Diatoms are a very important group of algae that live in great abundance just about anywhere in the world where there is water and light. These microscopic organisms range in size from about 5 micrometers (or less) to up to 2 millimeters in length. Most species live as single cells, although some form colonies, and they are capable of rapid reproduction when conditions are suitable. Diatoms constitute such a large component of ocean plankton that they are responsible for creating a significant portion of the oxygen we breathe, perhaps accounting for up to 45% of Earth's primary production.
Diatoms serve as effective ecological tools because different species tolerate different environmental conditions. There are at least 100,000 species of diatoms living today, and they have been around since at least the time of the dinosaurs, as the earliest known diatom fossils date back to the Jurassic Period. The fossils survive well in aquatic sediments because their cell walls are made of silica, a type of glass. These glass shells are perforated by tiny holes that form elaborate patterns, which are used to differentiate species under the microscope. Their capacity for preservation, along with diatom community sensitivity to change, enables their use in paleoecological studies, which seek to reconstruct environmental conditions from the past. Similarly, monitoring of living diatom assemblages aids in studying modern environmental variations and detecting habitat degradation in its earliest stages.