Disclaimer: These postings were sent to us from a variety of media sources over the Internet. The content has not been reviewed for scientific accuracy or edited in any manner.
Bad tidings Every once in a while, normally benign plankton suddenly turns poisonous, killing everything in a vast expanse of ocean. A Florida marine scientist thinks he knows why.
By WILLIAM ILLSEY ATKINSON Saturday, July 17, 2004
Globeandmail.com
Plankton, a smorgasbord of microscopic species that floats freely near the surface of the sea, is usually pretty benign. But occasionally, it turns into a mass killer, poisoning billions of sea creatures over wide areas of open ocean.
These baffling events are called red tides, and a marine scientist at a Florida research laboratory believes that he is close to explaining why they happen.
Plankton has an ancient lineage, and may be the closest living approximation to the first living cells. With these organisms, the simplest categories break down. Some types are self-propelled, making them animals, yet they also trap solar energy via photosynthesis, technically making them plants.
In its myriad numbers and shapes, plankton underpins the global food chain. Shellfish strain them from the water, starfish eat shellfish, fish eat starfish, and so on.
But while plankton sits low on the ecological totem pole, it isn't always content to be fodder for more complex, multicellular forms. In a red tide, the organisms rise up and slaughter every species in vast tracts of ocean in a kind of slave rebellion.
Many plankton species produce potent neurotoxins that seem precisely tailored to cripple or kill higher life forms. Even primitive blue-green algae have been implicated in the mass death of alligators in a freshwater swamp. In the open ocean, red tides often involve a plankton species called dinoflagellates.
What makes these plankton go postal?
"The plankton in a red tide aren't really malevolent," says Gary Kirkpatrick, a senior scientist at the Mote Marine Laboratory in Sarasota, Fla., and a specialist in dinoflagellates. "It's innocent murder they commit."
Still, he says, for the deadly behaviour to have become entrenched in the plankton's DNA, something about red-tide behaviour must pay off. Why do dinoflagellates use up time and energy to make poisons exquisitely adapted to the biochemistry of higher forms? Dr. Kirkpatrick suspects a red tide is the plankton's method of dynamite fishing -- killing everything within an enormous area, then feasting on what it has destroyed.
Like much else in nature, red tides start with a population explosion. Major increases in unicellular marine creatures are called blooms. They occur with lightning rapidity: Coastal observers or deepwater ships suddenly report large areas so dense with plankton that they discolour the sea. The colour of the bloom may be yellow, khaki or red. It's in these harmful algal blooms that plankton performs its worst villainy.
"Dinoflagellate metabolism requires a lot of nutrients, such as free carbon," Dr. Kirkpatrick says. "At low dinoflagellate concentrations, these nutrients are provided by carbon dioxide dissolved in seawater, land runoff and the bacterial breakdown of dead marine life. But those sources may be insufficient to fuel blooms of red tide."
When plankton's nutrient source is depleted by high red-tide concentrations, Dr. Kirkpatrick says, this may signal the suddenly starving dinoflagellate population to make poison. In other words, as soon as gentler, slower processes can't provide them with the resources they need, the plankton hurries the process along.
Individual dinoflagellates (the Latin name means "terrible whip") are so tiny -- 30 microns across, on average -- that they cannot be seen by the naked eye. But magnified several thousand diameters in a scanning electron microscope, their exteriors are dazzlingly complex. Some dinoflagellate species make their exterior surfaces out of cellulose. Like medieval samurai, they clad themselves in wooden armour.
Each dinoflagellate has two of the "terrible whips" for which they are named. One flagellum propels the free-swimming plankton through the water. The other wraps around its equator, and spins it around. Each whip is powered by a molecular motor that measures only a few hundred nanometres in diameter.
The intricacy of the natural engineering fascinates the scientists studying them. Something about these little critters engenders awe.
Maybe it's their venom. Most plankton poisons derive from saxitoxin, a close relative of the botulin that causes food poisoning. A few molecules of these neurotoxins in the wrong place, and presto! no breath, no heartbeat, no life.
Red tides are prodigious in manufacturing such poisons. A red tide may carry 1,000 deadly plankton per cubic centimetre, over 2,000 square kilometres of surface area and an average depth of 20 metres. That's 4,000,000,000,000,000,000 zooplankton -- and a lot of neurotoxin.
So vast are some red-tide blooms, particularly in the Gulf of Mexico, that satellites routinely image them from space.
Red tides are not restricted to warm waters: They also occur in Canada's cold northern waters. In 1984, a plankton bloom occurred off the coast of Prince Edward Island.
Shellfish, which get their food by straining plankton from seawater, concentrated these zooplankton and the nerve toxin they contained. While the shellfish themselves were unaffected by the toxins, the people who ate them were not.
Plankton neurotoxins are amazingly stable and, like many synthetic pollutants, can be transmitted unchanged up the food chain. Dozens of Canadians became ill and several older people died. Even among those it did not kill, the toxin, called domoic acid, produced massive memory loss and symptoms like those of late-stage Alzheimer's disease.
The syndrome has since been named amnesic shellfish poisoning. While the PEI red tide was caused not by dinoflagellates but by much bigger plankton called diatoms, the poisonous effect was the same.
As disturbing as these facts are, humans are let off lightly. At sea, toxin concentrations in a red tide can destroy everything it encounters, plus everything that eats a creature that has succumbed to the primary kill.
In a 1946 red tide near Tampa Bay on the Gulf Coast of Florida, seawater was as viscous as motor oil with dinoflagellates and their decaying victims. A hundred kilometres of beach were fouled with dead sea creatures, including crab, fish, turtles and manatees.
The dinoflagellate toxins were so concentrated that they entered the marine aerosol and irritated human throats, lungs and eyes several kilometres inland.
Happily, a natural balance quickly reasserts itself: Within a few days, the deadly plankton itself becomes food. One of their predators is Noctiluca, a benign dinoflagellate. As its Latin name implies, this plankton species is bioluminescent: Like a rainbow after a thunderstorm, it signals a red tide's end with a radiant all-clear.
Then it's business as usual -- until the villainous plankton get hungry again.
William Illsey Atkinson's book Nanocosm was short-listed for the 2004 Canadian Science Writers' Science in Society Award.