Light micrograph of the marine diatom Mediopyxis helysia. This centric diatom has its many, lenticular, golden-brown chloroplasts contracted in the centre of the cell around the nucleus. 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.