Ncert Exercises & Solutions

Which one of the following is not a nitrogen-fixing organism?

1. Anabaena

2. Nostoc

3. Azotobacter

4. Pseudomonas

(d) Pseudomonas is a dentritying bacteria, which convert ammonia and nitrates into free

nitrogen, ie., 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,

Azospirillum 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 aquatio and terrestrial

environments, many of which can fix atmospheric nitrogen, e.g., Nostoc, Anabaena,

Page 188, XII NCERT