Light micrograph of two marine centric diatoms. The larger diatom on the left belongs to the genus Coscinodiscus, the one on the right to the genus Actinocyclus. Both genera have many brown chloroplasts and one nucleus each. The samples come from the North Sea, German Bight. The relatively cool northern seas harbour very large quantities of these primary producers, which autotrophically form the basis of the food chain. At the same time, diatoms in the oceans provide about 45% of the world's oxygen. Microscopic contrast method : Differential interference contrast. Magnification 400x at a print width of 10 cm.