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.