Coloured scanning electron micrograph (SEM) of a Tetracyclus cruciformis Andrews diatom, characterized by its cruciform (cross-shaped) morphology. They are found in freshwater habitats. 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. Magnification: x1250 when printed 10 centimetres wide.