Which one of the following is not a nitrogen-fixing organism?
a. Anabaena
b. Nostoc
c. Azotobacter
d. Pseudomonas

Solution:

Option (d) is the answer.

Nitrogen fixation is aided by Anabaena, Nostoc, and Azotobacter. Pseudomonas, on the other hand, execute denitrification, which is the conversion of nitrates to nitrogen gas.