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Other HAB Species


Of some 60 or so species that cause red tides, only a handful is known to be toxic. Dominant dinoflagellate HAB genera include Alexandrium, Karenia, and Pfiesteria. The diatom genus most commonly associated with HABs is Pseudo-nitzschia. Each of these genera produces a different toxin and thus has a different role on organisms further up the food chain. Next, we discuss some of the HAB species in detail.

Scanning electron microscope view of cysts of the toxic alga Alexandrium tamarense
Scanning electron microscope view of cysts of the toxic alga Alexandrium tamarense

Alexandrium spp. is the dominant taxon in coastal regions of New England and eastern Canada but it is also found from California to Alaska.

It is a heterotrophic dinoflagellate that produces a saxitoxin, one of the most powerful known types of neurotoxins. These toxins destroy the function of nerve cells and can thereby cause paralysis. Saxitoxins are most effectively concentrated by shellfish such as clams, quahogs, mussels, scallops and oysters that filter large volumes of seawater to acquire their nutrition. Although the saxitoxin does not harm these shellfish, even in small quantities, the toxin can be extremely dangerous for humans, resulting in a serious illness known as paralytic shellfish poisoning (PSP). The saxitoxin attacks the human nervous system within 30 minutes of ingestion with symptoms that may include numbness, tingling, weakness, partial paralysis, incoherent speech, and nausea. In severe cases, the toxin can lead to respiratory failure and death within a few hours. Alexandrium spp. toxins have also been harmful to whales, sea otters and birds.

The following videos describe the causes and impacts of red tides as well as possible antidotes for shellfish poisoning.

Video: Red Tide - Death Comes to Gasparilla Island Florida (5:59) This video is not narrated.

Video: Prized Science Episode 4: Taming the Red Tides (4:28)

Click here for a transcript of the Prized Science Episode 4: Taming the Red Tides video.

[MUSIC PLAYING] PRESENTER: "And all the waters that were in the river were turned to blood, and the fish that was in the river died and the river stank. And the Egyptians could not drink of the water of the river."

The Book of Exodus in the Holy Bible may give us the first account of a red tide. Red tides are a phenomenon in which certain pigmented algae, toxic algae, undergo population explosions. They bloom in enormous numbers, staining the water reddish brown. Toxins or poisons released by the algae periodically kill millions of fish and cause billions of dollars in losses to the global commercial fishing industry.

If health officials detect a red tide, they ban fishing for oysters, shrimp, and other shellfish. If they didn't, unsuspecting consumers would get hit with a virtual tidal wave of discomfort in the form of neurotoxic shellfish poisoning. This terrible form of food poisoning causes noxious nausea and vomiting, but tingling of the mouth, arms, and legs, as well as poor coordination and other very unpleasant symptoms. With no specific treatment, victims may suffer for days on end.

That may change thanks to the research of Michael Crimmins, a scientist at the University of North Carolina in Chapel Hill. His work focuses on brevetoxin A, the compound that causes neurotoxic shellfish poisoning. It could even lead to the world's first antidote for this painful condition. For that research, the American Chemical Society awarded Crimmins its 2010 Ernest Guenther award in the chemistry of natural products.

Natural products are chemical compounds produced by plants, animals, and other living things. Natural products or substances derived from them have been the source of almost one out of every three of our prescription drugs. For more, here's Dr. Crimmins.

MICHAEL CRIMMINS: So brevetoxin A is a very complex structure that was isolated from red algae. And this red algae, the scientific name is karenia brevis. It creates these massive algae blooms in the Gulf of Mexico and the Pacific in various places. And these massive growths of algae create what is called red tides. And these red tides are these red algae that have grown sort of out-of-control over miles and miles and miles of ocean. They can be 50, 100 miles wide or long. And these algae produce a series of neurotoxic compounds, of which brevetoxin A is one of those compounds.

PRESENTER: When shellfish encounter algae in these red tides, they take up the toxins. People who eat contaminated shellfish then become ill. The algae can also produce a toxic mist which is swept along by the wind, like fog. People along the beach or in ships can become sick simply by inhaling this contaminated sea mist. In their quest to develop medicines to treat neurotoxic shellfish poisoning, Crimmins and his colleagues have been gathering knowledge about brevetoxin A and related toxins.

MICHAEL CRIMMINS: What we've been trying to do is to develop a synthesis of these compounds in the laboratory. While they're incredibly toxic, they're not really available in very large quantities from the natural sources. So we've been trying to develop a chemical synthesis of the compounds in an effort to perhaps develop analogs that would be effective antidotes to these compounds.

PRESENTER: Dr. Crimmins is talking about a potential cure for neurotoxic shellfish poisoning. For those who might encounter the toxin, that could reduce an ancient health threat of biblical proportions to a mere ripple.

Karenia is the dominant taxon causing red tides in Florida and Texas, but rarely species of Karenia have also been found up the east coast in North Carolina.

Examples of Karenia

It produces a toxin known as a brevetoxin (named after a species of Karenia, K. brevis). Like saxitoxins, brevetoxins damage nerve cells, leading to disruption of normal neurological processes and causing neurotoxic shellfish poisoning (NSP). In humans, gastrointestinal symptoms and a variety of neurological ailments result, but there are no known fatalities. However, in fish, the brevetoxins attack the central nervous system and cause respiratory failure. Karenia dinoflagellates are responsible for massive fish and bird kills in the Gulf of Mexico. The brevetoxins are colorless, odorless, and heat and acid stable, thus they survive food preparation.

Close-up of Gambierdiscus toxicus
Gambierdiscus toxicus a species that can produce ciguatera toxins

The genus Gambierdiscus lives in tropical waters, usually in reefs, and produces a toxin known as Ciguatoxin that causes gastrointestinal problems followed by mild neurological symptoms. This syndrome is known as Ciguatera fish poisoning. Because the toxin is fat soluble, it gets concentrated up the food chain by bioaccumulation from seaweed to smaller fish than to larger fish. The larger fish, which are the most dangerous to eat because they have the highest toxin concentrations, include commercially available seafood such as grouper, snapper, and barracuda. Ciguatera is responsible for more human illnesses—estimated between 10,000 to 50,000 cases annually—than any other HAB toxin.

Scanning electron microscope image of Pseudo-nitzschia australis
Scanning electron microscope image of Pseudo-nitzschia australis

Pseudo-nitzschia and the species Nitzschia navis-varingica are common diatom genera in Californian red tides. These taxa produce domoic acid, which is concentrated by filter-feeding shellfish. This neurotoxin can also bioaccumulate in fish such as anchovies that feed directly on the diatoms. Domoic acid causes a variety of gastrointestinal ailments, memory loss and brain damage in humans and is hence referred to as amnesic shellfish poisoning. Rarely, the neurotoxin can be fatal. It can also affect marine mammals, causing seizures.

Scanning electron microscope image of Pfiesteria piscida
Scanning electron microscope image of Pfiesteria piscicida.

The species Pfiesteria piscicida and P. shumwayae have been the most common dinoflagellate species in red tides in estuaries and bays along the east coast of the US from Delaware to Florida. The species occur in environments where freshwater and saltwater mix and have not been reported from freshwater environments or the open ocean. Pfiesteria blooms are restricted to summer months. Species have been associated with massive kills of menhaden and other estuarine fish in the Chesapeake Bay and the Tar-Pamlico and Neuse River Estuaries in North Carolina. The fish in contact with Pfiesteria rapidly develop bleeding lesions and have skin actively flake off them, and it has been proposed that the presence of live fish stimulates the production of toxin in the dinoflagellate. Ultimately the open lesions may destroy gill function and lead to death. Nevertheless, the connection between fish mortality and Pfiesteria is still doubted by some scientists. In fact, the effects of Pfiesteria on fish and human health has been one of the largest and nastiest controversies in marine science over the last 25 years and it does not appear that the conflict is anywhere near over.

The following video explains some of the research on Pfiesteria.

Video: Pfiesteria Update: An Enduring Debate (4:57)

Click here for a transcript of the Pfiesteria Update: An Enduring Debate video.

PRESENTER: In the spring of 1997, sick fish began showing up in the pound nets of watermen working along the Pocomoke River.

And soon after, some of those same watermen began showing up in doctors' offices. In these pound nets, floating among the sick fish, was a microbe called Pfiesteria piscicida, first identified in the wild by a North Carolina scientist named JoAnn Burkholder. She claimed this one-celled organism could sometimes release a toxin in the presence of fish, and that toxin could make people sick. Pfiesteria could also change shape, shifting through 24 life cycle stages from tiny zoospores all the way to large amoeba forms.

The search for more answers about this new organism has led to conflicting, even contradictory findings, kicking off complicated debates among marine scientists. When Wayne Litaker investigated Pfiesteria, he found a very simple life cycle with no amoeba forms at all. He used sophisticated molecular RNA probes as well as computer-controlled photography.

WAYNE LITAKER: So, you actually have the computer turn the microscope on every so often, turn the lights on, and take a picture, turn it off, and then you put it all together in a movie. And you can actually follow what's happening through time. We never saw any transformations. We saw a normal seven stage dinoflagellate life cycle. So, we didn't see any of those transformations whatsoever.

PRESENTER: Other researchers contradicted Burkholder's claim that Pfiesteria used a toxin to kill fish. At the Virginia Institute of Marine Science, Wolfgang Vogelbein announced evidence that Pfiesteria kills fish by directly feeding on them, not by releasing a toxin.

WOLFGANG VOGELBEIN: OK, what you're seeing here is the tail fin of an anesthetized fish.

These fish and these dinoflagellates are directly attacking the fish and feeding on the skin. It's that simple.

PRESENTER: Vogelbein put a filter between his hungry Pfiesteria and his tiny test fish, a filter that would keep any Pfiesteria out, but let any toxin pass through. A simple experiment with repeatable results. None of his fish died.

WOLFGANG VOGELBEIN: We can actually see what's happening, and we're finding out that there isn't an exotoxin here, at least not in our cultures.

PRESENTER: Fish were not dying from Pfiesteria toxin, but from Pfiesteria feeding.

WOLFGANG VOGELBEIN: This happens. You've seen it today, we've shown it to you. Firsthand. You've seen it. It happens. This is how fish die in our cultures.

Others say toxins kill their fish. Fine, let's do a comparative study, let's get to the bottom of it.

JOANN BURKHOLDER: Toxic Pfiesteria is not on trial here. Toxic Pfiesteria exists. There is no question. It's been cross confirmed in multiple laboratories.

PRESENTER: Andrew Gordon runs one of the laboratories where a Pfiesteria connected toxin is killing fish. His technique runs water from a toxic culture through a filter removing all the Pfiesteria cells, then tests this filter in Pfiesteria-free water on small fish.

ANDREW GORDON: I have no doubt that in these systems in the presence of Pfiesteria, that there is a soluble toxin produced. And we've seen it. I've had undergraduates do the experiment; they're seeing it; they're all over the country now. And they're believers because they've done the experiment themselves. It's a very simple experiment to do.

PRESENTER: A simple experiment with repeatable results. In most of his experiments, all of his fish died.

ANDREW GORDON: The toxin that's produced is Pfiesteria-associated. You never see it in the absence of Pfiesteria, and you see it in the presence of Pfiesteria. So, that's pretty clear.

PRESENTER: What is not so clear so far from all the studies in the laboratory is how much harm these toxins can cause in our rivers, either to fish or to people.

JOANN BURKHOLDER: We have a long ways to go, and we just don't understand what turns on and off toxin production in these creatures. I wouldn't be surprised if the so-called nontoxic or non-inducible strains of Pfiesteria make toxin. But just can't somehow turn it on or activate it. And we'll be finding that out, I think, in the next couple of years.

PRESENTER: By the time Pfiesteria blooms again in the Chesapeake, scientists may have more answers to some of these contentious questions.

At least part of the debate has been fueled by the popular press, who have focused attention on the organism after studies suggested that it was carnivorous. These studies indicated that Pfiesteria species ingested the skin of fish after it flaked off. In fact, lab studies have shown that when fish were left in tanks with Pfiesteria, the fish died within hours. For this to occur, however, the dinoflagellate and fish must be in direct contact. Without contact, the same studies show that fish suffered no ill effects. Some scientists alternatively point to water molds or fungi including the species Aphanomyces invadans as the pathogen cause of the ulcerative lesions, skin loss, and damage to gills. A. invadans and other fungi are universally present in fish with ulcers and skin loss. Also weakening the case for Pfiesteria, this genus is still known to exist in North Carolina estuaries, but fish kills have become less frequent recently. Moreover, where lesions on the fish menhaden were observed, nearby fish including catfish, perch, and carp were unaffected. These disparities have cast some doubt on whether Pfiesteria is harmful to fish at all. In fact, great differences exist among public health professionals, and warnings from different state agencies are in conflict.

Examples of Pfiesteria

The health impact of Pfiesteria on humans is also uncertain, as is the method of transmission of the potential toxin. Scientists working with Pfiesteria in the laboratory have suffered from long-term neurological symptoms, such as memory loss, fatigue, and dermatological problems, and fisherman in contact with Pfiesteria-related fish kills have also suffered from similar ailments. However, other groups of fishermen who have come in close contact with lesion-covered fish have not reported adverse effects. There are reports that the hypothetical Pfiesteria toxin is transmitted via aerosols.

Until recently, the missing link in the Pfiesteria conundrum is that the toxin produced by this organism has been elusive. However, in 2007 scientists in a government lab claimed the first positive identification of a toxin associated with Pfiesteria. Even with this identification, questions remain; for one, the toxin is unstable in the natural environment, and second, it is not been proven to have adverse health effects. Thus, the controversy about Pfiesteria is far from over. In all reality, a number of factors may result in the fish kills; in particular, the fish in estuaries may have already been under great stress from other biological agents (bacteria, viruses, fungi, parasites), exposure to chemicals (pollutants, toxins), suboptimal water quality, and rapid water temperature change, that have the potential to cause lesions to form.

Far less controversial than the relationship of Pfiesteria with fish kills are studies that directly relate fish kills to low oxygen levels caused by algal blooms. In a number of estuaries along the eastern US, warmer waters in summer combined with increased production by algae, as a result of increased runoff and eutrophication, lead to severely decreased oxygen levels and major fish kills without the involvement of a toxin.