Question

A free-living anaerobic nitrogen fixer is
(a ) Rhodospirillum
(b) Rhizobium
(c ) Azotobacter
(d) Beijernickia

Answer 1

Hint: Nitrogen-fixing prokaryotes can be divided into those that carry out biological nitrogen fixation in a symbiotic relationship with a plant and those that fix nitrogen in a free-living state.

Complete answer:
 Rhodospirillum is a gram-negative, photosynthetic and free-living bacteria which can convert dinitrogen into usable forms for plants due to the presence of nitrogenase, an enzyme complex responsible for the reduction of dinitrogen into ammonia.

Additional Information:
 Nitrogen is the most abundant element in the air but can frequently become a growth-limiting factor for plants since it is not always available in a biologically usable form. The high energy triple bond between the nitrogen atoms is responsible for its unreactivity. Thus, it is first converted into usable forms such as ammonia or nitrate. Reduction of nitrogen to nitrate or ammonia by prokaryotic organisms is known as biological nitrogen fixation. Only a few prokaryotes which possess nitrogenase complexes are capable of nitrogen fixation, termed as nitrogen-fixing organisms or diazotrophs.
In the soil, most of the nitrogen-fixing bacteria are free living. They include aerobic (fix N$_2$ in presence of O$_2$) as well as anaerobic(fix N$_2$ in absence of O$_2$). Only a few nitrogen-fixing prokaryotes form a symbiotic association with higher plants.

Examples of free-living nitrogen-fixing are Aerobic nitrogen-fixing bacteria: Azotobacter, Beijernickia
and Anaerobic nitrogen-fixing bacteria: Rhodospirillum
-Examples of symbiotic nitrogen-fixing bacteria: Rhizobia, Frankia
So, the correct answer is ‘Rhodospirillum’.

Note: Rhizobia and Frankia live free as aerobes in the soil but are unable to fix nitrogen. They develop the ability to fix nitrogen only as symbionts where they become anaerobic. Rhizobia exhibits symbiotic association with the roots of several legumes such as alfa-alfa, sweet pea, lentils etc. and Frankia fixes nitrogen in non-leguminous plants such as Alnus .