Diatoms. Coloured scanning electron micrograph (SEM) of Ditylum brightwellii a species of cosmopolitan marine centric diatoms. It is a unicellular photosynthetic autotroph that has the ability to divide rapidly and contribute to spring phytoplankton blooms. The D. brightwellii has a high length to diameter ratio. The cell wall is silicified, as is characteristic of all diatoms. This hard, porous covering is known as the frustule and causes the cell to be more dense than the surrounding water. Ditylum brightwellii is found in all global oceans except in polar waters. Magnification: x600 when printed 10 centimetres wide.