Question

Which of these is a free-living aerobic and anaerobic nitrogen-fixing bacteria respectively?
(a)Rhizobium, Clostridium
(b)Azotobacter, Clostridium
(c)Azotobacter, Agrobacterium
(d)Agrobacterium, Clostridium

Answer 1

Hint: Organisms that are capable of surviving and growing in an oxygenated environment are known as aerobic organisms whereas the organism that does not require oxygen for their survival and growth is known as anaerobic organisms.

Complete answer:
Azotobacter and Clostridium are the free-living aerobic and anaerobic nitrogen-fixing bacteria respectively. Azotobacter lives in the presence of oxygen and hence is aerobic whereas Clostridium survives without the presence of oxygen and hence is anaerobic. Azotobacter is usually found in neutral to alkaline soils and is a free-living diazotrophic bacteria. They have a unique model of metabolism by which they fix nitrogen aerobically and are involved in the high respiration rate of the cells and allow normal oxygen-sensitive nitrogenase to experience limited oxygen exposure. Clostridium constitutes 250 species and is responsible for several diseases in human beings. It is a gram-positive bacteria that includes several human pathogens and causative agents. The biosynthesis of nitrogen-containing organic compounds is done by fixing inorganic nitrogen compounds. microorganisms carry out nitrogen fixation in soil and have symbiotic relationships with plant groups including legumes.

Additional Information: Lightning fixes nitrogen through which the nitrogen gas is converted into the Oxygen gas present in the atmosphere into nitrogen oxides. Plants use the nitrate which is the reaction of water with nitrogen oxides to produce nitrous acids or nitric acid and the nitrogen in the atmosphere is non-reactive and highly stable due to the bonds between the triple nitrogen molecules. This bond can be broken by the lightning which produces energy and allows nitrogen atoms to react with the oxygen which forms nitrogen oxides.

So, the correct answer is 'Azotobacter, Clostridium'.

Note: Hermann Hellriegel discovered biological nitrogen fixation which occurs when the atmospheric nitrogen gets converted into ammonia by the nitrogenase enzyme. Azotobacter is rod-shaped and is large in size around 2 microns. Some species of Azotobacter constitute flagella for motility.