Xanthophyceae

(botany) A class of yellow-green to green flagellate organisms of the division Chrysophyta; zoologists classify these organisms in the order Heterochlorida.

A class of plants, comprising the yellow-green algae, in the chlorophyll a–c phylectic line (Chromophycota). Alternate names are Tribophyceae, derived from Tribonema, a filamentous member of the class, and Heterokontae, referring to the presence of two kinds of flagella on each motile cell. The absence of starch is an important characteristic in distinguishing yellow-green algae from green algae, which they may resemble superficially in form and color. See also Algae; Chromophycota.

Motile cells have two unequal flagella borne apically or laterally. One (usually the longer) is directed forward and is pleuronematic (hairy); the other is directed backward and is acronematic (smooth). The compound zoospore (synzoospore) of Vaucheria is exceptional in having numerous pairs of slightly unequal smooth flagella. There are usually two chloroplasts in each motile cell, the ventral one with a reddish refractile eyespot at its anterior end. One or two contractile vacuoles occur at the anterior end of a motile cell. Cells in most Xanthophyceae are uninucleate, but in some genera they are multinucleate. The mitochondria have tubular cristae. Each vegetative cell contains one to many yellow-green, laminate or discoid, parietal chloroplasts.

Most Xanthophyceae occur in fresh water, especially soft water. Frequently, they are epiphytic on aquatic plants. The chief exception is Vaucheria, which has fresh-water, brackish-water, and marine species. Xanthophyceae also occur in and on soil and mud, in snow and ice, and on tree trunks and damp walls.