Polarised light micrograph of Navicula sp. diatom algae and pink bacteria. Diatoms are aquatic single-celled photosynthetic organisms. They produce approximately a quarter of the oxygen that exists in Earths biosphere. Navicula diatoms typically have an elongated shape that resembles a boat. They are highly motile and move through a gliding movement. Pink stains in freshwater environments are most commonly caused by the bacterium Serratia marcescens. This bacterium produces a red pigment called prodigiosin, which gives the stains their pink colour. Magnification: 186x when printed 10 centimetres wide.