. Free-living nitrogen fixing aerobic bacterium is

A: Rhodospirillum

B: Anabaena

C: Nostoc

D: Beijerinckia

E: Rhizobium

Best Answer


  • The free-living nitrogen-fixing aerobic bacterium is Beijerinckia. It is free-living and can fix nitrogen under aerobic conditions, with the help of the nitrogenase enzyme. 
  • The nitrogen-fixing bacteria are mostly prokaryotic which can use atmospheric nitrogen and change it into usable forms like ammonia, nitrates, and nitrites. 
  • Rhodospirillum is a facultative anaerobe, thus, depending upon the availability of oxygen, it can either undergo aerobic respiration or alcoholic fermentation. 
  • Anabaena is a filamentous bacterium (cyanobacteria), which is free-living or forms a symbiotic association with Azolla (water fern) and has heterocyst cells for nitrogen fixation.
  • Nostoc is unicellular cyanobacteria that are found either free-living or live in symbiotic association; for example, coralloid roots of cycas contain symbiotic cyanobacteria Nostoc.
  • Rhizobium is a nitrogen-fixing bacteria, commonly found in the root nodules of leguminous plants and fixes nitrogen under anaerobic conditions. 

Final Answer

Hence, the correct option is D i.e., Beijerinckia

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