Both Azotobacter and Beijernickia are free-living nitrogen-fixing aerobic bacteria. They can fi x atmospheric nitrogen into a usable
form for plants in the soil, without forming symbiotic associations with plant roots.
(1) Both Rhizobium and Frankia are nitrogen-fixing bacteria, but they are not free-living aerobic bacteria. Rhizobium forms a symbiotic relationship with leguminous plants and fi xes nitrogen inside root nodules. Frankia, on the other hand, forms symbiotic associations with certain non-leguminous plants and fi xes nitrogen.
(3) Anabaena is a cyanobacterium that can fi x nitrogen in specialized cells called heterocysts. It can form symbiotic relationships with certain aquatic plants. Rhodospirillum is a photosynthetic bacterium, but it does not fi x nitrogen.
(4) Some Pseudomonas species are involved in nitrogen cycling; they are not known as significant free-living nitrogen fixers. Thiobacillus is not a nitrogenii fixing bacterium. Instead, it is known for its ability to oxidise sulphur compounds