Nitrogen-fixing bacteria. Coloured scanning electron micrograph (SEM) of nitrogen-fixing soil bacteria (Rhizobium sp.) living on a root nodule of a bean plant. These bacteria (pink) have a symbiotic relationship with the plant, where both the plant and bacteria benefit from this arrangement. The bacteria are able to convert (fix) atmospheric nitrogen in the soil to ammonia, which the plant needs but is unable to make itself. In return, the plant passes carbohydrates produced during photosynthesis to the bacteria for them to use as an energy source. Magnification: x50 when printed 10 centimetres tall.