Members of the kingdom Phaeophyta are the brown algae or seaweeds, a group of photosynthetic protists. They contain chlorophylls a and c, not a and b as do plants. The brown colour comes from the pigment xanthophyll. Many brown seaweeds have a blade or thallus and a holdfast for anchorage. They form great forests in some shallow oceans and there are about 1500 known species.