Alveolates

Alveolates are a major group of eukaryotes within the SAR clade, consisting mainly of unicellular organisms but including some colonial species. They are defined by the presence of cortical alveoli—flattened vesicles just beneath the plasma membrane—whose function is not fully understood but which is a defining shared feature.

Three principal subgroups are commonly recognized: Dinoflagellata (dinoflagellates), Ciliophora (ciliates), and Apicomplexa (apicomplexans). Dinoflagellates often have two flagella oriented in perpendicular grooves and can be photosynthetic, mixotrophic, or heterotrophic; many have cellulosic thecal plates and inhabit marine and freshwater environments, with some species responsible for harmful algal blooms. Ciliates are characterized by numerous cilia used for movement and feeding and by a dual nuclear system comprising a macronucleus and one or more micronuclei; they possess complex oral structures and contractile vacuoles. Apicomplexa are mostly parasitic in animals and plants; they possess an apical complex used to invade host cells and typically retain a reduced plastid called an apicoplast, even when nonphotosynthetic.

Ecology and evolution: Alveolates display a wide range of lifestyles, from free-living protists to intracellular parasites. They play major roles in aquatic ecosystems as primary producers in some lineages and as consumer predators or parasites in others. Phylogenetically, alveolates form a distinct lineage within the SAR supergroup, united by the presence of cortical alveoli and related cellular features.