A Plankton's Penchant for Poisoning

CINCINNATI--In 1989, a researcher at North Carolina State University was studying a newly discovered one-celled marine organism when he developed persistent confusion and memory loss. Other scientists in the lab later became sick, too. Now researchers using the organism's toxins have triggered similar neurological symptoms in rats. The findings, presented here at the Society of Toxicology's annual meeting, could lead to a better understanding of the risks the sinister organism and others in its family pose to people.

In 1988, botanists JoAnn Burkholder and Howard Glasgow at North Carolina State first identified Pfiesteria piscicida, a dinoflagellate--one of a class of organisms that are the primary constituents of plankton--as a major cause of fish kills in North Carolina estuaries. When sewage spurs plankton blooms off the coast, Pfiesteria produces toxins that may account for 30% of fish kills in the state, Burkholder says. But interrogating the culprit was hazardous to their health. Burkholder, Glasgow, and several others were insidiously poisoned by the toxins in the lab. Their symptoms, which in some cases persist, include loss of short-term memory, disorientation, immune-system suppression, and skin lesions. The researchers are all back in the lab.

To get a better handle on how Pfiesteria attacks the nervous system, a team led by environmental toxicologist Edward Levin and neurologist Donald Schmechel of Duke University trained a group of rats over 6 weeks to walk down a series of planks radiating from a platform--like spokes on a wheel--each leading to a Froot Loop. The rats learned that after fetching their reward, there was no point in going down a plank again, and they remembered this lesson even after being injected with a Pfiesteria extract. But when rats were injected before training, they learned much more slowly--a deficit that increased with dose and persisted for up to 10 weeks. The researchers saw the same result with a more complex task in which different planks were baited each day. "We've nailed it down to learning" and not memory, says Levin.

But while scientists may know the organism's modus operandi, they haven't yet identified its weapon. "We just need to find the active ingredient," says neurotoxicologist Hugh Tilson of the U.S. Environmental Protection Agency. While other groups try to pinpoint the toxin or toxins, Levin's team plans to probe whether the dangerous dinoflagellate affects nonspatial learning in rats, such as their ability to detect differences between lightness and darkness. Problems of this sort may point to potential nonspatial deficits in people. "We need to determine what sort of risks are involved," Levin says, so government agencies can properly warn fishers, divers, and others who might encounter these perilous plankton.