Diatoms

Diatoms are a major group of microalgae within the phylum Bacillariophyta. They are predominantly unicellular photosynthetic organisms found in oceans, freshwater, and moist soils, and they form a substantial portion of the phytoplankton. Some species aggregate into colonies.

Their defining feature is the frustule, a cell wall made of hydrated silica with complex, species-specific patterns. The frustule consists of two halves that fit together, and as the cell grows, silica is deposited within and around these halves. When diatoms die, their rigid shells accumulate as diatomaceous earth, an important fossil reservoir and industrial resource.

Morphologically, diatoms are divided into centric diatoms, with radial or bilateral symmetry, and pennate diatoms, typically bilaterally symmetric and often capable of gliding motility via a raphe system. They inhabit a wide range of aquatic environments, from open seas to rivers and damp soils, and some tolerate brackish conditions.

Reproduction is predominantly asexual by binary fission, which gradually reduces the size of daughter cells. Sexual reproduction, involving the formation of auxospores and zygotes, restores cell size and completes the cycle.

Ecologically, diatoms are primary producers that convert light energy into organic matter and contribute to silica and carbon cycles. They respond rapidly to nutrient availability and can form seasonal or bloom events. Diatom fossils provide a rich record for paleoclimatology and biostratigraphy, and diatomaceous earth is mined for filtration, polishing, and industrial uses.