Question

The common nitrogen fixer in paddy fields is
A. Frankia
B. Rhizobium
C. Azospirullum
D. Oscillatoria

Answer 1

The mechanism by which atmospheric nitrogen is converted to a form of nitrogen such as ammonia by either natural or industrial means is nitrogen fixation. Most nitrogen in nature is harvested by microorganisms from the atmosphere to form ammonia, nitrites, and nitrates that can be used by plants. A paddy field is a flooded field of arable land for the cultivation of semi-aquatic crops, especially rice.

Complete answer:
Plants do not have available atmospheric nitrogen. Many bacteria fix nitrogen that turns atmospheric nitrogen into ammonium compounds and makes it accessible to plants. Examples include Azotobacter, Frankia, Azospirillum, and Rhizobium.
Frankia is a genus of nitrogen-fixing bacteria that live in symbiosis with actinoric plants, similar to the Rhizobium bacteria found in the tubers of legumes of the Fabaceae family. Frankia also initiated root tuber formation.
Rhizobium is a soil bacteria that fixes nitrogen belonging to the genus of gram-negative bacteria. Rhizobium species from endosymbiotic nitrogen-fixing associations with legumes. The bacteria colonize plant cells in the roots, where they use the nitrogenase enzyme to convert atmospheric nitrogen to ammonia and then provide organic nitrogen compounds such as glutamine.
In freshwater and soil environments, Azospirillum is found especially in close relationships with plant roots. It is assumed that interactions with plants are largely beneficial. It has been reported that over 113 plant species in thirty-five different plant families have benefited from the association with the Azospirillum species. The growth of the Chlorella Vulgaris algae was positively influenced by the presence of Azospirillum, in addition to vascular plants. Azospirillum is commonly used in paddy fields as a nitrogen-fixing organism. It is used in rice fields as a biofertilizer. It encourages the production of plants. Azospirillum is an important symbiotic bacteria and biological fertilizer used in rice fields. Treatment with Azospirillum is recommended for seed, seedling, and in the field. Azospirillum bacteria thrive in rice root areas and can fix more nitrogen in the air, which is absorbed by plants. Root exudates from cultivated plants provide nutrients for the survival and reproduction of bacteria. Azospirillum also dissolves phosphorus and silicon to the extent necessary for rice. Helps with drought tolerance when watering or rainfall is delayed. By applying Azospirillum, the use of inorganic nitrogen can be reduced by up to thirty per cent.
Oscillatoria is a genus of filamentous cyanobacteria named for vibrations in motion. The filaments in the colony can slide back and forth against each other until the entire mass is aligned with the light source. It is usually found in irrigated waters and is mainly blue-green or brown-green.

Hence, the correct answer is option (C).

Note: The genus Azospirillum belongs to the bacterial alpha-Proteobacteria class. Azospirillum is gram-negative, does not form spores, and has an oblong-rod shape that is slightly twisted. There is at least one flagellum in Azospirillum and sometimes several flagella, which they use to travel rapidly. Azospirillum is aerobic, but many may also act as microaerobic diazotrophs, which means that they can convert inert nitrogen from the air into biologically functional forms under low oxygen conditions.
Some nitrogen-fixing bacteria, such as Azospirillum, live in the vicinity of the roots of higher plants without forming a close relationship. Bacteria accept plant exudate and use it as part of their nutritional needs. Bacteria fix nitrogen and release some of the fixed nitrogen for use by plants. This phenomenon is called associative mutualism.