In diatoms, the pigmented organelle that is the site of photosynthesis.

Plastids contain carotinoid pigments such as beta-carotene, diatoxanthin, diadinoxanthin, and fucoxanthin. They also contain a combination of chlorophylls a, c1, c2, and c3, depending on the species.

Most diatom plastids appear golden brown because the carotenoid pigments mask the color of the chlorophylls.

Alternative Terms

An older term, often used more in European literature.
A plastid can be defined as any organelle that is the site of manufacture and storage of important chemical compounds used by the cell. It may or may not contain pigments. A chloroplast is an example of a pigmented plastid, but the term, plastid, is preferred over chloroplast when referring to this organelle in diatoms.
Frustulia Live
Image Credit: S. Spaulding
Portion of a cell of Frustulia showing the golden brown plastids. Also visible are the transparent silica diatom valve and four distinct lipid vesicles.
Okoboji 6 3 12 9
Image Credit: M. Potapova
Image of a filament of the green alga Cladophora, with attached living diatoms. Cells of Cocconeis are lying prostrate on the surface of the Cladophora. Colonies of Rhoicosphenia abbreviata are attached by stalks. The golden brown plastids are visible in the diatom cells.
Disc Plastid
Image Credit: W. Smith (1856, fig. 330)
Digital illustration of a drawing by William Smith (1856) of the multiple discoid plastids within the filaments of Melosira moniliformis.