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| Dinoflagellates |
Dinoflagellates, phylumDinophyta, are unicellular and colonial algal organisms from the kingdom Protista named for the spinning motions that result from the movement of their flagella.
The two thousand to four thousand species that make up the Dinophyta phylum typically have two flagella. Dinokonts (Dinophyceae) have one flagellum running in a groove that cuts transversely across the cell and another flagellum, the sulcus, that runs backward in a longitudinal groove and is more or less perpendicular to the transverse one. In desmokonts (Desmophyceae), both flagella arise from a point at the front of the cell.
The motions of the flagella, which make dinoflagellates spin like a top, help give rise to the name, for dinos in Greek means “whirling,” and flagella means “whip.” Single-celled species are the most common, but colonial species exist. The largest dinoflagellate, Noctiluca, may grow as large as 2 millimeters in diameter.
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In the active phase of their life cycles, dinoflagellates come in two forms, unarmored (naked) or armored (the cate). All species have a complex outer covering, consisting of an outer membrane, flattened vesicles in the middle, and a continuous inner membrane.
In thecate forms, however, the vesicles contain plates made of cellulose or some other polysaccharide. The plates may form a structure as simple as a bivalve-type shell; however, in some species they form wings and other appendages that give the beholder the appearance of some fantastic alien spaceship.
Cell Characteristics
Dinoflagellates are not members of the kingdom Plantae but rather are protists, and they have both plant and animal characteristics. Some species are autotrophic—in other words, they have their own chloroplasts and can produce their own sugars and organic materials through photosynthesis.
Other species are heterotrophic—they have no chloroplasts and typically must prey on or parasitize other organisms or consume organic detritus in order to obtain nourishment. Some of the heterotrophic species, however, can acquire the chloroplasts of prey and become photosynthetic themselves.
These two characteristics, when considered with the large number of nonphotosynthetic dinoflagellates, have led some to argue that dinoflagellate chloroplasts have been secondarily acquired from a eukaryotic endosymbiont. Photosynthetic pigments include chlorophylls a and c, carotenoids, and xanthophylls.
Photosynthetic dinoflagellates often form symbioses with other aquatic organisms such as sponges, cnidarians (jellyfish, sea anemones, and corals), molluscs (bivalves, gastropods, octopuses, and squids), turbellarians, and tunicates. These symbiotic dinoflagellates, or zooanthellae, lack armor. They carry outmost of the photosynthesis that fuels the productivity of coral reefs.
Although classified as protists, dinoflagellates have cellular nuclei with characteristics intermediate between those of prokaryotes and those of eukaryotes.
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| Cell Characteristics |
As in eukaryotes, the nucleus is surrounded by a nuclear membrane and contains a nucleolus. However, the chromosomes are attached to the nuclear membrane in such a way that chromosomes remain attached to the inner wall of the cell membrane in prokaryotes.
Dinoflagellates are also highly unusual in that they have permanently condensed chromosomes—and dinoflagellate deoxyribonucleic acid (DNA) does not form a complex with proteins, as in typical eukaryotic cells. There remains disagreement over whether dinoflagellate characteristics represent some ancient evolutionary lineage or more recent derivation.
Life Cycle
Arguably the most accomplished shape shifters in the living world, dinoflagellates have incredibly complex life cycles. For example, Pfiesteria has at least twenty-four distinct stages in its life cycle, one of several reasons why scientists who work with the organism call it the “cell from hell.”
Dinoflagellate life cycles may include dormant cysts, cells without flagella (including amoeba-like stages), and more typical biflagellated cells.
Dinoflagellates may reproduce sexually or asexually. The cells are generally haploid, except for a zygote produced by the union of two gamete cells during sexual reproduction. The zygote undergoes meiosis shortly after fertilization.
Dinoflagellate cells divide asexually in three ways: The parent cell of a naked dinoflagellate simply constricts and pinches off into two daughter cells; some armored types shed the theca prior to or during division; and other armored types split the parental theca, dividing the portions between the daughter cells.
Unfavorable environmental conditions may trigger sexual reproduction as well as the formation of dormant cysts. Cysts may be transported large distances by currents, which in large part explains the dispersal of toxic dinoflagellate blooms up and down coastlines.
Pfiesteria exhibits all three forms, with the amoeboid and flagellated stages being toxic to fish. Encysted stages lie dormant in the bottom sediments of estuaries. The active amoeboid and flagellated stages are usually nontoxic.
Amoeboid stages, which either inhabit the sediments or are free swimming, consume bacteria, algae, small animals, or bits of fish tissues. Flagellated stages may ingest prey in a similar fashion but often siphon off the tissue of their prey through a cytoplasmic extension called the peduncle.
In the presence of an environmental trigger—such as substances given off by a school of live fish—amoeboid, flagellated, and encysted cells activate into toxic forms that swim toward the prey. The toxic forms then secrete toxins that immobilize and injure the prey with ulcerated, bleeding sores.
Pfiesteria then feeds off materials that leak from the sores. Once the fish die, flagellated cells transform into amoeboid forms that feed on the carcass. If conditions suddenly become unfavorable, the active forms encyst and sink to the bottom. The entire cycle can take place in a matter of hours.
Red Tides and Toxins
Dinoflagellates are responsible for most of the red tides or brown tides that sicken and kill aquatic organisms and humans worldwide. Red tides are known from biblical times; one of the ten plagues reported to have been visited upon Egypt in the Book of Exodus (8:20-21) was most likely a red tide. Red tides were also known in ancient China and among Native American in Alaska and the Pacific Northwest.
Typically, the organisms that cause red and brown tides cause no harm until their populations explode or bloom. Adverse effects to other organisms result from oxygen depletion by irritation to skin and other organs, by the blocking of sunlight (in cases where the bloom is visible), or by the production of toxic substances, as in the case of Pfiesteria.
In general, it is the toxic substances that sicken or kill humans and other vertebrates, such as manatees and birds. There is some controversy over whether human activities in estuarine and coastal waters have caused an increase in the frequency of these algal blooms.
In the mid-1990’s a spate of horrific fish kills in the estuaries of North Carolina, Virginia, and Delaware began raising alarms up and down the East Coast of the United States.
Ghastly lesions appeared on the affected fish, as if they were being eaten alive. People who spent a lot of time on or near the afflicted waters were affected, too, with symptoms ranging from memory loss to skin lesions. The single-celled culprit turned out to be Pfiesteria piscicida.
Several toxic syndromes that affect humans are caused by dinoflagellates: ciguatera fish poisoning, caused by toxins produced by Gambierdiscus and other species; paralytic shellfish poisoning, caused by toxins produced by Alexandrium and other species; neurotoxic shellfish poisoning, caused by toxins produced by Gymnodinium breve; diarrhetic shellfish poisoning, caused by toxins produced by Dinophysis species; and Pfiesteria-associated syndrome.
Bioluminescence
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| Bioluminescence |
The light is created through the reaction of oxygen with a substrate, luciferin (which has no relation to luciferins responsible for phosphorescence in other organisms such as lightning beetles), which is catalyzed by an enzyme, luciferase. Bioluminescence in dinoflagellates follows a circadian rhythm in which the maximum occurs at night.