alveolata

Alveolata is a major supergroup of eukaryotes within the SAR (Stramenopiles, Alveolates, Rhizaria) clade. It comprises three well-defined lineages: Ciliates (Ciliophora), Dinoflagellates (Dinophyta), and Apicomplexa (Sporozoa). The defining feature of alveolates is the presence of cortical alveoli—flattened vesicles just beneath the plasma membrane—that form a supportive cell layer and are visible in many members under light microscopy.

Most alveolates are unicellular, though some form colonies or exist as symbionts. Ciliates use numerous cilia for locomotion and feeding, and they typically possess a macronucleus and micronucleus with a complex sexual process called conjugation. Dinoflagellates commonly have two flagella and, in many species, thecal plates that create an armored appearance. Apicomplexa are primarily parasitic and feature an apical complex used to invade host cells; most live at a sessile or intracellular stage in their hosts.

Genetically and metabolically, alveolates show considerable diversity. Dinoflagellates often have plastids acquired through secondary endosymbiosis and may be photosynthetic, mixotrophic, or heterotrophic. Apicomplexans retain nonphotosynthetic plastids called apicoplasts. Ciliates carry two kinds of nuclear genomes and a variety of organelles adapted for feeding and digestion. Mitochondria typically possess tubular cristae, a feature shared by many alveolates.

Ecologically, alveolates play key roles in aquatic ecosystems. Dinoflagellates include important primary producers and some species responsible for harmful algal blooms. Ciliates recycle nutrients and form a link between microbial prey and larger predators. Apicomplexans include several medically important parasites, such as Plasmodium (malaria) and Toxoplasma, illustrating the clinical relevance of this group. The exact phylogenetic relationships among the alveolate lineages are active areas of research, but the alveolate clade is widely recognized as a cohesive lineage within SAR.