Chrysophyta

Abstract

The chrysophytes (more than 1,200 described species) are unicellular or colonial algae characterized by heterokont flagella and chloroplasts with chlorophyll a and c, and by their endogenous silicified stomatocysts. They occur mainly as phyto- plankton in temperate freshwaters, and their distribution is ecologically deter- mined, mainly by temperature and pH. Cells are naked or in many cases surrounded by an envelope, e.g., of species- specific silica scales manufactured from the chloroplast ER and Golgi vesicles and transported to the cell membrane and extruded. Photoreceptor systems include a swelling on the short flagellum and a corresponding stigma in one of the chloroplasts. Photosynthesis results in chrysolaminaran. But in many species, e.g., in colorless species, organic compounds can be taken up from the water or by phagocytosis. Life history includes mitotic divisions and encystment. In many species, sexuality - cell fusion followed by encystment of the zygote - has been observed. Classification was traditionally based on morphological criteria, including ultrastructure, but in recent years molecular methods have resulted in profound changes in our concepts of relationships and evolution.