All plants need nitrogen to synthesize proteins, but for this purpose they are unable to utilize atmospheric nitrogen. Nitrogen fixation is brought about by two types of bacteria which are known as nitrogen fixing bacteria. One type is symbiotic nitrogen fixers that are associated with plants e.g., Rhizobium and Azospirillum. The other type of these bacteria are free living in the soil e.g., Azotobacter and Nos toe. Nostoc is photosynthetic and Azotobacter is non- photosynthetic. So that, the free living aerobic non-photosynthetic nitrogen fixing bacterium is Azotobacter.