Dinoflagellates and Red Tides

Dinoflagellates are the most common sources of bioluminescence at the surface of the ocean. This section describes the life history and ecology of dinoflagellates, and explains how and why they produce bioluminescence.

What is a dinoflagellate?

Neither plant nor animal, dinoflagellates are unicellular protists; most exhibit the following characteristics:

  • They are planktonic.
    Ninety percent of all dinoflagellates are marine plankton. Others are benthic, symbiotic, or parasitic.
  • They are small.
    Although many of them are microscopic and range from 15 to 40 microns in size, the largest,Noctiluca, may be as large as 2 mm in diameter!
  • They are motile.
    Dinoflagellates swim by means of two flagella, movable protein and microtubule strands that propel the cell through the water. The longitudinal flagellum extends out from the sulcal groove of the hypotheca (posterior part of cell); when it whips back and forth it propels the cell forward. The flattened transverse flagellum lies in the cingulum, the groove that extends around the equator of the cell. Its motion provides maneuvering and forward movement. As a result of the action of the two flagella the cell spirals as it moves.
  • Many are thecate, having an internal skeleton of cellulose-like plates.
    The peripheral part of the cell has a series of membranes called the amphiesma. In “armored” species polysaccharide deposited between the membranes forms rigid plates called thecae. “Naked” cells lack thecae.
  • Their chromosomes are always condensed.
    Dinoflagellate DNA always exists in a crystalline form in the nucleus, unlike other eukaryotes. In addition, lack proteins called histones that in other eukaryotic cells helps organize the chromosomes. Dinoflagellates contain a lot of DNA, which explains the large size of the nucleus. The metabolic requirements of supporting the large amount of DNA may explain the low growth rates of dinoflagellates compared to other unicellular protists.
  • Not all dinoflagellates are photosynthetic.
    Many dinoflagellates are photosynthetic, manufacturing their own food using the energy from sunlight, and providing a food source for other organisms. The photosynthetic dinoflagellates are important primary producers in coastal waters.Some photosynthetic dinoflagellates are symbiotic, living in the cells of their hosts, such as corals. Called zooxanthellae, they are found in many marine invertebrates, including sponges, corals, jellyfish, and flatworms, as well as within protists, such as ciliates, foraminiferans, and colonial radiolarians.Approximately half of all species are heterotrophic, eating other plankton, and sometimes each other, by snaring or stinging their prey. Non-photosynthetic species of dinoflagellates feed on diatoms or other protists (including other dinoflagellates); Noctiluca is large enough to eat zooplankton and fish eggs. Some species are parasites on algae, zooplankton, fish or other organisms.
  • Dinoflagellates usually reproduce asexually.
    The most form of reproduction is asexual, where daughter cells form by simple mitosis and division of the cell. The daughter cells will be genetically identical to that of the original cell. The thecal plates may either be divided, or completely shed and then reformed.

Red tides

Red tides are conditions when a dinoflagellate population increases to such huge numbers that it discolors the water. This “bloom” may be caused by nutrient and hydrographic conditions, although the environmental conditions which result in red tides are not completely understood. For dinoflagellate red tides, the water is discolored red or brown due to as high as 20 million cells per liter. These red tides are composed primarily of one species of dinoflagellate that has been rapidly growing and accumulating.

Some red tides are luminescent; most in southern California create dramatic nighttime displays of bioluminescence in the wakes breaking on the beach. A synopsis of the putative mechanisms responsible for these red tides is kindly provided by Prof. Wolfgang Burger, a geologist and former Interim Director of SIO:

“My understanding is this: if, after an upwelling or mixing event (storm?) there is plenty of sunshine, which warms the water and makes for stable stratification, conditions are right for a bloom. If, in addition, Gonyaulax cysts waiting around on the bottom have been stirred up into the water, and have by some means (change in temperature? light?) detected that the time is good for popping open, sufficient seeds are released to start the process. Rapid reproduction ensues (by cell division; these are unicellular organisms) and crowds out everything else, by taking away the light (the water was brown!) and perhaps also by chemical means.”

Some but not all red tides are toxic. In toxic red tides, the dinoflagellates produce a chemical that acts as a neurotoxin in other animals. When the dinoflagellates are ingested by shellfish, for example, the chemicals accumulate in the shellfish tissue in high enough levels to cause serious neurological affects in birds, animals, or people which ingest the shellfish.

There are several types of neurotoxins produced by dinoflagellates. These chemicals may affect nerve action by interfering with the movement of ions across cell membranes, thus affecting muscle activity. The toxin saxitoxin, produced by Gonyaulax off the west coast of North America, and Alexandrium off the northeast coast, accumulates in shellfish. Eating contaminated shellfish causes paralytic shellfish poisoning (PSP). The worst cases of PSP result in respiratory failure and death within 12 hours. Another toxin that accumulates in shellfish is brevetoxin, produced by the dinoflagellate Karenia brevis. Brevetoxin is unique in that it becomes aerosolized when the dinoflagellates end up in the surf zone and then blows onto the beach causing respiratory irritation in humans. If you are on a beach on the Gulf coast of Florida and notice asthma-like breathing symptoms, chances are you are experiencing toxicity from a Karenia bloom. A toxin produced by the dinoflagellate Dinophysis causes diarrhetic shellfish poisoning (DSP), which results in digestive upset but which is not fatal. Ciguatera is another form of dinoflagellate toxicity in tropical areas caused by eating fish contaminated by toxins of the dinoflagellate Gambierdiscus toxicus.