frustules

Frustule is the term for the hard, siliceous cell wall of diatoms, a diverse lineage of microalgae found in oceans, freshwater, and moist soils. The frustule consists of two overlapping halves called the thecae: the epitheca (upper) and the hypotheca (lower). When a diatom divides, each daughter cell reconstructs a new half, and the two halves come together to enclose the cell again.

Composition and morphology: The frustule is made of hydrated silica and is surrounded by a layer of organic material. It displays species-specific ornamentation, including pores (areolae), slits, ribs, and perforations that facilitate transport of nutrients and waste. The formation occurs in a specialized intracellular compartment, the silica deposition vesicle (SDV); within the SDV, templating proteins such as silaffins and polyamines guide silica polymerization to create the intricate patterns.

Function and significance: The frustule provides mechanical protection and structural support while allowing selective exchange with the environment through its pores. The ornate patterns influence optical properties and interaction with light, and in some diatoms the raphe enables gliding motility. Frustules contribute to the sinking and sedimentation of diatom cells, affecting carbon and silica cycles in aquatic ecosystems.

Ecology and fossil record: After death, frustules accumulate as opaline silica in sediments and form diatomaceous earth, a widely exploited material for filtration, polishing, and industry. Fossil frustules are abundant in sedimentary rocks and are used in paleoclimatology and paleoceanography to interpret past productivity and silica availability.

Taxonomy and research: The diversity of frustule morphology supports diatom classification; ongoing research explores the molecular mechanisms of silicification and potential applications of biomimetic diatom-inspired nanostructures in photonics and materials science.