Question

In the following list, how many free-living diazotrophs are there?
Azobacter, Beijerenckia, Rhodospirillum, Bacillus, Anabaena, Nostoc.

• A. Four
• B. Six
• C. Two
• D. Three

Answer 1

The correct option is A Four

Diazotrophs are the bacteria and archaea that fix atmospheric nitrogen gas into a more usable form such as ammonia. A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen.
Azotobacter is aerobic, gram negative nitrogen fixing bacteria. The special feature of Azotobacter is, it can form cysts (not spores). It is a symbiotic free living nitrogen fixing bacteria. Beijerenckia is afree living nitrogen fixing aerobic microbe. It has more amount of nitrogenase enzyme for nitrogen reduction.
Rhodospirillum is a free living gram negative, facultative anaerobe, it can therefore be used in alcoholic fermentation under low oxygen conditions or aerobic respiration in aerobic conditions. Bacillus is a free living anaerobic or aerobic gram positive rod shaped bacterium widely found in soil and water.
Anabaena and Nostoc fix nitrogen in association with coralloid roots of Cycas. Anabaena living in association with fronds of Azolla and Nostoc living in association with thallus of Anthoceros and also fix atmospheric nitrogen. So these are not free living.
So the correct option is 'four'.