Question

The algae bringing about nitrogen fixation:
A. Nitrosomonas
B. Nostoc
C. Pseudomonas
D. Chlamydomonas

Answer 1

Hint: Nostoc is a cyanobacterium and such organisms are also called blue-green algae. This bacterium oxidizes ammonia into nitrite as a metabolic process.
It is known as nitration (a step of Nitrification).

Complete answer: Nitrosomonas is a genus of Gram-negative and rod-shaped chemoautotrophic bacteria. It cannot fix atmospheric nitrogen.
Nostoc is a genus of cyanobacteria in the family Nostocaceae.
It has common names such as star jelly, mare's eggs, facia, etc.
It is a microorganism composed of beaded filaments that aggregate to form a gelatinous mass. These grow in water and damp places.
Nostoc species are able to fix nitrogen from the atmosphere.
These species establish nitrogen-fixing symbiotic associations with representatives of the four types of terrestrial plants: some bryophytes (hornworts and liverworts), the pteridophyte Azolla, gymnosperm cycads, and the angiosperm Gunnera.
Nostoc, Anabaena are cyanobacteria but also are known as blue-green algae.
Hence, these are the “algae” which can take part in nitrogen fixation.

Pseudomonas bacterium is a genus found widely in the environment, e.g. in soil, water, and plants. These can reduce the nitrates back into nitrogen gas ($$N_2$$) and so they are called denitrifying bacteria.
Chlamydomonas is a genus of single-celled green algae. These are found in soil and freshwater. These are unable to fix atmospheric nitrogen.

So, the correct answer is B. Nostoc.

Note: In Nostoc sp., heterocyst is the cellular site of nitrogen fixation.
It protects nitrogenase from inactivation by $$O_2$$.