Diatoms

Diatoms are single-celled algae with cell walls made of glass-like silica. They are among the most abundant and productive of the phytoplankton, and are thought to carry out roughly a fifth of all photosynthesis on Earth.

Each cell lives inside a frustule, a two-part silica shell that fits together like a box and lid, with delicate patterns of pores and ribs. Centric diatoms are radially symmetrical and mainly float in the water, while pennate diatoms are elongated and often glide over surfaces. Diatoms take up dissolved silicon as silicic acid and are limited by it in many areas. They are a group of the algae that arose by secondary endosymbiosis, a process described in the article on endosymbiotic theory.

Diatoms bloom when nutrients are supplied by upwelling or mixing, as in the spring in temperate seas, and they feed zooplankton and fish. Because their shells are heavy, they sink quickly, which helps to carry carbon to the deep sea through the biological pump. Their needs for nitrogen, phosphorus and silicon shape the nutrient ratios discussed in the Redfield ratio.

Dead diatoms accumulate on the seafloor and lake beds and form a soft rock called diatomite, or diatomaceous earth, used for filtration, abrasives and as an additive. The shells are used by scientists to reconstruct past climates, and some diatoms contain toxins that cause harmful blooms. Together with coccolithophores they dominate the microscopic life of the sea, and they are central to the carbon cycle.