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Dinoflagellates are common organisms in all types of aquatic ecosystems. Roughly half of the species in the group are photosynthetic (Gaines and Elbrächter 1987), the other half is exclusively heterotrophic and feeds via osmotrophy and phagotrophy. As a consequence, they are prominent members of both the phytoplankton and the zooplankton of marine and freshwater ecosystems. They are also common in benthic environments and in sea ice.
Noctiluca scintillans is a very large marine, planktonic, phagotrophic, athecate dinoflagellate that can cause pinkish red or greenish red tides, that is able to be bioluminescent, and that can contain green eukaryotic endosymbionts (Pedinomonas noctilucae). The shown specimen is from a temperate region (without endosymbionts) but the species can also be found in subtropical and tropical areas. © Mona Hoppenrath
In terms of morphology, dinoflagellates can be as varied and complex as any unicellular eukaryote. Complex organelles found in the group include structures reminiscent of a full-fledged vertebrate eye (but in a unicellular organism that lacks a brain), nematocysts comparable to those of cnidarians, and a bewildering array of plastid types in the photosynthetic forms. Dinoflagellates exist as plasmodia (i.e. multinucleate organisms), biflagellated cells, coccoid stages and even, in one small group, as cell arrays that approach multicellularity.
Genetically, dinoflagellates are also unique. The nucleus of a large majority of dinoflagellates (the so-called dinokaryotes) is so different from other eukaryotic nuclei that it has been given its own name, the dinokaryon. Dinokarya lack nucleosomes, and DNA content is orders of magnitude larger than that of other eukaryotic cells, for example those of humans. These dinokarya divide via a unique form of mitosis. Recent research is starting to show just how unique dinoflagellate genetic systems are. For example, gene products of all dinoflagellate nuclei (not only dinokarya) are processed in a unique way: a spliced leader is trans-spliced to all mRNA molecules. The genomes of plastids and mitochondria of the group are also unique: they are atomized (i.e. the genome is split into very small fragments), and gene content is much, much lower than that of comparable organelles in other organisms.
Approximately 4500 species assigned to more than 550 genera have been described, nearly three quarters of the genera and more than half of the species being fossil. Of the ca. 2000 living species, more than 1700 are marine and about 220 are from freshwater (Taylor et al. 2008). These numbers are sure to grow substantially in the future. Between the years 2000 and 2007 three new dinoflagellate families, 22 new genera, and 87 new species were described (Centre of Excellence for Dinophyte Taxonomy CEDiT). Recent molecular analyses have shown that there are large numbers of undescribed dinoflagellate species in environments like marine picoplankton (e.g. Moreira and López García 2002, Worden 2006) or as symbionts (‘zooxanthellae’) in many types of protists and invertebrates like corals (Coffroth and Santos 2005).
Dinoflagellates are perhaps best known as causers of harmful algal blooms (webpages about this topic: ISSHA, WHOI, IOC). About 75-80% of toxic phytoplankton species are dinoflagellates (Cembella 2003), and they cause “red tides” that often kill fish and/or shellfish either directly, because of toxin production, or because of effects caused by large numbers of cells that clog animal gills, deplete oxygen, etc. (Smayda 1997). Dinoflagellate toxins are among the most potent biotoxins known. They often accumulate in shellfish or fish, and when these are eaten by humans they cause diseases like paralytic shellfish poisoning (PSP), neurotoxic shellfish poisoning (NSP), diarrheic shellfish poisoning (DSP) and ciguatera (Lehane and Lewis 2000). They also have been linked to major human health concerns, especially in estuarine environments (Pfiesteria). Some syndinians, notably Hematodinium, are parasites of economically-significant crustacean species.
The main ecological significance of dinoflagellates lies elsewhere, though. They are second only to diatoms as marine primary producers. As phagotrophic organisms they are also important components of the microbial loop in the oceans and help channel significant amounts of energy into planktonic food webs. As zooxanthellae, dinoflagellates have a pivotal role in the biology of reef-building corals.