Which one of the following fixes the atmospheric nitrogen but is not an autotroph ?

The correct option is Rhizobium.

To understand why this is the correct answer, let us break down the concepts of nitrogen fixation and autotrophic nutrition, and then analyze each of the options provided.

1. Understanding the Key Concepts:
• Nitrogen Fixation: This is the chemical process by which atmospheric nitrogen (N₂) is converted into inorganic nitrogen compounds like ammonia (NH₃) that plants can readily absorb and utilize.
• Autotrophs: These are organisms that can synthesize their own food from simple inorganic substances, typically using light energy (photoautotrophs).
• Heterotrophs: These are organisms that cannot synthesize their own food and must obtain energy and carbon by consuming or absorbing organic substances from other organisms.

2. Analyzing the Options:
• Anabaena and Nostoc: These are genera of cyanobacteria (blue-green algae). They are photoautotrophic, meaning they perform photosynthesis to produce their own food. They also possess specialized cells called heterocysts that allow them to fix atmospheric nitrogen. Because they are autotrophs, they do not satisfy the condition of the question.
• Oscillatoria: This is also a genus of filamentous cyanobacteria. Like Anabaena and Nostoc, it is photoautotrophic and performs photosynthesis.
• Rhizobium: This is a genus of soil bacteria that forms a symbiotic relationship with the roots of leguminous plants (such as peas, clover, and alfalfa). Within the root nodules, Rhizobium fixes atmospheric nitrogen for the plant. In return, the plant provides the bacteria with carbohydrates and other organic nutrients. Because Rhizobium cannot manufacture its own food and relies entirely on the host plant for its nutrition, it is a heterotroph.

Therefore, Rhizobium is the organism that fixes atmospheric nitrogen but is not an autotroph.