Rhizobium nitrogen-fixing bacteria. Coloured scanning electron micrograph (SEM) of nitrogen-fixing soil bacteria (Rhizobium sp.). These bacteria are often found living symbiotically in the roots of leguminous plants. When doing so, they are able to convert (fix) atmospheric nitrogen in the soil to ammonia. The plants are unable to carry out this process, although it is vital for the production of amino acids, the building blocks of proteins. In return the plant passes carbohydrates produced during photosynthesis to the bacteria for use as an energy source.