Question

Which one of the following is not a nitrogen-fixing organism?

• A. Pseudomonas
• B. Azotobacter
• C. Nostoc
• D. Anabaena

Hint:

Important nitrogen-fixing microorganisms: \[ \textit{Rhizobium} \] \[ \textit{Azotobacter} \] \[ \textit{Nostoc} \] \[ \textit{Anabaena} \] Remember: \[ \boxed{\textit{Pseudomonas}} \] is commonly associated with phosphate solubilization rather than nitrogen fixation.

Answer 1

The Correct Option is A

Step 1: Recall the meaning of nitrogen fixation. Nitrogen fixation is the process by which atmospheric nitrogen \[ (N_2) \] is converted into biologically usable forms such as ammonia. Certain bacteria and cyanobacteria possess this ability.

Step 2: Examine the given organisms.
• Azotobacter is a free-living nitrogen-fixing bacterium. \(\checkmark\)
• Nostoc is a nitrogen-fixing cyanobacterium. \(\checkmark\)
• Anabaena is a nitrogen-fixing cyanobacterium commonly found in paddy fields. \(\checkmark\)
• Pseudomonas is mainly known as a phosphate-solubilizing bacterium and is not a nitrogen-fixing organism. \(\times\)

Step 3: Identify the correct answer. Among the given options, \[ \boxed{\text{Pseudomonas}} \] does not fix atmospheric nitrogen. Therefore, \[ \boxed{\text{(A)}} \] is the correct answer.