Bacillariophyta

Bacillariophyta is a division of mostly unicellular, photosynthetic algae commonly known as diatoms. They are distinguished by their unique siliceous cell walls, called frustules, composed of two halves that fit together like a petri dish—the epitheca and hypotheca. The frustule displays intricate, species-specific patterns of pores and ridges and fossilizes well.

Most diatoms are marine or freshwater plankton, though many are benthic. They come in centric (radially symmetric) and pennate (bilaterally symmetric) forms. Pennate diatoms may produce a raphe, a slit used for lateral movement by gliding. Diatoms are photosynthetic and contain pigments such as chlorophyll a and c and fucoxanthin, giving them a brownish hue. They store energy as chrysolaminarin.

Ecology and role: Diatoms contribute substantially to global primary production and the marine carbon cycle. They require dissolved silica for frustule construction, and silica availability can limit growth. Diatoms form diatomaceous earth when fossilized.

Reproduction and life cycle: Diatoms reproduce primarily asexually by mitosis, with daughter cells inheriting one half of the frustule; this leads to gradual size reduction over generations. When cells reach minimum size, sexual reproduction occurs, producing an auxospore that enlarges to the initial size and rebuilds the frustule.

Fossil record and applications: Diatom frustules are abundant in sediments from the Jurassic onward and provide valuable proxies in paleolimnology and paleoceanography. Economically, diatomaceous earth is used in filtration, polishing, and as absorbents. Because of their sensitivity to environmental changes, diatoms are used as bioindicators of water quality.