Frustulids

Frustulids are an informal, non-taxonomic term used to refer to a group of unicellular, photosynthetic protists characterized by siliceous frustules—the two-part cell walls that enclose the organism. In practice, the term is often applied to diatoms (Bacillariophyta), though it is not a formal taxonomic grouping and may be used more broadly in ecological and paleontological contexts to describe frustule-bearing protists.

They possess a silica-based frustule composed of two valves that fit together. Valve ornamentation is highly diverse, with pores and striae forming species-specific patterns. Some members have silica raphe systems that enable gliding motility, while others are less mobile. The frustule provides protection, influences buoyancy, and shapes light capture for photosynthesis.

Frustulids occur in nearly all aquatic environments, from freshwater lakes and rivers to marine plankton and benthic habitats, including soils and sediments. They are major primary producers and contribute to carbon fixation and the global silica cycle. Their frustules, when deposited, accumulate as silica-rich sediments and crystals.

Reproduction is typically asexual by cell division, which gradually reduces the size of the frustules and cells. When size reaches a threshold, sexual reproduction restores typical cell size and genetic diversity. Environmental conditions influence the balance of reproductive modes.

The robust silica frustules fossilize well, yielding extensive microfossil records. These remains support paleoenvironment reconstructions, biostratigraphy, and studies of historical silica fluxes. While not a formal taxonomic group, frustulids describe a widespread and ecologically important lineage of frustule-bearing protists.

See also: Frustule, Diatoms, Silica cycle.