Which one of the following bacteria has potential for nitrogen fixation?
Correct option is D. Rhizobium
Plants cannot consume atmospheric nitrogen, directly because in atmosphere nitrogen occurs in stable diatomic form. The atmospheric nitrogen needs to be converted into plant usable forms like nitrate, nitrite and ammonium ion. This process is called as nitrogen fixation. Rhizobium is a microaerobic bacteria, which occurs in root nodules of legume plants in symbiotic association. Rhizobium bacteria reduces atmospheric nitrogen into ammonium ions using reducing power obtained from host plant.
So, the correct answer is option D.