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Applied and Environmental Microbiology, January 2005, p. 519-529, Vol. 71, No. 1
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.1.519-529.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Characterization of Ichthyocidal Activity of Pfiesteria piscicida: Dependence on the Dinospore Cell Density

Tomás Drgon ,1,{dagger},{ddagger} Keiko Saito,1,{dagger} Patrick M. Gillevet,2 Masoumeh Sikaroodi,2 Brent Whitaker,3 Danara N. Krupatkina,1 Federico Argemi,1,§ and Gerardo R. Vasta1*

Center of Marine Biotechnology, University of Maryland Biotechnology Institute,1 Department of Animal Health, The National Aquarium in Baltimore, Baltimore, Maryland,3 Environmental Biocomplexity, Department of Environmental Sciences and Policy, George Mason University, Manassas, Virginia2

Received 9 June 2004/ Accepted 23 August 2004

The ichthyocidal activity of Pfiesteria piscicida dinospores was examined in an aquarium bioassay format by exposing fish to either Pfiesteria-containing environmental sediments or clonal P. piscicida. The presence of Pfiesteria spp. and the complexity of the microbial assemblage in the bioassay were assessed by molecular approaches. Cell-free water from bioassays that yielded significant fish mortality failed to show ichthyocidal activity. Histopathological examination of moribund and dead fish failed to reveal the skin lesions reported elsewhere. Fish larvae within "cages" of variable mesh sizes were killed in those where the pore size exceeded that of Pfiesteria dinospores. In vitro exposure of fish larvae to clonal P. piscicida indicated that fish mortality was directly proportional to the dinospore cell density. Dinospores clustered around the mouth, eyes, and operculi, suggesting that fish health may be affected by their direct interaction with skin, gill epithelia, or mucous surfaces. Molecular fingerprinting revealed the presence of a very diverse microbial community of bacteria, protists, and fungi within bioassay aquaria containing environmental sediments. Some components of the microbial community were identified as potential fish pathogens, preventing the rigorous identification of Pfiesteria spp. as the only cause of fish death. In summary, our results strongly suggest (i) that this aquarium bioassay format, which has been extensively reported in the literature, is unsuitable to accurately assess the ichthyocidal activity of Pfiesteria spp. and (ii) that the ichthyocidal activity of Pfiesteria spp. is mostly due to direct interactions of the zoospores with fish skin and gill epithelia rather than to soluble factors.

* Corresponding author. Mailing address: Center of Marine Biotechnology, University of Maryland Biotechnology Institute, 701 East Pratt St., Baltimore, MD 21202. Phone: (410) 234-8826. Fax: (410) 234-8896. E-mail: vasta{at}umbi.umd.edu.

{dagger} T.D. and K.S. contributed equally to this study.

{ddagger} Present address: National Institute on Drug Abuse, Baltimore, MD 21224.

§ Present address: Instituto de Limnología, Universidad Nacional de La Plata, 1888 Buenos Aires, Argentina.

Applied and Environmental Microbiology, January 2005, p. 519-529, Vol. 71, No. 1
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.1.519-529.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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