Although it is very abundant in Earthˇs atmosphere, gaseous nitrogen (N2) cannot be assimilated directly by living beings. It must first be fixed in a mineral form through combination with other chemical elements. These transformations are sometimes produced in the atmosphere by the action of solar radiation, combustion, or thunderbolts, but usually micro-organisms fix the nitrogen in the soil by producing ammonium and nitrate ions. Plants can then absorb the nitrogen and use it to make amino acids, the basic constituents of proteins. This organic nitrogen is transmitted to animals through the food chain. Certain micro-organisms transform the organic nitrogen contained in excrement and dead organisms into mineral nitrogen, then into gaseous nitrogen through a denitrification reaction.