Question

Which one is the most active and efficient nitrogen fixer?
A. Anabaena
B. Nostoc
C. Aulosira
D. Trichodesmium

Answer 1

Hint: Cyanobacteria/Blue-green algae are nitrogen-fixing bacteria. They are found in both aquatic and terrestrial habitat. Some examples are Anabaena, Azolla, Nostoc, Aulosira, Oscillatoria, Trichodesmium, etc.

Step by step answer:Cyanobacteria are a group of photosynthetic bacteria. These bacteria also help in the nitrogen-fixation. They mainly live in damp environments like moist soil or water. Sometimes they are also found in symbiotic relationships with plants and fungi.
-Cyanobacteria range from unicellular forms to filamentous forms to colonial forms. Under harsh conditions, they form climate-resistant spores known as the akinetes.
-Cyanobacteria fix the atmospheric nitrogen. Cyanobacteria have specialized cells called heterocysts that help in fixing the atmospheric nitrogen in anaerobic conditions. But sometimes heterocysts also form under suitable and favorable environmental conditions if the fixed nitrogen is scarce.

-Cyanobacteria fix or convert the nitrogen gas into ammonia, nitrites, or nitrates that can be easily absorbed by plants and converted to protein and nucleic acids.
-Anabaena or Anabaena azollae is phototrophic cyanobacteria (blue-green algae) that fixes the atmospheric nitrogen to provide nutrients to plants.
-Nostoc is a genus of cyanobacteria that also fixes atmospheric nitrogen.
-Aulosira is a genus of filamentous cyanobacteria that are active nitrogen fixers in the rice fields.
-Trichodesmium is a filamentous cyanobacterium that is found in the aquatic habitat.
Aulosira is the most active and efficient nitrogen fixer.
Hence option C is correct.

Note: Cyanobacteria are commonly known as blue-green algae or BGA. Not every BGA is bluish-green in colour, for example, Batrachospermum. They were kept along with algae but later it was discovered that they are actually bacteria.