Which among the following is autotrophic microbe which can fix atmospheric nitrogen ?

All of these

a-Anabaena

b- Nostoc

C-Oscillatoria

d- All of the above is the right answer.

Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc. In paddy fields, cyanobacteria serve as an important biofertiliser. Blue green algae also add organic matter to the soil and increase its fertility.

All of the given options (Anabaena, Nostoc, and Oscillatoria) are autotrophic microbes that are capable of fixing atmospheric nitrogen. These microbes belong to a group of bacteria called cyanobacteria, also known as blue-green algae. Cyanobacteria have the ability to perform photosynthesis and are able to convert atmospheric nitrogen gas (N2) into a usable form for plants and other organisms through a process called nitrogen fixation.

Nitrogen fixation is carried out by specialized cells called heterocysts, which are present in the filamentous structure of cyanobacteria. These heterocysts provide an anaerobic environment necessary for the activity of the nitrogenase enzyme, which catalyzes the conversion of atmospheric nitrogen into ammonia (NH3) through a series of biochemical reactions.

Anabaena, Nostoc, and Oscillatoria are commonly found in various aquatic environments such as freshwater bodies, ponds, and wetlands. They form symbiotic associations with plants like Azolla (a floating fern) and cycads, providing them with a source of fixed nitrogen. These autotrophic microbes play a crucial role in the nitrogen cycle and contribute to the overall fertility of the ecosystems they inhabit.