Soil bacteria are essential for various geochemical material cycles because they act as decomposers and provide the plants with nutrient salts. A large group of bacteria are the so-called cyanobacteria which belong to the prokaryotes and not to the algae. They are photosynthetic organisms and therefore independent of organic nutrients, but require light for their energy supply. Together with green algae (Chlorophyta) and other algae groups, they form the phytoplankton of ocean and freshwaters and are the basic food source of many ecosystems.