Microbe may kill fish by skinning, not poisoning
Arguments have taken a strange turn over how to isolate toxins from the
Pfiesteria microbes accused of killing fish by the millions and threatening
human health. Two research teams now say that in the Pfiesteria strain they've
examined, there's no toxin to find. Instead, explains one of the teams, the
single-celled dinoflagellate alga kills by swarming over a fish and eating
away its skin.
A laboratory sample of Pfiesteria shumwayae, one of two named Pfiesteria
species, slays fish only by direct contact, says Robert Gawley of the
University of
Miami in Coral Gables. By centrifuging a solution of P. shumwayae growing in
the
lab, the scientists removed microbes from the mix. The leftover liquid, which
would contain any released poisons, proved harmless, Gawley's team reports in
an upcoming Proceedings of the NationalAcademy of Sciences.
Results from other tests prompted the same conclusion from Wolfgang Vogelbein
of the Virginia Institute of Marine Science in Gloucester Point and his
colleagues. They report that fish stayed healthy when bathing in the same
liquid as P. shumwayae, as long as filters prevented direct contact.
Also, microscope images of fish in con- tact with P. shumwayae showed algal
cells gouging the fish's skin, Vogelbein's team reports in an upcoming
Nature.
Debates over Pfiesteria biology have heated up since 1992 when JoAnn Burk-
holder of North Carolina State University at Raleigh and her colleagues blamed
this genus for filling North Carolina rivers with dead fish with bloody skin
ulcers (SN: 9/6/97, p. 149). In the late 1990s, alarm rose that breathing
spray from Pfiesteria-laden waters, in the lab or outdoors, impairs people's
mental functioning.
Gawley says that when he started his current work, he intended to find the
toxins causing such havoc. Besides testing cen- trifuged samples, his team
looked for genes encoding enzymes that other dinoflagel- lates seem to use to
make toxins, but none turned up in P. shumwayae.
Vogelbein and his colleagues tested the same strain of P. shumwayae that
Gawley did. "The only time we saw fish dying was when they were in contact
with the Pfies- teria," says Vogelbein's collaborator Jeffrey Shields of the
Virginia institute.
Burkholder criticizes both papers for not using a sufficiently toxic strain of
Pfiesteria. The ones in her lab knock out the fish much faster, within a few
hours instead of a day or two. She says that when her team exposed shellfish
larvae to Pfiesteria con tained within a dialysis sac, the larvae died rapidly
even though they had no direct con- tact with the algae. Burkholder adds that
the chemists with whom she works have found a toxin.
The two new papers join other recent work to suggest that fears of Pfiesteria
have been overblown, says WayneUtaker of the National Oceanic and Atmospheric
Adminis- tration in Beaufort, N.C. He's seenPfiesteria feed- ing directly on
fish in laboratory tanks, but he cautions that wild, free swimming fish may
avoid lethal densities of these algae. Some 40 other men- aces-from low
oxygen concentrations to a :e of the alga water fungus-can give >ble a fish to
death. fish bleeding skin ulcers, Utaker says. In June, his lab reported a
7-stage lifecycle for Pfiesteria, instead of the unusually complex sequence of
more than 20 stages that Burkholder has described.
Litaker says that the microbe once seen as an extraordinary poisoner may turn
out to be just "a normal, everyday dinoflagellate."-S. MILIUS
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