Red Tide

Introduction

Red tide is a type of algal bloom, which is a sudden, large increase in the number of microscopic organisms living in a body of water. These organisms are called dinoflagel-lates and are a type of phytoplankton that live in marine and freshwater habitats. Sudden large increases in their numbers are thought to be attributable to changes in salinity, temperature, and water depth, and sometimes to human activities such as the addition of nutrients to water from agricultural runoff.

When dinoflagellates that contain various natural neurotoxins—such as brevetoxin, domoic acid, and saxi-toxin—occur in large enough concentrations, they can cause heavy and murky spots of reddish, discolored surface waters. This reddish discoloration is the source of the term “red tide.” When these toxins are present in the dinoflagellates causing the “red tide,” it is termed a harmful algal bloom (HAB). In such cases, these HABs can be eaten by fish, birds, marine mammals, and humans, causing adverse symptoms and, sometimes, death. Toxins can become airborne, as well; when winds set inland during a large red-tide algal bloom off the coast of Florida, for example, asthma attacks send an increased number of people to hospital emergency rooms.

Historical Background and Scientific Foundations

Most species of phytoplankton are not harmful to humans. In fact, they are beneficial as a food source for larger fish and mammals. However, sometimes phytoplankton grow (bloom) so fast that they become visible on the surface of the water. When this happens, the term “algal bloom” is used to describe the various species of phytoplankton that are involved.

Because various phytoplankton species contain pigments of different colors—ranging from green to brown to red—they can turn water different colors depending on which types of phytoplankton have grown to large enough concentrations. The water can appear to be colored from white to nearly black, but usually the water looks reddish or brown, depending on the species. Because of this color variance, scientists usually prefer the term “algal bloom” to the term “red tide.” Red tide is used most frequently in the United States, while algal bloom is more often used in other parts of the world.

A particular species of dinoflagellate called Karenia brevis is often responsible for the red tides that occur in the waters of the eastern Gulf of Mexico. Red tides also occur in the northeastern part of the United States. The dinoflagellate species Alexandrium spp., which are especially common along the coasts of the North Atlantic Ocean and the Pacific Ocean of North America, are associated with red tides in these areas. These dinoflagellate species are eaten by a wide range of organisms, including mussels, soft-shell clams, sea scallops, oysters, lobsters, crabs, salmon, herring, mackerel, whales, sea birds, and sea otters.

Studies by researchers at the Woods Hole Oceanographic Institution and the U.S. National Oceanographic and Atmospheric Administration (NOAA) show that the frequency of algal blooms has increased along the coasts of the United States and other countries since the 1970s. Although the cause of the increased number of blooms is not absolutely certain, a general consensus among scientists is that the documented warming of the coastal oceans has made conditions more favorable for algal growth. If so, a consequence of global warming could be more algal blooms and more cases of marine toxin-related illness.

WORDS TO KNOW

ALGAL BLOOM: Sudden reproductive explosion of algae (single-celled aquatic green plants) in a large, natural body of water such as a lake or sea. Blooms near coasts are sometimes called red tides.

BREVETOXIN: Any of a class of neurotoxins produced by the algae that cause red tide (coastal algae blooms). Brevetoxins can be concentrated by shellfish and poisonous to humans who eat the shellfish.

DINOFLAGELLATE: Small organisms with both plant-like and animal-like characteristics, usually classified as algae (plants). They take their name from their twirling motion and their whip-like flagella.

DOMOIC ACID: A neurotoxin produced by the algae that cause red tide (coastal algae blooms). Domoic acid can be concentrated by shellfish and poisonous to humans who eat the shellfish.

NEUROTOXIN: A poison that interferes with nerve function, usually by affecting the flow of ions through the cell membrane.

PHYTOPLANKTON: Microscopic marine organisms (mostly algae and diatoms) that are responsible for most of the photosynthetic activity in the oceans.

SALINITY: Measurement of the amount of sodium chloride in a given volume of water.

SAXITOXIN: A neurotoxin found in a variety of dinoflagel-lates. If ingested, it may cause respiratory failure and cardiac arrest.

Impacts and Issues

Scientists are unsure about the cause of red tides, and whether they are natural occurrences or due to human activities. Some parts of the world only experience red tides at certain times of the year, while other regions seem only to have red tides when human activities promote them.

Some scientific studies have found that any pollution that increases the temperature of the water can increase the frequency of red tides. Other studies show that the nitrates and phosphates found in agricultural runoff can cause red tides. Still other studies show an increase in iron concentration as the cause of red tides. However, currently no definitive link has been found between human activities and the incidence of red tides.

Organisms that consume HABs are harmed by the neurotoxin that the dinoflagellates contain. One such neurotoxin—saxitoxin—is found in shellfish that ingest the harmful dinoflagellates. This toxin can produce paralytic shellfish poisoning (PSP) in humans. Patients who eat shellfish containing this toxin experience symptoms—such as amnesia, diarrhea, disorientation, memory loss, nausea, paralysis, and vomiting—that can eventually worsen to severe problems with the respiratory, gastrointestinal, and neurological systems. Death usually comes from respiratory failure. According to the Woods Hole Oceanographic Institution, a person weighing 165 lb (75 kg) can die from digesting just 0.02 oz (0.5 g) of saxitoxin.

Brevetoxin, another neurotoxin, causes neurotoxic shellfish poisoning in humans, while domoic acid produces amnesic shellfish poisoning. Okadaic acid causes diarrhetic shellfish poisoning. Ciguatoxin—from the algae genus Gambierdiscus, which lives in tropical waters such as those around coral reefs—produces ciguatera fish poisoning.

The incidence of red tides has increased over the past 30 or so years, possibly implying that human activity is responsible. Scientists have suggested that climate change and increased pollution levels could make red tides more pronounced. The increased incidence of red tides could have an adverse effect on the availability of food sources for marine animals. However, at the present time, scientists do not know conclusively what causes red tides.

Why some dinoflagellate species produce harmful toxins while others do not also is unknown. Scientists continue to study red tides, especially in an attempt to identify the specific genes that are involved in producing red tides.

Scientists urge governments to use better and more frequent monitoring and detection systems for red tides. Although the annual incidence of death and serious illnesses in the United Sates due to fish and shellfish poisoning is relatively low, new toxins appear periodically that make it more difficult to protect the environment and human health.

The artificial creation of non-toxic algal blooms by adding powdered iron to the ocean surface has been proposed as a way of combating global climate change. The idea is that phytoplankton would absorb carbon dioxide from the atmosphere, incorporating its carbon into their tissues; when the phytoplankton died and sank, they would take the carbon with them to the bottom of the sea, sequestering it (isolating it for a long time) from the atmosphere. However, both the efficacy and safety of this technique, called iron fertilization, have been questioned by many scientists. Small-scale tests have seemed to show little carbon sequestration from artificially induced algal blooms.

In 2007, researchers announced that Karenia brevis not only produces a number of brevetoxins that can constrict bronchial passages and so interfere with breathing, but also produces an antidote to its own toxins, a substance called brevenal. Brevenal, first discovered in 2004, can be made synthetically and is being evaluated for

treatment of some of the symptoms of cystic fibrosis, a genetic disorder that afflicts about 30,000 people in the United States. Brevenal may also be used in Florida to save manatees (rare water-dwelling mammals found only in certain tropical coastal waters) that are threatened with poisoning by red tides.

See Also Algal Blooms; Coral Reefs and Corals; Iron Fertilization

BIBLIOGRAPHY

Books

Okaichi, Tomotoshi, ed. Red Tides. Dordrecht, Netherlands: Kluwer, 2004.

Subba Rao, D. V. Algal Cultures, Analogues of Blooms and Applications. Enfield, NH: Science Publishers, 2006.

Periodicals

Potera, Carol. “Florida Red Tide Brews Up Drug Lead for Cystic Fibrosis.” Science 316 (2007): 1561–1562.

Web Sites

Fathom (Columbia University) and Woods Hole Oceanographic Institution. “Toxic Blooms: Understanding Red Tides.” http://www.fathom.com/course/10701012/index.html (accessed March 29, 2008).

Woods Hole Oceanographic Institution. “Harmful Algae and Red Tides.” http://www.whoi.edu/page.do?pid=11913 (accessed April 2, 2008).

William Arthur Atkins