Diatoms arranged on a Victorian-era antique microscope slide, produced by J & T Jones. The diameter of this minature work of art is under two millimetres. Diatoms are single-celled algae whose cell walls are constructed from silica and are known as a frustule. Diatoms are the most dominant and widespread group of eukaryotes on Earth and come under the term phytoplankton (microscopic photosynthetic plankton) and as such provide the organic biomass that most of ocean life depends on. Diatoms are major oxygen generators as well as being central to the global carbon cycle, responsible for 20% of global carbon fixation through photosynthesis. Additionally they play major roles in the cycling of other nutrients such as silicon and nitrogen. Diatoms are thought to be major contributors to crude oil deposits, following their deposition and burial on the ocean floor. Specific diatom biomarkers are currently used to locate potential sites for crude oil drilling.