Red Tide

Saxitoxin is a natural but potent neurotoxin that is synthesized by certain species of marine dinoflagellates. Saxitoxin causes paralytic shellfish poisoning, a toxic syndrome that affects humans who consume contaminated shellfish. Other biochemicals synthesized by dinoflagellates are responsible for diarrhetic shellfish poisoning, another toxic syndrome. Some red tide dinoflagellates produce reactive forms of oxygen—superoxide, hydrogen peroxide, and hydroxyl radical—which may be responsible for toxic effects. A few other types of marine algae also produce toxic chemicals. Diatoms in the genus Nitzchia synthesize domoic acid, a chemical responsible for amnesic shellfish poisoning in humans.

Paralytic, diarrhetic, and amnesic shellfish poisoning all have the capability of making large numbers of people ill and can cause death in cases of extreme exposure or sensitivity. Because of the risks of poisoning associated with eating marine shellfish, many countries routinely monitor the toxicity of these foods using various sorts of assays. One commonly used bioassay involves the injection of laboratory mice with an extract of shellfish. If the mice develop diagnostic symptoms of poisoning, this is an indication of contamination of the shellfish by a marine toxin. However, the mouse bioassay is increasingly being replaced by more accurate methods of determining the presence and concentration of marine toxins using analytical biochemistry. The analytical methods are generally more reliable and are much kinder to mice.

Marine animals can also be poisoned by toxic chemicals synthesized during blooms. For example, in 1991 a bloom in Monterey Bay, California, of the diatom Nitzchia occidentalis resulted in the accumulation of domoic acid in filter-feeding zooplankton. These small animals were eaten by small fish, which also accumulated the toxic chemical and then poisoned fish-eating cormorants and pelicans that died in large numbers. In addition, some humans who ate shellfish contaminated by domoic acid were made ill.

In another case, a 1988 bloom of the planktonic alga Chrysochromulina polylepis in the Baltic Sea caused extensive mortalities of various species of seaweeds, invertebrates, and fish. A bloom in 1991 of a closely related species of alga in Norwegian waters killed large numbers of salmon that were kept in aquaculture cages. In 1996, a red tide killed 149 endangered manatees (Trichechus manatus latirostris) in the coastal waters of Florida.

Even large whales can be poisoned by algal toxins. In 1985, 14 humpback whales (Megaptera novaeangliae) died in Cape Cod Bay, Massachusetts, during a five-week period. This unusual mortality was caused by the whales eating mackerel (Scomber scombrus) that were contaminated by saxitoxin synthesized during a dinoflagellate bloom. In one observed death, a whale was seen to be behaving in an apparently normal fashion, but only 90 minutes later it had died. The symptoms of the whale deaths were typical of the mammalian neurotoxicity that is associated with saxitoxin, and fish collected in the area had large concentrations of this very poisonous chemical in their bodies.