The rise in toxic tides: what's behind the ocean blooms? - toxic microorganisms - includes related articles - Cover Story

Science News, Sept 27, 1997 by Christine Mlot

In 1984, a small herd of cows in Montana collapsed and died 10 minutes after drinking from a pond coated with a pea green film.

In 1987, three people died and more than 100 became ill after eating blue mussels from Prince Edward Island.

In 1996, 149 manatees perished mysteriously off the coast of Florida. Autopsies of these endangered marine mammals revealed biological poisons in their brains and other tissues.

In the last 2 months, thousands of fish have sickened and died in tributaries of the Chesapeake Bay. The kills resemble a decade-long pattern in North Carolina estuaries, where a billion or more crabs and fish, especially menhaden, have been killed.

Researchers and the news media have recorded an increasing number of water-related poisonings of both wildlife and people worldwide since the 1970s. The culprits have turned out to be potent toxins produced by any of several single-celled aquatic organisms--some well known, others completely new and dumbfounding in their biology.

A droplet of pond or ocean water contains myriad numbers and species of these organisms. They live in an ancient microscopic realm that is neither wholly bacterial nor animal nor plant, even though they are often classified with algae.

"There's clearly been an increase in both the frequency and extents of harmful algal blooms in coastal waters," says Frances M. Van Dolah of the National Marine Fisheries Service in Charleston, S.C.

The question is, why?

The toxins causing the Chesapeake and North Carolina fish kills have come from Pfiesteria, an organism that has attracted the attention of scientists only in this decade. Its treacherous ways have recently caught the public's attention as well. Last week, in an interim report, a medical team appointed by Maryland Governor Parris N. Glendening linked intensive exposures to toxic Pfiesteria-containing waters with memory and health problems in 13 people.

Yet Pfiesteria is only one of the toxin producers that are dramatically making their presence known. These organisms have traditionally been called algae or plankton, specifically phytoplankton. When they reproduce into large populations, they are informally known as tides or harmful blooms. Recent textbooks call most of these single-celled microorganisms protists. During evolution, protists gave rise to macroscopic plants and animals.

Pfiesteria, with its 24-stage life cycle, defies neat categorization. At some stages, it swims about powered by two flagella and so is considered a dinoflagellate.

Dinoflagellates are key players in aquatic ecosystem large numbers of them live only in the pores of ice, for example, where they photosynthesize and provide food for other organisms. Pfiesteria can photosynthesize, but only if it has stolen the green organelles called chloroplasts from true algal cells. It uses a hoselike attachment to acquire these "kleptochloroplasts."

Although Pfiesteria can subsist in this way says JoAnn M. Burkholder of North Carolina State University in Raleigh, it changes its eating habits depending on what's available. When fish are around, it can turn into a predator. It abandons its usual swirling swim pattern and makes a beeline for its prey in response to an unknown cue, perhaps in fish oil or excrement. Then Pfiesteria churns out its toxic cocktail. It emits at least one compound that seems to kill the fish and another that opens up the fish's skin, allowing Pfiesteria to feed on the tissue inside (SN: 9/6/97, p. 149). Burkholder, a codiscoverer of Pfiesteria, has personally experienced its toxic effects and knows the organism inside out. "It's plant-and animal-like," she says.

The dinoflagellates responsible for so-called red tides behave more like plants. The pigments they use in photosynthesis can tint the water red during a bloom. Beachcombers notice red tides in other ways-airborne toxins sting the insides of their noses and throats.

Red tides have been around at least since biblical times. In Exodus, a plague turned the water bloodred and destroyed fish. In recent times, such tides have become more frequent and more noticeable. In 1972, New England experienced its first red tide, with devastating effects on the shellfish industry. In Florida, where red tides usually last from 3 to 5 months, one persisted for 18 months, culminating in the manatee deaths in 1996.

Another group of plankton, surprisingly, was responsible for the Prince Edward illness. Canadian researchers traced the outbreak to a toxin, domoic acid, produced by a diatom. Until then, diatoms had been known mainly as benign photosynthesizers. Lacking flagella, they tumble about in the surf or waves, protected by often spectacular filigrees; of silica.

In 1991, dozens of California pelicans and cormorants died after feeding on anchovies found to contain the toxin from these diatoms. The toxic diatoms have since been found around the world.

The cows' demise was less mysterious. They were felled by toxins from the oldest photosynthesizers on earth, cyanobacteria, formerly called blue-green algae. They are not always blue-green. One kind blooms frequently in the Red Sea and is probably responsible for its name. Other cyanobacteria grow into bright green films, like the one on the lethal lake in Montana.