Diatoms are some of the most important and influential organisms that exist. Every person should have an appreciation for them at an early age.
Diatoms account for 23% of the primary productivity of the world. That means that of all the photosynthesis going on at any one time, almost one-quarter of it is happening inside a diatom.
All that photosynthesis happening means there must be a whole lot of diatoms out there. It's true: diatoms live throughout the lighted zone of every ocean, as well as in freshwater streams and lakes. Some species live everywhere, but most have temperature, salinity, or other environmental preferences that restrict their ranges. Even so, any watery point on the globe contains several to many different species of diatoms living in an assemblage, or community.
As primary producers, diatoms play a key role in marine food webs. They are an important food item for a larger class of animal plankton (zooplankton) such as copepods, as well as for all kinds of filter and suspension feeders from worms to anemones to mollusks like clams and mussels.
Chaetoceros colony (golden-brown rectangle, upper left-center) approached by the jaws of a copepod (lower right half of photo). The diatom's long spines, faintly visible here, might protect it from the copepod.