Coloured scanning electron micrograph (SEM) of a marine diatom frustule (round). The pointed structure with radiating arms is a sponge spicule. Diatoms are microscopic, unicellular algae that are abundant in marine and fresh water. There are many phytoplankton diatom genera that are an important part of the food chain. The cell walls contain silica dioxide (glass). Sponges are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them. Sponge spicules are structural elements found in most mesophyll sponges. They provide structural support and deter predators. Large spicules that are visible to the naked eye are referred to as megascleres, while smaller, microscopic ones are termed microscleres. Magnification: x500 when shortest axis printed at 25 millimetres.