Diatoms are tiny microalgae which make up a large proportion of phytoplankton (plankton which is more closely related to plants than to animals) living in the world’s waters. More than 100,000 living species of diatoms form the basis of countless food chains, and are eaten by everything from tiny zooplankton (which includes small animals such as krill) to snails, fishes and sea cucumbers. Diatoms are unicellular (having one cell only), and the cells are protected by a two-part cell wall, with one slightly wider half and one smaller, fitting together like a box and lid. Unusually, these casings are made of silica. When the diatom divides in half to reproduce, each ‘daughter’ cell inherits one of the casings and grows one new, smaller half. This means that as the generations continue, the average size of the diatoms gets gradually smaller. Once they get to their smallest possible size, they form an auxospore instead of dividing. This grows much larger and reverses the reduction in size, before beginning division once more.

Image courtesy of Matthias Burba and the Nikon Small World competition.