Algae is a general category of one-celled plants that are sometimes called phytoplankton. Some harmful algae are dinoflagellates -- strange plants that propel themselves through the water with one or more whip-like organs.
Noctiluca algae produced a spectacular display of color when the bloom
occurred near the water's surface off the coast of California. Such blooms are caused by
high concentrations of sometimes toxic algae species. Photo by Peter J.S. Franks, Scripps Institution of Oceanography.
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Attack of the killer algae
Within the past 20 years, massive blooms of harmful algae (often called "red tides") have occurred along all three U.S. sea coasts. Some of these algae make toxins that interfere with nerve signals in fish and people. Sometimes shellfish seem to accumulate these toxins without serious harm, but humans get sick after eating them. Other people get sick after fishing in infested waters. And some get sick after simply breathing near infested waters!
Blue crab collected in a pfisteria-
Some harmful algae can exist in sediment in a dormant form called a cyst. Under the right conditions, these cysts can germinate and turn into the swimming stage that divides to form masses of algae that can be dense enough to tint seawater. Although the common name "red tide" reflects the color of some toxic algal blooms, they can take other colors as well (such as green or brown), or be colorless. Sometimes water that is perfectly blue or clear can be more dangerous than a visible red tide because even a small number of cells of certain species represent a lot of toxin.
In fact, scientists are moving away from the bloodcurdling image wrought by "red tide" in favor of the more accurate, but infinitely more boring, "harmful algal bloom," with the obligatory acronym HAB.
Whatever the name, these harmful blooms can carry potent nerve toxins. When people eat shellfish contaminated with these toxins, they can develop paralysis, amnesia and other neurological problems.
Will you have the mussels?
Shellfish that make their living by filtering tiny bits of food from the sea are often the first victims of a harmful algal bloom. JoAnn Burkholder, professor of aquatic botany and marine sciences at North Carolina State University, says some toxins are so powerful that just 5000 cells in a liter of seawater are enough to force closure of shellfish beds to prevent human poisoning. Burkholder helped discover one harmful alga, Pfiesteria, which has caused repeated shellfish closings and fish kills in Chesapeake Bay and North Carolina's Albemarle-Pamlico estuary.
The Pfiesteria toxin can cause blurred vision and sleepiness. Longer exposure can cause severe headaches and memory loss and what Burkholder calls a "very striking" syndrome in which victims have trouble forming sentences.
While the Food and Drug Administration says people have not been harmed by eating Pfiesteria-contaminated shellfish, there are reports of neurological problems among people who swam or water-skied through Pfiesteria blooms. In 1996, Pfiesteria caused peculiar holes in fish skin in parts of Chesapeake Bay. At the same time, fishermen began complaining of memory losses. In 1998, a research group (see "Learning and Memory..." in the bibliography) reported that fishermen with the most hours on the water had the most difficulty forming memories. The problems resolved after three to six months.
More, but not merrier!
On the Gulf Coast, a dinoflagellate called Gymnodinium breve causes fish to suffocate when the toxin enters gill tissue. Shellfish accumulate the G. breve toxins with no ill effects, but humans who consume those shellfish can get very sick. The organism has plagued the Gulf Coast since the 1800s and has been blamed for a 1996 die-off of manatees.
"At this time, there are more than 50 [known] species or strains of harmful algae," says Don Anderson, a senior scientist at the Woods Hole Oceanographic Institution. "The truth is that no one knows how many harmful species there are, since the list keeps growing, and no one has kept track very well," he adds.
A new hazard?
One clue to the origin of the toxins is the fact that the algae produce them all the time, not just when stressed. "It could be a metabolite [normal chemical byproduct]," Anderson says, that just happens to be toxic to us."
There's no question that more attention has been paid to the toxic algae in recent years, and that is responsible in part for the increasing numbers of reports of toxic species or toxic outbreaks. Nonetheless, only a few of the toxic blooms in the United States are being caused by unknown organisms. Says Anderson, "We're not talking about mutations or anything strange like that."
The toxic algae are just a few dozens of the tens of thousands of algae living in the ocean. Anderson adds that it's not clear why they make toxins. "For virtually all of them, we don't know the ecological role of the toxin." A few algae make toxins that are poisonous to the tiny zooplankton animals that graze on them, but many of these are slow-acting. "The effect may not show up until [the grazing animal] tries to lay eggs. It works, but this is not the same strategy as one finds with some plants on land, which releases toxins as a sudden chemical defense when they are bitten."
What about the long term?
Nonetheless, Burkholder says "small but serious impacts are already quietly affecting fish and human health in our increasingly urbanized coasts." As she indicates, some scientists think red tides are spurred by rises in coastal pollution, which feed nitrogen and phosphorous to the massive blooms.
More widespread ecological changes, such as global warming, may also underlie the harmful blooms. "Population explosions of nuisance organisms (e.g. harmful algae), may be viewed as symptoms of failing ecosystem health," wrote the authors of a three-year study of ocean health (see "Marine Ecosystems:..." in the bibliography).
Swimming against this tide of opinion are observations of many HAB problems that occur in pristine waters, such as the paralytic shellfish poisoning problems in Alaska or the Gulf of Maine. Some of these blooms develop in offshore waters, far from human pollution.
To Anderson, this just demonstrates that many natural and artificial factors are feeding the expansion of toxic blooms. The simple fact is that more scientists, with more instruments, are paying attention, which conspires to make a large problem look even larger.
Algae feed the animals that make coral. But coral reefs are getting sick...
In addition to the obvious problem of acute human health effects, the long-term, chronic exposures to algal toxins are worrisome, but remain largely unstudied. Further hampering efforts to forecast, analyze and treat the diseases, scientists have not yet identified some of the toxins involved -- in particular the Pfiesteria toxins.
