11 Dinoflagellates

Most dinoflagellates are photosynthetic or heterotrophic unicells having two distinctive flagella whose motion causes the cells to rotate as they swim. In fact, the term dinoflagellate originates from the Greek word dineo, meaning “to whirl.” However, dinoflagellates also occur as several types of nonflagellate single-cell or filamentous forms whose relationships to other dinoflagellates are revealed by the characteristic structure of their flagellate reproductive cells. One fascinating example is Blastodinium—a multicellular, worm-shaped parasite occurring in the guts of marine copepods. This parasite’s reproductive cells are recognizable as dinoflagellates by their close resemblance to the common free-living flagellate Peridinium, an important primary producer in fresh or brackish waters.

Many other examples of dinoflagellates’ ecological importance occur in diverse aquatic habitats. These include photosynthetic dinoflagellates that live as endosymbionts within the reef-building corals so prominent in coastal tropical and subtropical oceans. Stimulated by chemical signals from their animal hosts, dinoflagellate partners provide corals with essential organic food. This relationship explains why corals are so vulnerable to the loss of their photosynthetic symbiont—a process known as coral bleaching that occurs as a result of climate warming. Because coral reefs harbor a very high diversity of additional marine species, many of which have not been adequately catalogued and their ecological roles defined, conservation biologists are very concerned about the impact of coral bleaching on global biodiversity. Certain planktonic dinoflagellates are also globally important as toxin producers in coastal marine waters that have been affected by human influences. Dinoflagellate toxins have been responsible for the deaths of sea birds and mammals and can affect the health of humans.

In order to better understand the ecological functions of dinoflagellates, in this chapter we first survey dinoflagellate relationships to other protists and the unusually complex structure of dinoflagellate cells. Such information helps to explain the importance of dinoflagellates in today’s world.