Haptophyta, also known as Prymnesiophyta, is a phylum of kingdom Chromista that is composed primarily of aquatic, planktonic, unicellular organisms. All haptophytes are photosynthetic, and thus are at the base of aquatic food chains. Most, but not all haptophytes live in tropical oceans.
The defining characteristic of Haptophyta is a flagellum-like organelle called a haptonema. The haptonema protrudes from a spot near the origins of the two flagella common to all chromists, and initially was thought to be a third flagellum. However, the haptonema has a different morphology; often it is coiled like a proboscis. Haptophytes use the haptonema to acquire food--algae and bacteria--and to attach to solid surfaces.
Haptophyta is significant for several reasons. Haptophytes often produce algal blooms that are toxic to fish. The blooms also produce dimethyl sulfide, which oxidizes to form foul-smelling (to both humans and fish) sulfuric acid. Coccolithophorids are probably the most important haptophytes. The Golgi apparati of the coccolithophorids secrete ornate plates (coccoliths) made of calcium carbonate. The coccoliths are excreted to the exterior of the cells, where they become scales. As the cells die, their scales accumulate on the ocean floor in the form of chalk. Coccolithophorid deposits make up most of the calcareous rock of the Cretaceous period.
Haptophytes have complex life cycles that include alternating motile and non-motile stages. Asexual reproduction is dominant among them. Fossils of Haptophyta date back to the Jurassic period, and reached their largest abundance in the Cretaceous period. At the end of the Cretaceous period, the haptophytes suffered from a mass extinction that wiped out two thirds of the 50 genera.
- Green Plants by Peter Bell & Alam Hemsley