The diatoms are one of the largest and ecologically most significant groups of organisms on Earth. They are also one of the easiest to recognize, because of their unique cell structure, silicified cell wall and life cycle. They occur almost everywhere that is adequately lit (because most species need light for photosynthesis) and wet - in oceans, lakes and rivers; marshes, fens and bogs; damp moss and rock faces; even on the feathers of some diving birds. Some have been captured by other organisms and live as endosymbionts, e.g. in dinoflagellates and foraminifera. Because of their abundance in marine plankton, especially in nutrient-rich areas of the world’s oceans, diatoms probably account for as much as 20% of global photosynthetic fixation of carbon (~ 20 Pg carbon fixed per year: Mann 1999), which is more than all the world’s tropical rainforests.