Fish killer microbe may not be toxic, after all
Date: 06-Aug-02
Country: USA
Author: Maggie Fox, Health and Science Correspondent
The scientists said they found no evidence that Pfiesteria, blamed for high-profile fish kills over the past decade, secretes a deadly toxin, as had been previously believed.
In fact, the strange one-celled organisms do not produce any known poison, chemist Robert Gawley of the University of Miami and colleagues reported in the Proceedings of the National Academy of Sciences.
A second team at the College of William and Mary in Virginia said Pfiesteria may kill by eating little holes in the skin, weakening the fish and leaving them open to infections.
Gawley's team tested Pfiesteria by bursting open the organisms in a centrifuge and checking the water they were in.
"The cultures that we centrifuged had been living and killing tilapia (a kind of fish) for eight months," Gawley said in a telephone interview. "All you have to do is take a scoop of that water and centrifuge it and all of a sudden it won't kill fish any more."
Gawley said his team also considered the findings of a marine biologist at North Carolina State University who has argued that the organisms secrete toxins only after becoming excited by a large group of fish.
"To be honest, if there were a toxin in there, it would have killed the fish after we centrifuged all the cell mass out," he said. "If there was a toxin in there, it would be killing the fish. End of story."
NO EVIDENCE OF TOXIN
Gawley's team also looked at the DNA of the Pfiesteria and found no genetic sequence that matched that of any known fish poison. "It may be that at least some of the effects attributed to Pfiesteria are due to other algae," they wrote in their report.
Gawley said he did not doubt reports that fishermen and some scientists who work with Pfiesteria have been made sick.
The researchers have reported symptoms such as lethargy, sores, blurred vision, severe headaches, kidney and liver dysfunction, difficulty breathing and disorientation.
Fish that have been exposed to Pfiesteria also acted disoriented and many had big, open sores.
"Once you have got a bunch of dead fish in there ... there is all sorts of stuff in there besides fish and Pfiesteria," Gawley said. "There are bacteria, there are fungi and all these things could be making God-knows-what."
Wolfgang Vogelbein and colleagues at William and Mary and the U.S. Environmental Protection Agency said Pfiesteria probably ate little holes in the mucus layer that protects fish and their skin.
They tested Pfiesteria by putting them in a tank with fish, using a permeable membrane to separate them. The Pfiesteria were unable to get to the fish, but any toxin should have been able to pass through the barrier.
No fish died, they reported in the journal Nature. "Our findings indicate that fish mortality after Pfiesteria shumwayae exposure results from micropredatory feeding," they wrote.