Diatom, a member of the largest group of algae in the golden algae phylum. Diatoms are unicellular but may occur as stalklike or branching colonies. Their cell walls consist of almost identical halves, which fit together much as a box fits into its lid. The walls contain some cellulose, but are primarily composed of silica, which gives them rigidity and also produces elaborately sculpted patterns of grooves that often serve as identifying features. The cytoplasm contains the green pigment chlorophyll, but other pigments, especially the yellowish xanthophyll, give the organisms a golden-brown appearance.

Reproduction is usually by cell division. The shell separates, and each half secretes a slightly smaller shell that fits inside the old one. Successive cell divisions result in ever smaller daughter cells until a minimum size is reached. Periodically, sexual reproduction by means of fertilization of haploid gametes produces cells that are the original size of the organism.

More than 8000 species exist, mostly in fresh ponds or on the surface layers of the oceans, where they are a major component of the plankton on which marine life depends. The fossil remains of diatom shells are called diatomaceous earth.

Scientific classification: Depending upon the classification system used, diatoms either make up the class Bacillariophyceae in the phylum Chrysophyta, or they make up the separate phylum Bacillariophyta.