Coloured scanning electron micrograph (SEM) of fossilised diatoms, Coscinodiscus marginatus (left) and Eupyxidicula sp. (right). The image shows the intricate silica-based cell walls of two fossil diatoms and the complexity of these microscopic organisms. Diatoms contribute significantly to the oxygen in the atmosphere and are a fundamental link in the aquatic food chain. They are among the most diverse species on earth. Estimates of the number of diatom species range from 20,000 to 2 million. Scientists are discovering new species every year.200.000. Scientists are discovering new species every year. Magnification: x600 when printed 10 centimetres wide.