In a sense, protists include all eukaryotes that lack the distinguishing characteristics of plants, animals, or fungi. Protists include simple eukaryotes such as amoebas, and some unusual multicellular forms such as the brown algae, kelp. Protists are mostly microscopic, unicellular organisms and probably share common ancestry with fungi, multicellular plants, and animals. Algae are photosynthetic, eukaryotic organisms typically lacking multicellular sex organs. Thus, the major groups are referred to as green algae, brown algae, golden-brown algae, and red algae.
Red algae owe their color to water soluble pigments called phycobilins.
Other algal pigments such as chlorophylls and carotenoids are insoluble in water but can be extracted with organic solvents such as acetone and alcohol. Algae are also distinguished by their cellular organization. Unicellular algal species occur as single, unattached cells that may or may not be motile. Filamentous algal species occur as chains of cells attached end to end. These filaments may be few to many cells long and many be unbranched or branched in various patterns.