Diatoms on a microscope slide. Diatoms are single-celled algae whose cell walls are constructed from silica and are known as a frustule. Diatoms produce approximately 25% of the oxgen we breathe as well as being central to the global carbon cycle, responsible for approximately 20% of global carbon fixation through photosynthesis. 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. Focus-stacked, inverted image. This circular composition is 0.9 mm across. Image is x67 magnification when printed at 10 cm across.