Cyanobacteria are the oldest forms of life on earth. They are bacteria containing leaf green, which means they can obtain energy from photosynthesis. Cyanobacteria are also called blue-green algae. Fossil specimen from this group have been found dating back more than three billion years. Cyanobacteria in the sea plays a more minor role than other marine plankton. Some species on the mudflats make use of solar energy while others can survive where there is no oxygen or light by degrading chemical compounds. An important function of the land-living cyanobacteria in the ecosystem is its capacity for fixing nitrogen. Plants such as clover and sea buckthorn add nitrogen to the soil from the air by working together with cyanobacteria. Lichens are an intimate relationship between fungi and an alga or a cyanobacteria. That is why thy are able to survive in very dry or nutrient-poor places.