Dinoflagellata: Life History and Ecology

Dinoflagellates may be planktonic, or may live within another organism.

Ninety percent of all dinoflagellates are marine plankton. There are also many freshwater species, some of which have been found growing in snow! They may be photosynthetic or non-photosynthetic; about half the species fall into each category. The photosynthetic dinoflagellates are second only to diatoms as primary producers in coastal waters.

A number of photosynthetic dinoflagellates take up residence within other organisms as symbiotic partners. These zooxanthellae may be found in many marine invertebrates, including sponges, corals, jellyfish, and flatworms, as well as within protists, such as ciliates, foraminiferans, and colonial radiolarians. In each case, the host organism is able to swallow the dinoflagellate and incorporate it into its own tissues without harming it. The dinoflagellate then divides repeatedly to increase its numbers, and begins to manufacture carbohydrates which are provided to the host. The degree of interdependence varies greatly -- the sea anemone Anemonia can survive quite well without its zooxanthellae, while certain corals rely almost exclusively on the food from their symbionts, and build reefs much faster with the dinoflagellates present in their tissues.

Non-photosynthetic species of dinoflagellates feed on diatoms or other protists (including other dinoflagellates); the genus Noctiluca is large enough to eat fish eggs, and is able to swallow protists larger than itself. Some species will parasitize other organisms, such as zooplankton and other protists, filamentous algae, or fish.

Dinoflagellates may produce their own light chemically.

The Dinoflagellata are sometimes called Pyrrhophyta , meaning "fire plants". This is because some species are capable of bioluminescence, in which chemicals made by the organism produce light in a chemical reaction. The dinoflagellates begin to glow as it gets dark, but will brighten considerably when agitated, such as in the wake of a ship. The phenomenon was first noted in the genus Noctiluca, which resulted in its name ("night light"), but the reaction is now known to occur in several marine species.

The chemical reaction itself occurs when the compound luciferin (a substrate chemically similar to a chlorophyll precursor), is oxidized by the enzyme luciferase in the presence of ATP and oxygen. This reaction and similar ones occur in a number of unrelated organisms, both prokaryotic and eukaryotic.

Blooms of dinoflagellates produce "red tides" which injure marine life.

The most dramatic effect of dinoflagellates on their environment occurs in coastal waters during the warmer season, usually mid to late summer. At this time, an upwelling occurs in the ocean, bathing the surface plankton in nutrients from the bottom of the ocean. The surplus of nutrients triggers a "bloom" of photosynthetic dinoflagellates, whose population density may jump to more than 20 million per liter along some coasts. This high density may color the water golden or red, and is called a "red tide".

In some species these blooms are associated with the production of neurotoxins, poisons which injure the nerves of marine life that feed on the dinoflagellates. The result may be massive kills of fish and shellfish, as well as other forms of marine life. If animals containing these toxins are eaten by humans, the result may be illness or even death.

The neurotoxins affect muscle function, preventing normal transmission of electrochemical messages from the nerves to the muscles by interfering with the movement of sodium ions through the cellular membranes. Humans may be poisoned by eating fish, a condition known as ciguatera, or by eating shellfish, such as clams or mussels, and is then called paralytic shellfish poisoning, or PSP. The resulting condition is serious but is not usually fatal. Lethal concentrations lead to death from respiratory failure and cardiac arrest within twelve hours of consumption.

The most common dinoflagellate toxin is saxitoxin, a neurotoxin 100,000 times more potent than cocaine. It has been found in North American shellfish from Alaska to Mexico, and from Newfoundland to Florida. The most notorious producer of saxitoxin on the west coast of North America is Protogonyaulax catenella, and on the east coast Gessnerium monilatum. Both have been known to cause PSP.

A second form of toxin, found in the dinoflagellate Ptychodiscus brevis, is brevitoxin, an assemblage of various polyether alcohols which produce fish kills, and may also cause poisoning in humans when it accumulates in the tissues of shellfish. The rule of thumb is that shellfish should only be eaten during months with an "R" in them, and not during May to August.

Dinoflagellates are primarily asexual.

The most common means of producing more dinoflagellates is asexual cell division ( mitosis). This process "splits" the organism, producing two identical copies. The theca may be shed (and regrown in each of the daughters), or it may be divided, with each daughter receiving half and regrowing half. A few genera grow as filaments. These are formed when the cells don't separate after dividing.

Mature dinoflagellates are haploid, so when the sexual cycle begins, gametes are formed by simple mitosis. They may be naked or armored, looking like small versions of the "parent". The "male" and "female" gametes may or may not look alike, but they are always free-swimming. Upon fusion of the two gametes, a planozygote may be formed. This is an actively swimming zygote, as compared to the "usual" non-motile zygote of plants and animals.

It is the zygote stage which may form a cyst called an hystrichosphere under unfavorable conditions. This is a dormant capsule which protects the dinoflagellate until favorable conditions return.

For additional information:
Information concerning ciguatera fish poisoning and shellfish poisoning in the United States, from the U.S. Food and Drug Administration's handbook of foodborne pathogenic microbes and natural toxins.