Xanthophyceae

Abstract

The Xanthophyceae is a clade of stramenopilan photoautotrophs containing about 118 genera and 600 species. Morphology ranges from free-living or attached unicells to colonies and unbranched or branched filaments and siphons. A large majority are found in freshwater and soil, while some occur in brackish and marine habitats. Although abundant growth of a few species can occur in nature, none are known to be of practical importance. They are characterized by posses- sion of chlorophylls a, c1, and c2 and a range of xanthophylls, but not fucoxanthin, in generally yellowish-green, discoidal, parietal chloroplasts. Thylakoids are in groups of three, and most species investigated have a single thylakoid forming a girdle band around the periphery of the chloroplast. Chloroplasts are surrounded by chloroplast endoplasmic reticulum. Pyrenoids, when present, are typically semi-immersed and are not associated with granules of storage products. A cell wall consisting of two overlapping parts occurs in some coccoid and filamentous species. Reproduction is generally asexual but some, e.g., Vaucheria, exhibit sexual reproduction. The taxonomic status of a significant number of species is uncertain, especially those that are rarely observed, e.g., species of Chloramoebales, Heterogloeales, and Rhizochloridales. Transfer of species to the Eustigmatophyceae and other groups is likely. There are molecular phyloge- netic data for fewer than 20% of species. Four major clades are recognized. Two of these contain both coccoid and filamentous species. Many traditional orders, families, and genera are paraphyletic or polyphyletic. It is presently convenient to retain the traditional classification of seven orders based on morphology until these difficulties are resolved following the inclusion of more species in phylo- genetic analyses.