Which of following organisms cannot fix nitrogen?
A. Azotobacter
B. Oscillatoria
C. Anabaena
D. Volvox
E. Nostoc
Choose the correct answer from the options given below:

The correct answer is D only

Concept:

• Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into a form that plants can absorb and utilize, such as ammonia (NH₃).
• This process is carried out by certain microorganisms, including free-living and symbiotic bacteria, as well as some cyanobacteria. These organisms possess an enzyme called nitrogenase, which catalyzes the nitrogen fixation process.
• Not all organisms have the ability to fix nitrogen. Some organisms, despite being photosynthetic or aquatic, lack the enzyme nitrogenase and therefore cannot participate in nitrogen fixation.

Explanation:

• Bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil.
• Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc.
• Volvox (Option D): Volvox is a green algae that forms colonies and is photosynthetic. It does not have the enzyme nitrogenase and lacks the capability to fix atmospheric nitrogen.Volvox depends on already available forms of nitrogen in its environment for its nutritional needs.