Question

Which one of the following fixes CO${}_2$ into carbohydrates?

• A. Rhizobium
• B. Nitrobacter
• C. Bacillus
• D. Rhodospirillum

Answer 1

Answer: (D)

Rhodospirillum

The organisms that can obtain their energy and carbon from sunlight and organic compounds respectively are termed as photoautotrophs. Chemoautotrophs/chemolithotrophs are the organisms that require neither sunlight nor organic compounds but derive their carbon and energy from oxidation of simple inorganic substance. The electrons from these inorganic compounds are used to oxidize carbon dioxide and hydrogen with release of modest amount of energy. Rhizobium is a nitrogen fixing bacteria while Nitrobacter is nitrifying and Bacillus is denitrifying bacteria. They use inorganic compounds (nitrogen, nitrite and nitrates) as their energy source and thus, are chemoautotrophs. This makes options A, B and C incorrect. Rhodospirillum is an anaerobic photosynthetic bacterium that use carbon dioxide as carbon source and convert it into carbohydrates. Thus, the correct answer is option D.