Pfiesteria is a genus of heterotrophic dinoflagellates that has been associated with harmful algal blooms and fish kills. Pfiesteria complex organisms (PCOs) were claimed to be responsible for large fish kills in the 1980s and 1990s on the coast of North Carolina and in tributaries of the Chesapeake Bay. In reaction to the toxic outbreaks, six states along the US east coast have initiated a monitoring program to allow for rapid response in the case of new outbreaks and to better understand the factors involved in Pfiesteria toxicity and outbreaks. New molecular detection methods have revealed that Pfiesteria has a worldwide distribution.
Discovery and naming
Pfiesteria was discovered in 1988 by North Carolina State University researchers JoAnn Burkholder and Ed Noga. The genus was named after Lois Ann Pfiester (1936–1992), a biologist who did much of the early research on dinoflagellates: "“The new family and genus are named in honor of the late Dr. Lois A. Pfiester, a pioneer in describing and unravelling the sexual life cycles of freshwater dinoflagellates and who unselfishly shared her knowledge, expertise, and enthusiasm with all who asked for assistance.”. An in-depth story of the discovery can be found in And the Waters Turned to Blood (1998) by Rodney Barker.
There are two species described, Pfiesteria piscicida (from Latin Pisces, fish; cida, killer.), which has a complex life cycle  and the species Pfiesteria shumwayae, also with a complex life cycle. The type locality of Pfiesteria piscicida is Pamlico River Estuary, North Carolina, U.S.A.
Early research resulted in the hypothesis that Pfiesteria acts as an "ambush predator" and utilizes a "hit and run" feeding strategy by releasing a toxin that paralyzes the respiratory systems of susceptible fish, such as menhaden, thus causing death by suffocation. It then consumes the tissue sloughed off its dead prey.
- Life cycle: Early research suggested a complex lifecycle of Pfiesteria piscicida, but this has become controversial over the past few years due to conflicting research results. Especially contested is the question of whether toxic amoeboid forms exist.
- Toxicity to fish: The hypothesis of Pfiesteria killing fish via releasing a toxin in the water has been questioned as no toxin could be isolated and no toxicity was observed in some experiments. Toxicity appears to depend on the strains and assays used. The lesions observed on fish presumed killed by Pfiesteria have been attributed to water molds by some researchers. However, it has also been established that Pfiesteria shumwayae kills fish by feeding on their skin through micropredation. In early 2007, a highly unstable toxin produced by the toxic form of Pfiesteria piscicida was identified.
- Human illness: The effects of PCOs on humans have been questioned, leading to the "Pfiesteria hysteria hypothesis." A critical review of this hypothesis in the late 1990s concluded that Pfiesteria-related illness was unlikely to be caused by mass hysteria. Concluding that there was no evidence to support the existence of Pfiesteria-associated human illness, the National Institutes of Health discontinued funding for research into the effects of Pfiesteria toxin on humans shortly after a CDC sponsored Pfiesteria conference in 2000. A subsequent evaluation, however, concluded that PCOs can cause human illness. The controversy about the risk of Pfiesteria exposure to human health is still ongoing.
- A fictional Pfiesteria species dangerous to humans featured in James Powlik's 1999 environmental thriller Sea Change.
- The fictional species Pfiesteria homicida was one of the antagonists in Frank Schätzing's 2004 novel The Swarm, planned as a Hollywood movie starring Uma Thurman for release in 2015.
- The Pfiesteria hysteria controversy is a likely inspiration for Barry Levinson's film, The Bay.
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- CDC National Conference on Pfiesteria: From Biology to Public Health October 18–20, 2000, Atlanta GA
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