Question

What do cyanobacteria do in nitrogen fixation?

Answer 1

Hint: A photosynthetic bacterium can convert the nitrogen in the atmosphere into usable organic form with the help of some special cells called heterocysts. This bacterium is predominantly found in water and moist soils.

Complete answer:
Nitrogen is present in the atmosphere in a molecular form, which cannot be used up by plants. So, with the help of nitrogen fixation, this form of nitrogen gets converted into ammonia and nitrate. It is easier for plants and other organisms to metabolize ammonia than its molecular form.

Cyanobacteria are photosynthetic bacteria that can be found in water or moist soils. They are bluish-green due to the presence of phycocyanin (blue pigment) and chlorophyll-a (green pigment). Both these pigments help in capturing the light energy from the sun.

Nitrogen fixation cannot be done in the presence of oxygen, hence anaerobic conditions are required. This is achieved with the help of heterocysts. Heterocysts are special nitrogen-fixing cells that are formed during nitrogen starvation. These cells fix nitrogen with the help of an enzyme called nitrogenase.
a) The cyanobacteria are free-living blue-green algae present in the water of rice paddies.
b) They are growing as epiphytes on the surface of the green algae.
c) They can fix not only atmospheric nitrogen but also carbon from the carbon dioxide during photosynthesis.

Note:
Nostoc, Anabaena, and Oscillatoria are nitrogen-fixing algae; the three are nitrogen-fixing algae, also termed cyanobacteria. They live in water and can manufacture their food through the process of photosynthesis and releasing oxygen. They can fix atmospheric nitrogen as free-living forms and in mutual association with the roots of plants. Other than nitrogen cyanobacteria can also fix carbon from carbon dioxide during photosynthesis.