question_answer

A free living nitrogen-fixing cyanobacterium which can also form symbiotic association with the water fern Azolla is

A)  Tolypothrix                

B)  Chlorella

C)  Nostoc                       

D)  Anabaena

Correct Answer: D

Solution :

The freshwater fern Azolla harbours Anabaena (a blue green alga) in its leaf cavities. Chlorella is simply a green alga. Nostoc is a free living cyanobacteria involved in fixation and so is Tolypothrix. Hence, the correct option is [d].