Q25.The prokaryotic microorganism which fixes atmospheric nitrogen in symbiotic association under aerobic condition in aquatic ferns is-
(A) Rhizobium
(B) Klebsiella
(C) Anabaena
(D) Rhodospirillum
The correct answer is (C) Anabaena. Anabaena azollae, a filamentous cyanobacterium, forms a unique symbiotic association with aquatic ferns like Azolla, fixing atmospheric N₂ under aerobic conditions within leaf cavities.
Its heterocysts protect nitrogenase from oxygen, enabling efficient fixation in this oxygenated environment.
Option Analysis
Rhizobium (A): Forms root nodules with leguminous plants for symbiotic N₂ fixation, but operates under legumes (not aquatic ferns) and typically microaerophilic conditions.
Klebsiella (B): Free-living, facultative anaerobic bacterium capable of N₂ fixation, but lacks specific symbiosis with aquatic ferns.
Anabaena (C): Specifically Anabaena azollae resides in Azolla fern leaf cavities, differentiates heterocysts for aerobic N₂ fixation, and supplies fixed nitrogen to the fern.
Rhodospirillum (D): Photosynthetic purple bacterium that fixes N₂ anaerobically, not associated with aquatic ferns or aerobic symbiosis.
Anabaena represents the prokaryotic microorganism fixing atmospheric nitrogen in symbiotic association with aquatic ferns under aerobic conditions. This cyanobacterium thrives in Azolla leaf cavities, providing biofertilizer value.
Symbiosis Details
Anabaena azollae differentiates heterocysts to protect nitrogenase from oxygen while receiving fixed carbon from Azolla. Rhizobium targets legume roots. Klebsiella fixes freely; Rhodospirillum requires anaerobiosis.
