Assertion (A): The cyanobacterium Anabaena is a diazotroph.
Reason (R): Heterocysts of Anabaena lack photosystem II.

Question

Assertion (A): The cyanobacterium Anabaena is a diazotroph. 
Reason (R): Heterocysts of Anabaena lack photosystem II.

Accepted Answer

The correct option is (2): Both (A) and (R) are correct but (R) is not the correct explanation of (A).

Students, let's analyze these statements individually and then evaluate their relationship. The Assertion (A) states that the cyanobacterium *Anabaena* is a diazotroph. This is absolutely correct. A diazotroph is any organism capable of converting atmospheric dinitrogen gas ($N\_{2}$) into a biologically usable form like ammonia ($NH\_{3}$), a process called nitrogen fixation. *Anabaena* is a classic example of a filamentous cyanobacterium that performs this vital ecological function, primarily within specialized, thick-walled cells known as heterocysts. The Reason (R) states that heterocysts of *Anabaena* lack photosystem II (PSII). This is also a correct biological fact. Photosystem II is the component of the photosynthetic machinery responsible for the photolysis of water, which generates oxygen as a byproduct. Since the nitrogenase enzyme, which catalyzes nitrogen fixation, is extremely sensitive to oxygen and is irreversibly damaged by it, the heterocyst must maintain an anoxic (oxygen-free) environment. The absence of PSII is the key structural adaptation that ensures no oxygen is internally produced, thereby protecting the nitrogenase enzyme and allowing the process to proceed. Therefore, both (A) and (R) are true statements. However, (R) is not the *correct explanation* of (A). Assertion (A) states the fundamental nature of the organism—that it fixes nitrogen (diazotroph). This classification is due to the *presence* of the genetic information (the *nif* genes) and the *nitrogenase enzyme itself*. Reason (R) describes a necessary *adaptation* (oxygen protection) that *enables* the enzyme to function effectively. The lack of PSII is the "how" of protection, not the fundamental "why" of its being a diazotroph. Therefore, there is no direct causal link between the two statements in the required sense.

The incorrect option is (1): Both (A) and (R) are correct and (R) is the correct explanation of (A).
This option is invalid because, as explained, the relationship between (A) and (R) is one of adaptation, not direct causation. The core reason *Anabaena* is a diazotroph is its possession of the nitrogenase enzyme system, which is a fact independent of the protective mechanism. While the lack of PSII is necessary for the enzyme to *work*, it is not the *reason* for the organism’s inherent biological capacity or classification as a diazotroph. The Reason explains an accessory mechanism, not the central trait.

The incorrect option is (3): (A) is true but (R) is false.
This option is incorrect because Reason (R) is biologically accurate. The absence of Photosystem II in the heterocysts is a non-negotiable requirement for nitrogen fixation in an oxygen-producing organism like a cyanobacterium. If PSII were present and functional, the resulting oxygen would immediately inhibit and destroy the nitrogenase, halting the nitrogen fixation process. Therefore, the statement in the Reason is a well-established and essential fact of cyanobacterial physiology.

The incorrect option is (4): (A) is false but (R) is true.
This is incorrect because Assertion (A) is undoubtedly true. *Anabaena* is a widely known and studied example of a nitrogen-fixing microorganism, playing a critical role in aquatic and terrestrial ecosystems, particularly in rice paddies. The term "diazotroph" perfectly defines this capacity. Given that (A) is a true statement, this option, which claims (A) is false, must be rejected.