Consider the following statements regarding the role of bacteria in the nitrogen cycle: 1. Some bacteria fix atmospheric nitrogen into soil compounds. 2. Bacteria convert nitrogenous compounds in the soil into solid carbon. 3. Blue-green algae are also involved in fixing atmospheric nitrogen. Which of the statements given above is/are correct?

The correct option is (c). Explanation The nitrogen cycle is a critical biogeochemical process through which nitrogen is converted into various chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. Microorganisms, particularly bacteria and cyanobacteria, play a central role in driving these transformations, including fixation, nitrification, and denitrification. Statement-wise Analysis: - Statement 1 is Correct: Nitrogen-fixing bacteria are responsible for converting inert atmospheric nitrogen ($N_2$) into reactive nitrogen compounds like ammonia ($NH_3$) or nitrates. Examples include symbiotic bacteria like Rhizobium (found in legume root nodules) and free-living bacteria like Azotobacter. - Statement 2 is Incorrect: Bacteria in the nitrogen cycle do not convert nitrogenous compounds into solid carbon. Instead, denitrifying bacteria (such as Pseudomonas) convert nitrates present in the soil back into atmospheric nitrogen gas ($N_2$). Carbon fixation is a separate biological process associated with photosynthesis, not the nitrogen cycle. - Statement 3 is Correct: Blue-green algae, also known as Cyanobacteria (e.g., Nostoc and Anabaena), are prokaryotic organisms capable of fixing atmospheric nitrogen. They often possess specialized cells called heterocysts that facilitate this process, making them important biofertilizers in aquatic and terrestrial environments. Key Takeaway: Biological nitrogen fixation involves the conversion of atmospheric nitrogen into ammonia by bacteria and cyanobacteria. Denitrification returns nitrogen to the atmosphere. There is no pathway in the nitrogen cycle where nitrogenous compounds are converted into solid carbon.

NCERT Class 8 Science — Chapter 2: Microorganisms : friend and foe

mcqmedium

Consider the following statements regarding the role of bacteria in the nitrogen cycle:

Some bacteria fix atmospheric nitrogen into soil compounds.
Bacteria convert nitrogenous compounds in the soil into solid carbon.
Blue-green algae are also involved in fixing atmospheric nitrogen.

Which of the statements given above is/are correct?

Official previous-year question

Verified 1 Aug 2026.

Options

A1 and 2 only
B2 only
C1 and 3 only
D1, 2 and 3

Did you get this right?

Solution

The correct option is (c).

Explanation

The nitrogen cycle is a critical biogeochemical process through which nitrogen is converted into various chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. Microorganisms, particularly bacteria and cyanobacteria, play a central role in driving these transformations, including fixation, nitrification, and denitrification.

Statement-wise Analysis:

Statement 1 is Correct: Nitrogen-fixing bacteria are responsible for converting inert atmospheric nitrogen ($N_2$) into reactive nitrogen compounds like ammonia ($NH_3$) or nitrates. Examples include symbiotic bacteria like Rhizobium (found in legume root nodules) and free-living bacteria like Azotobacter.
Statement 2 is Incorrect: Bacteria in the nitrogen cycle do not convert nitrogenous compounds into solid carbon. Instead, denitrifying bacteria (such as Pseudomonas) convert nitrates present in the soil back into atmospheric nitrogen gas ($N_2$). Carbon fixation is a separate biological process associated with photosynthesis, not the nitrogen cycle.
Statement 3 is Correct: Blue-green algae, also known as Cyanobacteria (e.g., Nostoc and Anabaena), are prokaryotic organisms capable of fixing atmospheric nitrogen. They often possess specialized cells called heterocysts that facilitate this process, making them important biofertilizers in aquatic and terrestrial environments.

Key Takeaway:
Biological nitrogen fixation involves the conversion of atmospheric nitrogen into ammonia by bacteria and cyanobacteria. Denitrification returns nitrogen to the atmosphere. There is no pathway in the nitrogen cycle where nitrogenous compounds are converted into solid carbon.

Your note

More from Chapter 2: Microorganisms : friend and foe

With reference to microorganisms, consider the following statements: 1. They are classified into four major groups: bacteria, fungi, protozoa, and algae. 2. They can survive only in moderate climates and die in ice-cold or hot spring environments. 3. Some microorganisms are unicellular, while others are multicellular. Which of the statements given above is/are correct?

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Which one of the following characteristics distinguishes viruses from other microorganisms?

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Consider the following statements regarding the habitat of microorganisms: 1. They are found in air, water, and soil but not inside the bodies of animals. 2. They can exist freely as well as grow on other organisms. 3. They are capable of surviving in deserts and marshy lands. Which of the statements given above is/are correct?

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Which one of the following human diseases is caused by a protozoan pathogen?

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Consider the following pairs of microorganisms and their classification: 1. Aspergillus : Algae 2. Paramecium : Protozoa 3. Penicillium : Fungi 4. Spirogyra : Bacteria How many of the above pairs are correctly matched?

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The correct option is (c).

Explanation

Statement-wise Analysis:

Statement 1 is Correct: Nitrogen-fixing bacteria are responsible for converting inert atmospheric nitrogen ($N_2$) into reactive nitrogen compounds like ammonia ($NH_3$) or nitrates. Examples include symbiotic bacteria like Rhizobium (found in legume root nodules) and free-living bacteria like Azotobacter.
Statement 2 is Incorrect: Bacteria in the nitrogen cycle do not convert nitrogenous compounds into solid carbon. Instead, denitrifying bacteria (such as Pseudomonas) convert nitrates present in the soil back into atmospheric nitrogen gas ($N_2$). Carbon fixation is a separate biological process associated with photosynthesis, not the nitrogen cycle.
Statement 3 is Correct: Blue-green algae, also known as Cyanobacteria (e.g., Nostoc and Anabaena), are prokaryotic organisms capable of fixing atmospheric nitrogen. They often possess specialized cells called heterocysts that facilitate this process, making them important biofertilizers in aquatic and terrestrial environments.