Five divisions of unicellular algae are considered in microbiology because of their microscopic form and their unicellular characteristic. These organisms are classified in the kingdom Protista.
Division Chlorophyta. Algae of the division Chlorophyta possess green chlorophyll pigments and carotenoid pigments. A representative member is Chlamydomonas, which is often used in research and as a laboratory specimen. Chlamydomonas produces zoospores, which are flagellated. Organisms such as Chlamydomonas are believed to be evolutionary ancestors of other species. Other organisms in the division are Volvox and Spirogyra.
Division Charophyta. Members of the division Charophyta are stoneworts. Stoneworts cover the bottoms of ponds and may be a source of limestone.
Division Euglenophyta. Members of the division Euglenophyta include the common organism Euglena. These organisms have chlorophyll and carotenoid pigments for photosynthesis and flagella for movement. They share many characteristics with both plants and animals and are believed to be a basic stock of evolution.
A typical Euglena cell has a large nucleus and nucleolus. Contractile vacuoles help empty water from the organism, and two flagella arise at one end of the cell. Reproduction occurs by binary fission in the longitudinal plane.
Division Chrysophyta. Members of the division Chrysophyta are brown and yellow‐green algae. These organisms contain chlorophyll pigments as well as special carotenoid pigments called fucoxanthins. Fucoxanthins give the golden‐brown color to members of the division. Members of the division include the diatoms, oceanic photosynthetic algae found at the bases of many food chains. Diatoms contribute immense amounts of oxygen to the atmosphere and occupy key places in the spectrum of living things because they convert the sun's energy into the energy in carbohydrates.
Division Pyrrophyta. Members of the division Pyrrophyta are pigmented marine forms that include the dinoflagellates, amoeboid cells with flagella as well as protective cellulose plates that surround the cells. They have chlorophyll, carotenoid, and xanthophyll pigments. Dinoflagellates often have a brown or yellow color, and they reproduce by longitudinal division through mitosis. Dinoflagellates make up a large portion of marine plankton and are essential to many of the ocean food chains. Certain species are luminescent. Others have red or orange pigments; when these organisms multiply at abnormally high rates, they cause the “red tides.”