ALGAE, TOXIC. Marine phytoplankton, or single-cell algae, are the base of the marine food chain. Phytoplankton use sunlight to convert simple inorganic molecules, such as water and carbon dioxide, to complex organic compounds, such as protein, carbohydrates, and lipids. The ocean waters that surround continental coastlines are the home of a large number of marine algae, making them the most productive areas for the harvest of marine finfish and shellfish. Much of the time, algae are present at very low numbers, but when conditions are right, they can grow rapidly and explosively, producing a noticeable discoloration in the water called a "bloom." These blooms can cover very large areas of the coastal ocean that are often visible from satellites. Large numbers of "blooming" algae can, at times, produce toxins or result in lower oxygen levels in seawater, thereby creating significant problems for shore-based businesses (e.g., hotels, restaurants), the seafood industry, and consumers.
While many of these algal blooms are merely noxious (i.e., producing smelly odors, discolored water, gelatinous masses), some produce marine toxins that enter the marine food web when consumed by shellfish or finfish. Typically, these finfish and shellfish (particularly mussels) have a capacity to concentrate and accumulate very large amounts of these toxins without apparent harm to themselves. Toxin levels can be so high that consuming only a few mussels could be lethal. When blooms produce such deleterious effects, they are called harmful algal blooms, or HABs.
A variety of human intoxication syndromes have been characterized from the consumption of seafood contaminated with algal toxins. Five will be described here. For a detailed historical and descriptive treatment of the myriad seafood intoxications, please see the extensive monograph by Halstead, the chapter by Wekell and Liston, and Seafood Safety (pp. 87–110) in the bibliography. In the temperate zone, three HAB intoxications are of concern to seafood consumers and industry: paralytic shellfish poisoning (PSP), amnesiac shellfish poisoning (ASP, more correctly domoic acid poisoning), and diarrhetic shellfish poisoning (DSP). In semitropical and tropical waters, two syndromes have major impacts: neurotoxic shellfish poisoning (NSP) and ciguatera fish poisoning (CFP).
PSP and ASP present the greatest health risks due to the following factors: potency of the toxins, wide geographic distribution, and large coastal populations exposed to the toxins, thereby elevating the risk of human exposure to these toxins. In addition, shellfish production has increased to meet increasing worldwide demands. Perhaps the tropical poisonings (NSP and CFP) have disproportionately greater impacts because some of the "at-risk" populations in tropical communities are poor and dependent on seafoods for daily subsistence. In these regions, illnesses from these toxins have greater impact on daily lives than those in more developed and wealthier countries due to loss of work time and limited access to medical care. Since no antidotes are available, medical treatment for all seafood intoxications is limited to providing emergency measures (e.g., gastric lavage, breathing assistance) and treating symptoms (Wekell and Liston, pp. 111–155).
The Symptoms, Toxins, and the Organisms That Produce Them
Paralytic shellfish poison (PSP). Symptoms include paralysis of the peripheral muscles including the chest, making breathing difficult (in mild cases) to impossible in the most severe cases. Along the West Coast of North America, several deaths have been reported over the past decade from recreational and subsistence shellfish consumers. The only treatment is immediate medical intervention, that is, the use of artificial respiration until the toxins are purged from the victim's body. PSP is caused by the consumption of shellfish contaminated with a suite of toxins (about a dozen individual toxins have been identified). The first toxin to be chemically identified was named "saxitoxin" because it was found in the Alaskan butter clam (Saxidomus giganteus ). Since then, other toxins have been identified and named "gonyautoxins," after the historical name (Gonyaulax ) for the causative organism, currently known as the Alexandrium dinoflagellate. The PSP toxins and associated algae are found from the sub-Arctic (and sub-Antarctic) to tropical areas, but are most common in cooler temperate waters (both in the Northern and Southern Hemispheres). In the temperate zones, the toxins are produced by the dinoflagellates in the genus Alexandrium : specifically A. catenella and A. tamarense. In tropical and semi-tropical areas, the dinoflagellates Pyrodinium bahamense var. compressum and Gymnodinium catenatum have been implicated as producers of PSP toxins.
Amnesiac shellfish poisoning (ASP or domoic acid poisoning). The first identified outbreak occurred in 1987 in eastern Canada due to the consumption of mussels contaminated with domoic acid. Symptoms varied from mild gastroenteritis to the loss of short-term memory, and in the most severe cases, death. In this incident, the elderly appeared more susceptible than younger people. The toxin binds to parts of the brain responsible for memory and learning, resulting in nerve cell death. To date, in the United States, the most common vectors of lethal levels of the toxin have been sardines and anchovies—planktivorous fish that consume phytoplankton. Consumption of these fish has resulted in the stranding and death of marine mammals and sea birds on the West Coast of the United States. So far, these are the only known poisoning victims of this toxin. Domoic acid has seriously impacted razor clam recreational fisheries (and its support industries) in Washington State and commercial crab fisheries on the West Coast of the United States. In Nova Scotia, Canada, and Maine, very high levels of domoic have been detected in mussels. However, due to an extensive and successful monitoring and surveillance programs put in place since 1987, no human illnesses have so far been reported in these areas from consumption of commercially raised shellfish. Domoic acid is produced by some of the diatoms in the genus Pseudo-nitzschia. However, not all species have been implicated in major domoic acid poisoning outbreaks, so far these include: P. multiseries, P. australis, and P. pseudodelicatissima. Globally, these organisms are quite cosmopolitan and have been identified in temperate waters of both the Northern and Southern Hemispheres. The genus is also fairly common in marine estuaries.
Diarrhetic shellfish poisoning (DSP). As the name implies, human consumption of shellfish containing the toxins associated with this syndrome causes diarrhea. Because symptoms are mild and similar to those in other common illnesses (gastrointestinal distress, nausea), many cases probably go largely unreported so that the true incidence of DSP is likely much higher than current epidemiological data indicates. However, there is a particular human health concern associated with DSP because the toxins involved (the dinophysistoxins or DTXs) are known potent tumor promoters (Fujiki et al., pp. 232–240). Toxic shellfish and causative organisms have been found in the Mediterranean Sea, off the Galician coast of Spain, and Ireland. Dinophysis cf. acuminata and D. acuta are believed to be the causative organism and are found in temperate waters worldwide. Recently, DSP toxins have been found in North America in eastern Canada and the northeastern coast of the United States but the primary source of the toxins appears to be Prorocentrum lima (Reguera et al., pp. 78–80).
Neurotoxic shellfish poisoning (NSP). People consuming shellfish containing the toxins report tingling fingers, numbness of lips, and reversal of temperature sensation, along with gastroenteritis. During heavy blooms of the causative organism of NSP, the dinoflagellate Karenia brevis (older name: Gymnodinium breve ), aerosol dispersions of both the organism, and toxins can be caused by breaking waves and wind resulting in upper respiratory problems and distress in people. In some cases, victims may need to move inland to avoid exposure to the aerosols. The toxins involved are high-molecular-weight polyether toxins known as brevetoxins. Karenia brevis inhabits both temperate and tropical waters. In the United States, outbreaks are common in the Gulf of Mexico, where long-lasting blooms have been observed along the west coast of Florida. In the early 1990s, outbreaks of NSP were also reported on the North Island of New Zealand.
Ciguatera fish poisoning (CFP, or tropical fish poisoning). Symptoms, including gastroenteritis, that in most cases occur within a very short period of consuming the tainted fish. In severe cases, paresthesia or numbness occurs around the mouth, lips, and tongue. In extreme cases, burning sensations in mouth and death occur. Some victims report a reversal of the sensations of hot and cold. One victim was observed blowing on ice cream to cool it down. In some cases, symptoms reoccur years after the initial intoxication, usually during times of stress. In the most extreme cases of intoxication, death can occur within hours of consuming tainted fish. The toxin, ciguatoxin, and its congeners are complex fat-soluble, high-molecular-weight, polycyclic polyether compounds. The toxins are produced by the benthic dinoflagellate Gambierdiscus toxicus that grows on corals and other surfaces in tropical reefs. Virtually all tropical reef-dwelling fish are suspect (Halstead, pp. 325–402). Coral-eating fish (e.g., parrot fishes) consume algae on the coral including the associated G. toxicus, thereby accumulating toxin in their fatty tissues. Predators of these fish then concentrate the toxin. Top reef predators, such as barracuda in the Caribbean and moray eels (in the Pacific), are considered high-risk vectors of ciguatoxin. Tropical Pacific outbreaks of Ciguatera appear to be the most intense and have caused rapid deaths, while outbreaks in the Caribbean seem to be relatively mild. Treatment is largely symptomatic, entailing fluid replacement and other life support measures. Unfortunately, monitoring tests for Ciguatera toxins are complex and costly and have not been widely implemented.
Prevention, Control, and Mitigation
With all these intoxication syndromes, shellfish and fish show no clear outward signs that they might be poisonous. The presence of toxins can only be determined by appropriate chemical testing. Cooking, cleaning, or organ removal in fish and shellfish may reduce the toxin levels in some cases but cannot guarantee that the seafood has been rendered safe. Therefore, both recreational and commercial fishers must rely on monitoring and surveillance programs, usually operated by governmental agencies. Commercial seafoods, particularly shellfish, are usually monitored and tested for toxins (when reliable methods exist) as part of government safety and sanitation programs. It is wise for consumers to purchase only inspected shellfish from reliable, licensed retailers, since these businesses are usually required to maintain certification records for their shellfish stock.
For recreational fishers, it is vital that they obtain information about local closures or safe beaches. Necessary safety information should be posted on signs or made available by telephone "hotlines," websites, or news media. Unfortunately, depending on agency resources, recreational and private beaches are monitored in a more limited fashion, placing these fishers at a higher risk. Nevertheless, it is the recreational fishers' responsibility to obtain the needed information that will allow them to safely enjoy the sea's bounty.
See also Crustaceans and Shellfish; Fish; Fishing; Mammals, Sea .
Fujiki, Hirota, Masami Suganuma, and Horoko Suguri, et al. "New Tumor Promoters from Marine Natural Products." In Marine Toxins: Origin, Structure, and Molecular Pharmacology, edited by Sherwood Hall and Gary Strichartz, pp. 232–240. Washington, D.C.: American Chemical Society, 1990.
Halstead, Bruce W. Poisonous and Venomous Marine Animals of the World. Revised edition. Princeton, N.J.: Darwin Press, 1978.
Lassus, Patrick, Geneviève Arzul, and Evelyne Erard-Le Denn, et al., eds. "Harmful Algal Blooms." In Proceedings of the VI International Conference on Harmful Algae. Nantes, France, October 1993. Paris: Lavoisier Publishing, 1995.
"Naturally Occurring Fish and Shellfish Poisons." In Seafood Safety, edited by Farid E. Ahmed, chap. 4, pp. 87–110. Washington, D.C.: National Academy Press, 1991.
Reguera, Beatriz, Juan Blanco, Ma Luisa Fernández, and Timothy Wyatt, eds. "Harmful Algae." In Proceedings of the VIII International Conference on Harmful Algae. Vigo, Spain, 25–29 June, 1997. Vigo, Spain: Xunta de Galicia and Inter-governmental Oceanographic Commission of UNESCO, 1998.
Smayda, Theodore J., and Yuzuru Shimizu, eds. "Toxic Phytoplankton Blooms in the Sea." In Proceedings of the V International Conference on Harmful Algae. Newport, Rhode Island, November 1–28, 1991. New York: Elsevier, 1993.
Wekell, John C., and John Liston. "Seafood Biotoxicants." In Trace Substances and Health: A Handbook, edited by Paul M. Newberne, part II. New York: M. Dekker, 1982.
John C. Wekell