Also known as Haptophyta, the Prymnesiophyta includes about 500 living species in 50 genera, with many additonal fossil genera and species, most notably the coccolithophorids. Members of this group are primarily unicellular, and are photosynthetic. They are often important sources of food for aquatic communities. Prymnesiophyte algae are generally marine and are mostly tropical, though there are a few freshwater and terrestrial species reported. The group occurs worldwide, and several species have global distributions.
Some prymnesiophytes produce algal blooms which may cause serious problems for fish and for fishermen. Large blooms are problematic because of the mucilage surrounding the algal cells; it may clog fish gills, or render them permeable to dissolved toxins. Another problem is the production of dimethyl sulfide (DMS), a noxious-smelling compound which has been known to cause fish migrations to alter their normal routes.
Haptophytes are often a golden-brown color because of the presence of the yellow-brown accessory pigments, diadinoxanthin and fucoxanthin, a feature they share with other Chromista. These are contained in the one or two plastids that are present in the cell. Prymnesiophytes may have a complex life cycle, with an alternation between motile and non- motile phases of different morphologies.
The name Haptophyta was originally applied to the group because of the presence of the unique organelle, the haptonema. This is a peg-like structure that extends out from the cell near the point where the two flagella are attached. The haptonema was originally thought to be a third flagellum, but has since been found to have a quite different morphology, and its function is unknown.
Many prymnesiophytes are covered with scales, which can have rather complex architecture; they may have spines or an elaborated rim, and come in an amazing variety of shapes -- pentagons, muffin-shapes, baskets, donuts, or even trumpet-like shapes are known. The scales may remain unmineralized, and consist primarily of carbohydrates, or they may be calcified; silica plates are rare. These plates are formed by deposition within the Golgi apparatus, and are often embedded in mucilage.
Coccolithophorids are the best known members of the Prymnesiophyta. These delicate-looking organisms have external calcified plates (known as coccoliths), with a complex ornamentation. Cells may bear only one kind of plate or two. The plates accumulate on the bottom of the ocean as the organisms die, and there contribute to the formation of ocean sediments, carbonate oozes, and rocks such as the Mesozoic limestones and chalks.
Fossils of this group date back into the Jurassic, where they first become abundant, and some possible fossils of coccolithophores have been recovered from the Pennsylvanian. The group made a sudden and rapid appearance of new forms in the early Jurassic, and reached its greatest abundance in the Late Cretaceous. Near the end of the Cretaceous, the coccolithophores suffered a mass extinction of groups; two-thirds of the 50 genera disappear at that time, though many new groups appear in the Paleocene.
Extensive information on Emiliania huxleyi, a common marine coccolithophore, is available from the Ehux Web Site, including SEM and satellite pictures.
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