Frustul

The frustule, also called frustula, is the silica-based cell wall of diatoms, a diverse group of microalgae. It consists of two rigid halves, the thecae, that fit together like a lid and a box to enclose the cell. The two halves are typically referred to as the epitheca and the hypotheca, and the interface between them is a key feature for species identification. The outer surface shows species-specific ornamentation such as pores (areolae), ribs (costae), and sometimes a raphe for motility in certain pennate diatoms.

Composition and formation: The frustule is made largely of biogenic silica with a small amount of water, embedded in an organic matrix of proteins and polysaccharides that regulate silica deposition. Silica is deposited inside intracellular vesicles, and precise patterns are sculpted by proteins and other organic molecules. Growth occurs predominantly through asexual reproduction, where each daughter cell inherits one half of the parent frustule and constructs a new half within its cell. Over successive divisions, the average cell size decreases until sexual reproduction restores the original size.

Function and ecology: The frustule provides mechanical protection and defines the cell’s shape and light interactions. Diatoms are major primary producers in marine and freshwater systems, contributing substantially to global carbon fixation. When diatoms die, frustules accumulate in sediments, forming diatomaceous earth. Fossil frustules are used in biostratigraphy and paleoenvironmental reconstructions, and the silica cycles associated with frustules influence nutrient dynamics in aquatic ecosystems.

Taxonomic and material significance: Frustules are central to diatom taxonomy and are studied for insights into ecology, evolution, and materials science. Their intricate nanostructures have inspired biomimetic approaches to silica fabrication and filtration technologies.