Light micrograph of the marine diatom Biddulphia alternans. The single cells, triangular in apical view, form longer chains, the cells are connected with mucus drops. The specimens are from the North Sea, German Bight. The relatively cool northern seas harbour very large quantities of these primary producers, which autotrophically form the base of the food chain. At the same time the diatoms of the oceans form about 45% of the oxygen worldwide. Microscopic contrast method : Differential interference contrast. Magnification 400x at a print width of 10 cm.