Diatoms are microscopic algae composed of separate halves, with delicate siliceous cell walls. They're usually single-celled organisms, though some live colonially. They inhabit fresh and salt waters and are among the most abundant of all phytoplankton.
Coloured scanning electron micrograph (SEM) of the diatoms Mastogloia splendida (oval) and Navicula palpebralis (elongated). Magnification: when printed 10 centimetres wide. Credit: Power and Syred / Science Photo Library