Question

What benefit is derived from the mutualism of bacteria and legumes?

Answer 1

Mutualism, a relationship in which both species benefit from each other. This is a common behaviour found in nature.

Many legumes have root nodules in which bacteria called rhizobia resides. It's also called symbiotic nitrogen-fixing bacteria. This relationship is particularly useful and found common in nitrogen-limited conditions.
The role of Rhizobia is to convert nitrogen gas from the atmosphere into ammonia, which is then used in the formation of amino acids and nucleotides.

Hence Rhizobium converts atmospheric nitrogen into the soluble form for leguminous plants. However since Rhizobium cannot prepare their own food , it establishes in the roots of the plant and provide them with nutrients. Plants in turn provide it with food and shelter. Thus there exists a symbiotic relationship between the Rhizobium and leguminous plants.