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What is a dinoflagellate?

The name ’ refers to the forward spiraling swimming motion of these organisms. Free-living dinoflagellates are an ancient and successful group of aquatic organisms. They have adapted to pelagic (free-floating in the water column) and benthic (attached and associated with the bottom) habitats from arctic to tropical seas, from freshwater to estuaries and to hypersaline waters. Many species are cosmopolitan and can live in a variety of habitats: in the plankton or attached to sediments, sand, corals, or to macroalgal surfaces or to other aquatic plants. Some species are present as parasites in marine invertebrates and fish. Some even serve as symbionts, known as zooxanthellae, providing organic carbon to their hosts: reef-building corals, sponges, clams, jellyfish, anemones and squid. This group of unicellular microorganisms comprises a large number of unusual algal species of many shapes and sizes. There are about 130 genera in this group with about 2000 photosynthetic and 2000 heterotrophic species described so far.

Dinoflagellates exhibit a wide divergence in morphology and size (from 1 µm to > 1 mm). They have a common cell covering structure (theca) that differentiates them from other algal groups. In general, the theca is made up of two halves: the epitheca, which covers the top of the cell, and the hypotheca, which covers the bottom of the cell. The cingulum or girdle (the groove that wraps around the cell between the epitheca and hypotheca) houses the transverse flagellum, and the sulcus (found in the hypotheca) houses the longitudinal flagellum. Cells are either armored or unarmored. Armored species have thecal plates composed of cellulose or polysaccharides which are key features used in their identification. The thecal surface can be smooth and simple, or laced with spines, pores and/or grooves and can be highly ornamented. The cell covering of unarmored species is comprised of a membrane complex.

Dinoflagellates are free-swimming protists (unicellular eukaryotic microorganisms) with two flagella (longitudinal and transverse), a large nucleus with continually condensed chromosomes, chloroplasts, mitochondria, and golgi bodies. Biochemically, photosynthetic species have chlorophylls a and c, and the light-harvesting pigments peridinin, fucoxanthin and xanthophylls. Dinoflagellates primarily exhibit asexual cell division, but some species reproduce sexually, while others have unusual life cycles. Their nutrition varies from autotrophy (photosynthesis-nearly 50% of the known species) to heterotrophy (absorption of organic matter) to mixotrophy (autotrophic cells engulfing prey organisms including other dinoflagellates).

On the systematic level, dinoflagellates have been claimed by both botanists and zoologists. Dinoflagellates share features common to both plants and animals: they can swim, many have cell walls, and both photosynthetic and nonphotosynthetic species are known. Botanists have grouped them with the ’ and zoologists have grouped them with the ’ and both have produced classification schemes for this diverse and unusual group.

Dinoflagellates have attracted much attention from the general public throughout history. Dinoflagellate ’ (cell population explosions) can cause discoloration of the water (due to accumulation of carotenoid pigments), also known as red tides, which can have harmful effects on the surrounding sea life and their consumers. More intriguing and of public concern are the toxins that certain species produce. When these toxic species are in bloom conditions the toxins can be quickly carried up the food chain and indirectly passed onto humans via fish and shellfish consumption, sometimes resulting in gastrointestinal illness, permanent neurological damage, or even death. Socioeconomic stresses also result due to the closing of commercial fisheries and mariculture until the harmful algal bloom dissipates. Yet dinoflagellates are one of the most important members of the phytoplankton in marine and freshwater ecosystems. They are an integral part of the first link in the aquatic food chain: the initial transfer of light energy to chemical energy (photosynthesis). All other organisms are dependent upon this energy transfer for their subsequent existence (Steidinger and Tangen, 1996, Taylor, 1987a).