Question

Which of the following is an autotrophic bacterium?
A. Rhizobium
B. Azotobacter
C. Nitrobacter
D. Clostridium

Answer 1

Hint: The autotrophic bacterium is those organisms that use simple substances and produce complex organic compounds. Bacteria using light as their energy source are called photoautotrophs. Bacteria using inorganic substances as their energy source are called chemoautotrophs.

Complete answer:
Autotrophic bacteria are able to synthesize all of their cell constituents using carbon dioxide as the carbon source. The most common pathways for synthesizing organic compounds from carbon dioxide are the reductive pentose phosphate cycle, the tricarboxylic acid cycle, and the Acetyl-CoA pathway.
The two different types of autotrophic bacteria are-
Photoautotrophs- They derive energy from sunlight.
Chemoautotrophs -They use chemical energy to prepare their food.
Rhizobia are diazotrophic bacteria. They fix nitrogen after they get established inside the root nodules of legumes.
Azotobacter is a genus of free-living diazotrophic bacteria. Its resting stage is a cyst. It is found in neutral to alkaline soils, in aquatic environments, and on some plants.
Nitrobacter is a genus consisting of rod-shaped, gram-negative bacteria. These are chemoautotrophic bacteria. Nitrifying bacteria convert ammonia to nitrite and then it is further converted to nitrate. In this oxidation process, energy is released. Nitrate is utilized by plants. E.g. Nitrosomonas, Nitrobacter.
Clostridium botulinum is a soil-dwelling, heterotrophic bacteria. It metabolizes food strictly through fermentation.

Hence, the correct answer is option (C).

Additional information:
A diazotroph is a type of microorganism that is able to grow without external sources of fixed nitrogen. A heterotroph is a type of organism that cannot produce its own food. But takes nutrition from mainly plant or animal matter and other sources of organic carbon.

Note: