Nitrogen in the atmosphere is one of the most abundant chemical elements and makes up a large part of the earth's atmosphere. However, it is relatively inert, which means it does not readily react with other chemicals and produce compounds that are more readily available for use by plants, animals and other organisms. Only certain organisms are capable of a direct usage of nitrogen gas. Examples of these organisms are certain species of bacteria and archaea. They are referred to as diazotrophs. These bacteria are able to assimilate and then fix the nitrogen gas into a new nitrogen-containing compound (e.g. ammonia) that can be used more readily by other organisms such as plants. Rhizobia, Azospirillum, Klebsiella penumoniae, Clostridium and Azotobacter vinelandii are some of the microorganisms capable of nitrogen fixation. Rhizobium species are able to carry out nitrogen fixation only when they are in symbiotic association with leguminous plants. Rhizobium bacteria are found inside the root nodules of leguminous plants where they supply ammonia to the plant and the plant in turn provide for the organic acids (mainly dicarboxylic acids malate and succinate) as a carbon and energy source.