Correct Option
The correct option is (a).
Explanation
The Nitrogen Cycle is a biogeochemical process through which nitrogen is converted into multiple chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. This cycle is essential for life because nitrogen is a core component of amino acids, proteins, and nucleic acids (DNA and RNA).
Statement-wise Analysis
- Statement 1 is Correct: The atmosphere acts as the largest reservoir of nitrogen, constituting approximately 78% of the air. The nitrogen cycle ensures that nitrogen is continuously recycled from the atmosphere to the soil and organisms, and back to the atmosphere. Consequently, the total amount of nitrogen in the atmosphere remains more or less constant over time.
- Statement 2 is Incorrect: Atmospheric nitrogen exists as dinitrogen ($N_2$), which has a strong triple bond that renders it inert. Neither plants nor animals have the enzymes required to break this bond and utilize atmospheric nitrogen directly. Plants absorb nitrogen from the soil in the form of nitrates ($NO_3^-$) or nitrites ($NO_2^-$), while animals obtain nitrogen by consuming plants or other animals.
- Statement 3 is Incorrect: The conversion of atmospheric nitrogen into nitrogenous compounds (nitrogen fixation) is primarily carried out by certain prokaryotes, specifically bacteria (e.g., Rhizobium, Azotobacter) and blue-green algae (cyanobacteria). Viruses are non-living entities that require a host to replicate and do not possess the metabolic machinery to fix nitrogen.
Key Takeaway: Atmospheric nitrogen ($N_2$) is inert and cannot be used directly by most complex life forms. It must be "fixed" into usable forms like ammonia or nitrates, a process primarily driven by specialized bacteria, not viruses.