Most plankton are either slow-swimming or so small that their swimming speed is small. Diatoms grow by division with the frustule getting ever smaller, after which they go through a sexual stage that grows without a frustule. Diatoms often appear yellow-brown, and can color the water noticeably. These are mostly considered nutritious food for zooplankton. Diatom frustules are the major ingredient of diatomaceous earth, and are a significant source of siliceous sediments at high latitudes.
Diatoms are also the source of much of the world's petroleum deposits.
Diatoms produce oil, both to store chemical energy from photosynthesis and to increase buoyancy. When large coastal diatom blooms die and sink to the bottom, they can be buried under sediments and later compressed during rock formation, which squeezes out the oil, forming pools of oil under sections of impermeable rock. Relatively large, slow-growing cells with two flagella used for swimming. Red tides are not easy to predict, but seem to occur most often with calm water, stratification, and high nutrient concentrations. These characteristics make tropical areas near large population centers very vulnerable.