enzyme responsible for the biological reduction of dinitrogen (N₂) to ammonia (NH₃).

8. What is the standard carrier material used in inoculum production for biofertilizers?

A) Clay

B) Lignite or peat

C) Sand

D) Plastic granules

View Answer & Explanation

Correct Answer: B)

Explanation: Lignite or peat is commonly used as a carrier because they provide a stable environment, retain moisture, and support the survival of the bacteria until field application.

9. Which of these is a non-leguminous plant that forms a symbiosis with Frankia?

A) Pea

B) Casuarina

C) Soyabean

D) Groundnut

View Answer & Explanation

Correct Answer: B)

Explanation: Casuarina is a classic example of a non-leguminous actinorhizal plant that partners with Frankia to fix atmospheric nitrogen.

10. Cyanobacteria contribute to soil fertility primarily by:

A) Decomposing organic matter

B) Fixing atmospheric nitrogen

C) Increasing soil salinity

D) Producing antibiotics

View Answer & Explanation

Correct Answer: B)

Explanation: Many cyanobacteria are photoautotrophic and can fix atmospheric nitrogen, contributing significantly to nitrogen accumulation in soils, especially in wetlands.

12. What is the role of Congo Red in Rhizobium culture identification?

A) It stains the bacteria red

B) It helps differentiate Rhizobium from Agrobacterium (Rhizobium absorbs it weakly)

C) It promotes bacterial growth

D) It acts as an antibiotic

View Answer & Explanation

Correct Answer: B)

Explanation: Rhizobium colonies absorb Congo Red very weakly, appearing white or translucent, whereas contaminants like Agrobacterium often absorb it deeply, appearing pink or red.

13. Which of the following is a free-living cyanobacterium?

A) Nostoc

B) Anabaena

C) Both A and B

D) None of these

View Answer & Explanation

Correct Answer: C)

Explanation: Both Nostoc and Anabaena can exist as free-living organisms in the soil or in association with other plants (like Azolla).

14. What is the primary precursor for the biological synthesis of IAA?

A) Phenylalanine

B) Tryptophan

C) Methionine

D) Glycine

View Answer & Explanation

Correct Answer: B)

Explanation: The most common pathway for IAA biosynthesis in bacteria is the tryptophan-dependent pathway.

15. In rice cultivation, Azolla is often incorporated into the soil as:

A) A fungicide

B) A green manure

C) A synthetic fertilizer

D) A pesticide

View Answer & Explanation

Correct Answer: B)

Explanation: Azolla acts as green manure; when incorporated into the rice field, it decomposes and releases fixed nitrogen for the crop.

16. The symbiotic association between Frankia and actinorhizal plants is similar to the legume-Rhizobium symbiosis, but occurs in:

A) Root hairs of grasses

B) Modified roots of woody perennials

C) Leaf cavities of ferns

D) Stem nodules of shrubs

View Answer & Explanation

Correct Answer: B)

Explanation: Frankia infects roots of woody dicots, forming specialized actinorhizal nodules.

17. Which environmental factor is most critical for the survival of biofertilizers during field application?

A) High UV radiation

B) Soil moisture and temperature

C) Presence of heavy metals

D) High acidity

View Answer & Explanation

Correct Answer: B)

Explanation: Soil moisture, adequate temperature, and pH are crucial for the establishment and proliferation of microbial biofertilizers.

18. The "nod" genes in Rhizobium are responsible for:

A) Nitrogen fixation

B) Nodulation and infection process

C) IAA production

D) Antibiotic resistance

View Answer & Explanation

Correct Answer: B)

Explanation: Nodulation (nod) genes encode proteins necessary for the complex signaling process that leads to nodule formation on the host plant roots.

19. Which of the following is a characteristic of a successful biofertilizer strain?

A) High specificity to host plant

B) Ability to persist in the soil

C) Genetic stability

D) All of the above

View Answer & Explanation

Correct Answer: D)

Explanation: A commercial biofertilizer must be effective, persistent, stable, and competitive against native soil microbes.

20. What is the main advantage of seed treatment (coating) with biofertilizers?

A) It is cheaper than other methods

B) It ensures the microbes are in the rhizosphere right at germination

C) It prevents seed germination

D) It removes the need for soil tilling

View Answer & Explanation

Correct Answer: B)

Explanation: Coating seeds with the inoculum puts the beneficial bacteria in direct contact with the roots as soon as they emerge.

21. What is the typical shape of a Frankia colony on solid media?

A) Flat and spreading

B) Compact and lobed/spherical

C) Stringy and filamentous

D) Rapidly liquefying

View Answer & Explanation

Correct Answer: B)

Explanation: Frankia colonies are characteristically slow-growing and typically appear as compact, lobed, or spherical structures (often referred to as colonies with a mycelial "hairy" margin) on specialized agar media.

22. Which pigment, besides chlorophyll, is primarily involved in cyanobacterial photosynthesis?

A) Phycobiliproteins

B) Carotenoids

C) Xanthophylls

D) Anthocyanin

View Answer & Explanation

Correct Answer: A)

Explanation: Phycobiliproteins (like phycocyanin and phycoerythrin) are accessory pigments that capture light energy and transfer it to chlorophyll-a.

23. What is the primary method for mass multiplication of Azolla?

A) Seed germination

B) Vegetative fragmentation

C) Tissue culture

D) Spore induction

View Answer & Explanation

Correct Answer: B)

Explanation: Azolla reproduces rapidly by vegetative fragmentation, making it very easy to multiply in shallow water ponds for field application.

24. In the IAA biosynthesis pathway, which step is considered the rate-limiting step?

A) Conversion of Tryptophan to Indole-3-pyruvic acid

B) Carboxylation of Indole

C) Oxidation of glucose

D) Nitrogen uptake

View Answer & Explanation

Correct Answer: A)

Explanation: The conversion of L-tryptophan to indole-3-pyruvic acid (IPyA) by the enzyme tryptophan aminotransferase is typically the primary regulated step in this pathway.

25. Which of the following is NOT a benefit of using microbial biofertilizers?

A) Improvement in crop yield

B) Reduction in soil salinity

C) Increase in soil biodiversity

D) Immediate complete replacement of all synthetic fertilizers

View Answer & Explanation

Correct Answer: D)

Explanation: While biofertilizers significantly reduce the need for chemical fertilizers, they often act as supplements to, rather than total replacements for, high-input chemical farming.

26. Heterocysts are found in:

A) Unicellular cyanobacteria

B) Filamentous nitrogen-fixing cyanobacteria

C) All bacteria

D) Fungi

View Answer & Explanation

Correct Answer: B)

Explanation: Heterocysts are specialized differentiation structures specifically developed by certain filamentous cyanobacteria (like Anabaena and Nostoc) for N₂ fixation.

27. Which plant host is most commonly associated with Frankia for forest soil enrichment?

A) Rice

B) Alder (Alnus)

C) Wheat

D) Maize

View Answer & Explanation

Correct Answer: B)

Explanation: Alnus species (Alder) are well-documented actinorhizal plants that form symbiotic nitrogen-fixing associations with Frankia.

28. Which of these is a liquid formulation benefit for biofertilizers?

A) Longer shelf life

B) Easier to apply through irrigation systems

C) No carrier needed

D) All of the above

View Answer & Explanation

Correct Answer: D)

Explanation: Liquid biofertilizers provide higher counts, longer shelf life, and are very convenient for fertigation or seed treatment.

29. Rhizobium inoculum production requires:

A) High-salt concentration

B) Pure culture fermentation in a broth

C) Anaerobic conditions only

D) Presence of soil fungi

View Answer & Explanation

Correct Answer: B)

Explanation: To ensure efficacy, high-quality inoculum is produced by growing pure strains of Rhizobium in specialized fermentation broth.

30. What is the role of the "Leghaemoglobin" in root nodules?

A) To transport oxygen

B) To regulate oxygen levels for nitrogenase protection

C) To act as an enzyme

D) To provide nutrients

View Answer & Explanation

Correct Answer: B)

Explanation: Leghaemoglobin acts as an "oxygen scavenger," keeping the concentration of free oxygen low enough to prevent damage to the nitrogenase enzyme while providing oxygen for respiration.

31. What happens to the efficiency of Azolla if water temperature rises too high?

A) It increases

B) It decreases/inhibits growth

C) It remains constant

D) It converts to a parasite

View Answer & Explanation

Correct Answer: B)

Explanation: Azolla has an optimal temperature range (usually 20–30°C); temperatures exceeding this threshold generally inhibit its growth and N₂-fixing efficiency.

32. Which of the following bacteria is known for high IAA production?

A) Pseudomonas putida

B) Escherichia coli

C) Staphylococcus aureus

D) Streptococcus pyogenes

View Answer & Explanation

Correct Answer: A)

Explanation: *Pseudomonas* species, particularly *P. putida*, are widely studied and used as PGPR (Plant Growth Promoting Rhizobacteria) for their significant production of auxins like IAA.

33. Why are heterocysts thick-walled?

A) To store water

B) To resist high temperatures

C) To limit oxygen diffusion into the cell

D) To provide structural rigidity

View Answer & Explanation

Correct Answer: C)

Explanation: The thickened cell walls of the heterocyst create a micro-anaerobic environment, which is vital for the activity of the oxygen-sensitive nitrogenase enzyme.

34. What is the typical field application rate of Azolla for rice?

A) 1–2 kg/hectare

B) 100–500 kg/hectare (as a fresh weight mat)

C) 10 kg/hectare

D) 5000 kg/hectare

View Answer & Explanation

Correct Answer: B)

Explanation: For effective biomass coverage and nitrogen contribution, Azolla is generally applied as a fresh inoculum at rates ranging from 100 to 500 kg per hectare.

35. Which pathway of IAA production does NOT depend on tryptophan?

A) Tryptophan-independent pathway

B) IAM pathway

C) IPyA pathway

D) Tryptamine pathway

View Answer & Explanation

Correct Answer: A)

Explanation: While the tryptophan-dependent pathways are common, bacteria also possess a tryptophan-independent pathway (likely derived from indole-3-glycerol phosphate) to produce IAA.

36. Which of the following is a key requirement for successful Frankia inoculation in forest nurseries?

A) High phosphorus levels

B) Pure culture of Frankia or crushed nodule suspension

C) Sterilized sand only

D) High nitrate fertilizer

View Answer & Explanation

Correct Answer: B)

Explanation: Because Frankia species are often difficult to culture in large volumes and have slow growth rates, using a fresh crushed nodule suspension from an infected host plant is a practical and highly effective method for field or nursery inoculation.

37. How does the presence of combined nitrogen (e.g., urea) in soil affect the process of N₂ fixation by biofertilizers?

A) It stimulates it

B) It inhibits nitrogenase activity

C) It has no effect

D) It increases the number of nodules

View Answer & Explanation

Correct Answer: B)

Explanation: High levels of nitrogenous fertilizers suppress the expression of *nif* genes and inhibit the activity of nitrogenase, making the biofertilizer less efficient.

38. Which of the following is a "dual-purpose" biofertilizer that is also used for weed suppression in rice?

A) Rhizobium

B) Azolla

C) Phosphate Solubilizing Bacteria

D) Mycorrhiza

View Answer & Explanation

Correct Answer: B)

Explanation: Azolla forms a thick mat on the water surface, which prevents light from reaching weeds, thereby suppressing their growth while simultaneously fixing nitrogen.

39. The "IAA-producing" ability of microbes can be qualitatively screened using:

A) Gram staining

B) Salkowski reagent

C) Catalase test

D) Kirby-Bauer test

View Answer & Explanation

Correct Answer: B)

Explanation: The Salkowski reagent reacts with IAA to produce a characteristic pink-red color, providing a simple way to detect indole production in culture media.

40. Which of the following is a common host for Azolla?

A) Anabaena azollae

B) Rhizobium leguminosarum

C) Frankia alni

D) Azotobacter

View Answer & Explanation

Correct Answer: A)

Explanation: *Anabaena azollae* is the symbiotic cyanobacterium that lives within the leaf cavities of the water fern Azolla.

41. What is the shelf-life limitation of standard peat-based biofertilizer inoculants?

A) They never expire

B) Usually 6 months at room temperature

C) Only 24 hours

D) 10 years

View Answer & Explanation

Correct Answer: B)

Explanation: Microbial viability in carrier materials declines over time due to desiccation and nutrient depletion, typically limiting shelf life to around 6 months.

42. Why is Casuarina planting important in coastal areas?

A) It produces edible fruits

B) It is a salt-tolerant nitrogen fixer via Frankia symbiosis

C) It grows faster than any other tree

D) It fixes carbon only

View Answer & Explanation

Correct Answer: B)

Explanation: *Casuarina* is highly valuable for reclaiming sandy or saline coastal soils due to its symbiotic nitrogen fixation with *Frankia*.

43. Which technique is used to ensure the purity of Rhizobium isolates?

A) Streak plate method

B) Autoclaving

C) Soil drenching

D) UV sterilization

View Answer & Explanation

Correct Answer: A)

Explanation: The streak plate method is the fundamental microbiological technique used to isolate pure, single colonies of bacteria from a mixed population.

44. What is the primary reason for IAA-producing bacteria helping in stress tolerance?

A) By making the soil acidic

B) By increasing root surface area for better water/nutrient uptake

C) By killing all pests

D) By producing heat

View Answer & Explanation

Correct Answer: B)

Explanation: IAA stimulates lateral root development and root hair elongation, which increases the root's absorptive surface area, helping plants withstand drought and nutrient stress.

45. What does the "nif" gene cluster encode?

A) Photosynthesis proteins

B) Nitrogenase and associated regulatory proteins

C) Antibiotic production

D) Cell wall synthesis

View Answer & Explanation

Correct Answer: B)

Explanation: The *nif* (nitrogen fixation) gene cluster contains the blueprint for the nitrogenase enzyme complex and the necessary housekeeping proteins for its function.

46. Which of these is a disadvantage of chemical fertilizers compared to biofertilizers?

A) High cost-to-benefit ratio

B) Groundwater contamination and eutrophication

C) Quick results

D) Easy availability

View Answer & Explanation

Correct Answer: B)

Explanation: Excessive use of chemical nitrogen leads to runoff, causing eutrophication in water bodies and leaching into groundwater, whereas biofertilizers are naturally contained and biodegradable.

47. What is the function of the "mucilage" produced by Azolla?

A) It acts as a defense against predators

B) It hosts the cyanobacterial symbiont

C) It helps in photosynthesis

D) It prevents water loss

View Answer & Explanation

Correct Answer: B)

Explanation: The cavity in the Azolla leaf contains mucilage, which provides a protective, nutrient-rich niche for *Anabaena azollae* to reside and fix nitrogen.

48. Which of the following is NOT an advantage of using liquid biofertilizer over carrier-based forms?

A) Higher bacterial count

B) Less storage space required

C) Higher chemical fertilizer content

D) Easier shelf-life management

View Answer & Explanation

Correct Answer: C)

Explanation: Liquid biofertilizers do not contain chemical fertilizers; they are pure microbial suspensions, and the inclusion of chemicals would likely kill the microbial culture.

49. What is the "legume-Rhizobium" signaling process initiated by?

A) Flavonoids secreted by the plant roots

B) Bacterial oxygen production

C) Sunlight exposure

D) Soil temperature drop

View Answer & Explanation

Correct Answer: A)

Explanation: The plant releases specific flavonoids into the soil, which attract Rhizobium bacteria and trigger the expression of *nod* genes in the bacteria.

50. IAA production by PGPR is most beneficial during which stage of plant development?

A) Senescence

B) Early seedling establishment and root growth

C) Fruit ripening

D) Seed dormancy

View Answer & Explanation

Correct Answer: B)

Explanation: Because IAA stimulates initial root growth and root hair density, its presence is most critical during the early stages of plant growth when the root system is being established.

Related Topics

• Advantages and Disadvantages of Biofertilizers
• Difference Between Biofertilizer and Chemical Fertilizer