Chromalveolata

Chromalveolata was a proposed supergroup of eukaryotes, first outlined in a major 2005 classification as one of six primary eukaryotic lineages. It refined an earlier kingdom, Chromista, introduced by Thomas Cavalier-Smith in 1981. The group was thought to include all organisms that evolved from a single secondary endosymbiosis event, in which a bikont host engulfed a red alga. A key shared trait was the presence of plastids containing chlorophyll c.

However, the idea that Chromalveolata forms a single, natural evolutionary group (monophyly) has been abandoned. Two studies from 2008 produced evolutionary trees that split the chromalveolates apart, and more recent research continues to support this fragmentation.

Historically, many chromalveolates were classified as plants due to features like cell walls, photosynthesis, and sometimes a plant-like appearance. Under the five-kingdom system from 1969, most were placed in Protista, though water molds and slime nets went to Fungi, and brown algae remained in Plantae. Cavalier-Smith later united these diverse organisms under Chromalveolata, believing them to be monophyletic—a view now rejected.

The 2005 classification divided Chromalveolata into four main subgroups: Cryptophyta, Haptophyta, Stramenopiles (or Heterokontophyta), and Alveolata. Other groups sometimes linked to them include Centrohelida, Kathablepharidae, and Telonemia. While many chromalveolates are photosynthetic autotrophs, some—like ciliates and water molds—have lost this ability. All photosynthetic members use chlorophylls a and c, often with accessory pigments, and share similar glyceraldehyde 3-phosphate dehydrogenase proteins.

Doubts about monophyly emerged as early as 2005, and a 2006 review noted a lack of evidence for several of the six major eukaryotic groups, including Chromalveolata. By 2012, a consensus had formed that the group is not monophyletic. The four original subgroups now fall into at least two categories: one includes Stramenopiles and Alveolata, often joined by Rhizaria to form the SAR group; the other comprises Cryptophyta and Haptophyta. A 2010 study further split these: Cryptophyta appear as a sister group to SAR, while Haptophyta cluster with Archaeplastida (broadly defined plants). Kathablepharids are closely related to cryptophytes, and telonemids and centrohelids may be linked to haptophytes.

Various names have been used for different combinations of these former chromalveolate groups. Halvaria refers to the clade uniting Stramenopiles and Alveolata, identified in 2007 and 2008 analyses. The SAR group adds Rhizaria to Halvaria. Hacrobia was proposed for Haptophyta and Cryptophyta, though some analyses placed Hacrobia as a sister to Archaeplastida or even nested within it. More recently, Hacrobia has been split, with Haptophyta sister to SAR and Cryptophyta related to Archaeplastida.

Morphologically, chromalveolates lack many common features. Each major subgroup has unique traits—alveoli in Alveolata, haptonema in Haptophyta, ejectisomes in Cryptophyta, and two different flagella in Heterokontophyta—but none are universal. Shared features are limited to the common origin of chloroplasts and the presence of cellulose in most cell walls.

Ecologically, chromalveolates have major impacts. Some are harmful: dinoflagellates cause red tides that devastate fish and contaminate oysters; apicomplexans include successful animal parasites like *Plasmodium* (malaria); water molds cause plant diseases, such as *Phytophthora infestans*, which triggered the Irish potato blight. Others are vital: diatoms are key photosynthetic producers, generating much of Earth’s oxygen and absorbing carbon dioxide; brown algae, especially kelps, create underwater forest habitats and support coastal diets. Products from chromalveolates include algin from brown algae (used as a thickener in ice cream) and diatomaceous earth from diatom shells (used in reflective paint, toothpaste, and filters).