During the 1990s, massive fish kills plagued bays and estuaries along the East Coast of the U.S. People living near and working on these waters also complained of memory loss, headaches and other physical ailments. Scientists blamed these frightening phenomena on a microorganism namedPfiesteria piscicida (right), often referred to as the "cell from hell" in media coverage of the disturbing events. Studies indicated that the diminutive creature, which belongs to a group of free-living marine organisms known as dinoflagellates, had as many as 24 life-cycle stages, some extremely toxic. Now, however, researchers writing in the June issue of the Journal of Phycology assert that Pfiesteria¿s life cycle is much simpler than originally thought¿and that the organism is actually nontoxic.

Following up on earlier, unsuccessful attempts to confirm a number of the life-cycle stages originally reported for Pfiesteria, Wayne Litaker of the University of North Carolina at Chapel Hill and his colleagues pursued a rigorous study of each of the creature¿s transformations. Using new genetic techniques, the team compared a DNA sequence unique to Pfiesteria with sequences obtained from its putative developmental forms. They discovered that many of the previously described 24 phases¿notably the unusual amoeboid and cyst forms¿were actually entirely different organisms found in the same seawater. Furthermore, none of the true phases observed in their cultures were toxic. "We describe a very typical dinoflagellate life cycle," the researchers write. In explanation, they note that the amoebae detected in earlier studies may have been contaminants. As for what, if not Pfiesteria, killed the fish, Litaker says that a lack of oxygen brought about by bacterial overload and stratified water conditions during certain times of the year could be contributing factors.

But the Pfiesteria case isn't closed yet. In a commentary accompanying the report, D. Wayne Coats of the Smithsonian Environmental Research Center in Edgewater, Md., points out that other samples of the creature isolated under different circumstances might produce the mysterious amoeboid stages. "Resolution of such issues will require time, patience, and hard work," he writes. "In the meantime, [Litaker and his collaborators] have given very sound reasoning for approaching P. piscicida as a dinoflagellate with a relatively simple life cycle."

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