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Diatoms belong to a large group called the heterokonts, which include both autotrophs such as golden algae and kelp; and heterotrophs such as water moulds. They are a major component of marine phytoplankton and are the main primary producers of organic matter, thus forming a substantial part of the base of the food pyramid. As oxygen-producing phototrophs they generate a large part of the oxygen in the earth's atmosphere. There are around 6000 species today. However, it is believed that there are up to 100,000 species in total. Diatoms are mainly found in the sea and in freshwater in planktonic or benthic form, or they are found on stones or aquatic plants (epiphytes). Some species need pure and hardly polluted water and for this reason are also indicator organisms for unpolluted waters. Other species, in turn, which are also referred to as agricultural guilds, are typical of bodies of water that are particularly polluted by agricultural inputs, e.g. by overfertilization. Explanations in German. Wood engravings, published in 1893.