Phylum Dinophyta The Dinoflagellates

Occasionally, visitors to an ocean beach in midsummer may notice a distinctly reddish tint to the water, usually as a result of a phenomenon known as a red tide. Red tides are caused by the sudden and not fully understood multiplication of unicellular organisms called dinoflagellates (Fig. 18.24). There are over 3,000 species of dinoflagellates, with 300 of them known to be capable of producing red tides. When a red tide appears, some biologists dip a cup of sea water and save it for examination with a microscope. (The material can be preserved indefinitely with the addition of a few drops of formaldehyde, vinegar, or other weak acid.)

The cup of sea water just mentioned usually contains the thecae (shell-like remains) of hundreds of dinoflagel-lates, the best known representatives of Phylum Dinophyta. Some resemble armor-plated spaceships, while others may be smooth or have fine lengthwise ribs. The "armor plates" are located just inside the plasma membrane and are composed mostly of cellulose of varying thickness.

Dinoflagellates have two flagella that are distinctively arranged, usually being attached near each other in two adjacent and often intersecting grooves. One flagellum, which acts as a rudder, trails behind the cell. The other, which encircles the cell at right angles to the first groove, gives the cell a spinning motion as it undulates in its groove like a tiny snake.

Spaceship Dinoflagellate

Figure 18.24 Dinoflagellates. A. Ceratium. B. Gonyaulax.

Figure 18.24 Dinoflagellates. A. Ceratium. B. Gonyaulax.

Most dinoflagellates have two or more disc-shaped chloroplasts, which contain distinctive brownish xantho-phyll pigments in addition to various other pigments, including chlorophylls a and c2. About 45% of the species are, however, nonphotosynthetic, and some ingest food particles, whether or not chlorophyll is present. Some have an eyespot (a pigmented organelle that is sensitive to light), and all have a unique nucleus in which the chromosomes remain condensed and clearly visible throughout the life of the cell. The chromosomes contain a disproportionately large amount of DNA—as much as 40 times that found in human cells. The food reserve is starch, which in dinoflagellates is stored outside the chloroplasts.

Many dinoflagellates have tiny projectiles that are fired out of the cells when they are irritated. The most common form of projectile is a trichocyst, which is rod-shaped and is fired within milliseconds of irritation. The cells move rapidly in the opposite direction of the discharge, leading to speculation that they aid in escape, but there is also some evidence that they spear other organisms.

Dinoflagellates occur in most types of fresh and salt water, but those that cause red tides have received the most publicity because about 40 of the species also produce powerful neurotoxins that accumulate in shellfish such as oysters, mussels, scallops, and clams. The poisons apparently don't harm the shellfish, but about 2,000 persons become ill each year—15% fatally—from eating contaminated shellfish. Although fish in open waters can swim away from affected waters, large numbers still die and wash ashore after a red tide, and the devastation to caged fish can be enormous. Even pelicans, dolphins, whales, and manatees have been poisoned by dinoflagellate toxins in their food chains.

Three major types of dinoflagellate toxins have been studied for possible use in chemical warfare. They are so potent that as little as half a milligram (barely enough to cover two of the periods printed on this page) can be fatal. Symptoms in humans begin with a tingling feeling in the fingertips and lips and eventually progress to paralysis of the diaphragm, followed by death through suffocation. Other symptoms include nausea, abdominal cramps, muscular paralysis, amnesia, hallucinations, and diarrhea. The toxin does harm the shellfish and causes massive death of clams in many cases.

The havoc to the fishing industry caused by major red tides is so great that several laboratories have conducted research with dinoflagellates and their marine habitats to try to determine the causes of their blooms and to find a way of preventing such destruction in the future.

Until 1970, blooms of toxin-producing dinoflagellates were known only from temperate waters of the Northern Hemisphere. By 1990, however, the blooms were also occurring with increasing frequency throughout both temperate and tropical waters of the Southern Hemisphere. As mentioned in the introduction to this chapter and discussed in Chapter 11, dinoflagellates play a major role in oceanic bioluminescence.

Reproduction is by cell division. Sexual reproduction appears to be rare.

Cryptomonas Eyespot

flagella

Figure 18.25 Cryptomonas sp., a cryptomonad, x2000.

flagella ejectisome nucleomorph mitochondrion pyrenoid starch grain chloroplast periplast plate nucleus

Figure 18.25 Cryptomonas sp., a cryptomonad, x2000.

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Responses

  • SISKO
    Which direction to dinophyta spin?
    8 years ago
  • DANIELA
    What organisms are in the phylum dinophyta?
    9 months ago
  • vappu
    Which organisms with phylum of dinoflagellates?
    7 months ago

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