Xanthophyta, commonly called yellow‑green algae, is a group of photosynthetic stramenopiles characterized by their yellow‑green coloration. These organisms belong to the class Xanthophyceae within the heterokont algae and are distinguished by the dominance of carotenoids over chlorophylls in their plastids.
Features and classification
Xanthophytes exhibit a range of morphologies from unicellular flagellates to filamentous and siphonous forms. Their chloroplasts contain chlorophyll a and c and large amounts of carotenoids such as heteroxanthin and diadinoxanthin, giving them a yellow‑green color. Unlike many other algae, they lack fucoxanthin. Storage products include chrysolaminarin and lipids rather than starch. Cell walls, when present, are often composed of cellulose and sometimes impregnated with silica scales or longitudinal splits, as seen in the filamentous genus Tribonema. Flagellated cells bear two unequal flagella (heterokont), with the longer flagellum bearing tripartite hairs. Reproduction is primarily asexual by cell division, fragmentation or zoospores; sexual reproduction is rare and varies among genera. Taxonomically, xanthophytes are part of the phylum Ochrophyta and are related to brown algae and diatoms.
Ecology and representative genera
Yellow‑green algae are most diverse in freshwater habitats, including ponds, ditches and moist soils, although a few marine species are known. Many species occur as free‑living cells on submerged surfaces or form mats on wet soil. The coenocytic genus Vaucheria forms tubular, multinucleate filaments that can grow on damp earth and produce distinctive oogonia and antheridia during sexual reproduction. Tribonema produces filamentous colonies composed of cylindrical cells connected by H‑shaped pieces of wall. Botrydiopsis and Chlorobotrys are unicellular coccoid representatives. While xanthophytes do not form the large blooms associated with some algae, they contribute to primary production in small water bodies and soil crusts. Their heterokont flagella and pigment composition make them useful in studies of algal evolution within the stramenopiles. Xanthophyta illustrate the diversity of heterokont algae and occupy modest ecological niches in freshwater and terrestrial habitats. Related Terms: Heterokont, Algae, Chlorophyta, Stramenopiles, Carotenoids