question_answer 28)

  Which one of the following is not a nitrogen-fixing organism? (a) Anabaena                                     (b) Nostoc                           (c) Azotobacter (d) Pseudomonas

Answer:

                  (d) Pseudomonas is a denitrifying bacteria, which convert ammonia and nitrates into free nitrogen, i.e., these denitrifying bacteria are responsible to liberate free nitrogen in the environment through nitrogen in the environment through nitrogen cycle. Other options (a), (b) and (c) show nitrogen fixing organisms .Azospiriilum and Azotobacter are free-living bacteria, which absorb free nitrogen from soil, air and convert it into salts of nitrogen like amino acids and enrich soil nutrients. Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments, many of which can fix atmospheric nitrogen, e.g., Nostoc, Anabaena Oscillatoria, etc.