Question: 126: Which of the following pair represents free living nitrogen fixing aerobic bacteria?
(1) Rhizobium and Frankia
(2) Azotobacter and Beijernickia
(3) Anabaena and Rhodospirillum
(4) Pseudomonas and Thiobacillus
Answer: Option (2)
Explanation:
Free living nitrogen fixing bacteria are those that can fix atmospheric nitrogen independently without forming a symbiotic association with plants.
Azotobacter and Beijernickia are free living nitrogen fixing bacteria that require oxygen for their metabolism and are therefore aerobic.
They possess special mechanisms to protect the nitrogenase enzyme from oxygen.
Rhizobium and Frankia are symbiotic nitrogen fixing bacteria that live in association with plant roots.
Anabaena is a cyanobacterium and Rhodospirillum is a photosynthetic bacterium; they do not represent aerobic free living nitrogen fixing bacteria as a pair.
Pseudomonas and Thiobacillus are not nitrogen fixing bacteria.
Hence, the correct answer is Azotobacter and Beijernickia.