Gyrosigma diatoms. Light microscopy footage of two 90-micrometre-long specimens of Gyrosigma, a freshwater benthic diatom, after forming by cell division. Diatoms are single-celled algae that form an important part of the plankton at the base of the marine and freshwater food chains. Each diatom's siliceous cell wall is divided into two halves, one of which fits within the edges of the other like a box with a lid. Filmed in a freshwater sample from Rovinj, Croatia.