A nitrogen fixing prokaryote showing symbiotic association with Azolla belongs to the genus:

Question

Accepted Answer

The correct option is 3: Anabaena

The symbiotic relationship between the water fern *Azolla* and a nitrogen-fixing prokaryote is one of the most ecologically and agriculturally significant examples of mutualism. The prokaryote responsible for this crucial function is a cyanobacterium that belongs to the genus **Anabaena**, specifically the species *Anabaena azollae*. This microorganism resides permanently in specialized cavities within the dorsal lobes of the *Azolla* leaf. In this protected environment, *Anabaena* uses specialized cells called heterocysts to perform nitrogen fixation, converting atmospheric nitrogen ($\text{N}\_{2}$) into forms of ammonia and amino acids that the fern can readily absorb. In return, the fern provides the cyanobacterium with necessary carbohydrates produced through photosynthesis and a stable, sheltered habitat. This association makes the *Azolla-Anabaena* complex a highly effective biofertilizer, particularly in rice paddies across Asia, as the fixed nitrogen is released into the water upon the decomposition of the fern.

The incorrect option is 1: Rhizobium

The genus *Rhizobium* is composed of well-known nitrogen-fixing bacteria, but they form symbiotic relationships almost exclusively with the roots of plants belonging to the family Leguminosae, such as peas, beans, and clover. The *Rhizobium* bacteria colonize the host plant's roots, inducing the formation of specialized structures called root nodules, where they perform the nitrogen-fixing process. Unlike *Anabaena*, which is a cyanobacterium and colonizes the leaf cavities of *Azolla*, *Rhizobium* is a soil bacterium that does not associate with ferns or aquatic environments in the manner described by the question. Therefore, while it is a symbiotic nitrogen fixer, its host plant is fundamentally different from *Azolla*.

The incorrect option is 2: Azotobacter

*Azotobacter* is a genus of bacteria famous for its ability to fix atmospheric nitrogen; however, it is classified as a **free-living** nitrogen fixer. This means that *Azotobacter* performs nitrogen fixation independently in the soil, without forming any specialized, permanent symbiotic association with a plant host. These are highly aerobic microbes that must protect their nitrogenase enzyme from oxygen in a different manner than symbiotic organisms. Since the question explicitly asks for a prokaryote showing a **symbiotic association** with *Azolla*, a free-living genus like *Azotobacter* is an inappropriate choice.

The incorrect option is 4: Nostoc

*Nostoc* is another genus of nitrogen-fixing cyanobacteria, closely related to *Anabaena*, and it is indeed a prokaryote. However, the classical and primary symbiotic partner consistently found in the cavities of the water fern *Azolla* is *Anabaena azollae*. While *Nostoc* species are known to form symbioses, their typical partners include hornworts, liverworts, lichens, and the tropical plant *Gunnera*, not the fern *Azolla*. Though both are filamentous cyanobacteria capable of nitrogen fixation, the biological specificity for the *Azolla* partnership rests with the *Anabaena* genus, making it the most accurate answer.