Pfiesteria spp. are mixotrophic armored dinoflagellates populating the Atlantic coastal waters of the United States. They have been a focus of intense research due to their reported association with several fish mortality events. We have now used a clonal culture of Pfiesteria piscicida and several new environmental isolates to describe growth characteristics, feeding, and factors contributing to the encystment and germination of the organism in both laboratory and environmental samples. We also discuss applied methods of detection of the different morphological forms of Pfiesteria in environmental samples. In summary, Pfiesteria, when grown with its algal prey, Rhodomonas sp., presents a typical growth curve with lag, exponential, and stationary phases, followed by encystment. The doubling time in exponential phase is about 12 h. The profiles of proliferation under a standard light cycle and in the dark were similar, although the peak cell densities were markedly lower when cells were grown in the dark. The addition of urea, chicken manure, and soil extracts did not enhance Pfiesteria proliferation, but crude unfiltered spent aquarium water did. Under conditions of food deprivation or cold (4°C), Pfiesteria readily formed harvestable cysts that were further analyzed by PCR and scanning electron microscopy. The germination of Pfiesteria cysts in environmental sediment was enhanced by the presence of live fish: dinospores could be detected 13 to 15 days earlier and reached 5-to 10-times-higher peak cell densities with live fish than with artificial seawater or f/2 medium alone. The addition of ammonia, urea, nitrate, phosphate, or surprisingly, spent fish aquarium water had no effect.Pfiesteria piscicida is a mixotrophic dinoflagellate that has been the subject of intense research due to its presumed association with massive fish kills and a novel toxic exposure syndrome consisting of characteristic skin lesions, respiratory problems, and short-term memory loss (9,32,36,40,53). In the Chesapeake Bay region, the increased frequency of fish lesions, mass mortalities in several estuarine and marine fish species, and human health problems reported during the summer of 1997 led to intense media coverage and closures of public waterways to both commercial and recreational use (http://www.dnr.state.md.us/Bay/cblife/algae/dino/pfiesteria /hilltest.html). Because the fish kills and detrimental effects on human health observed in the Chesapeake Bay estuary closely resembled those reported in North Carolina and attributed to P. piscicida blooms (17, 64), it appeared that P. piscicida might be responsible for events in the bay (47).Although the development of molecular approaches for the environmental detection of P. piscicida and related species (11,21,22,50,58,61,62,68) has contributed significantly to the accurate assessment of their geographic distributions, the limited information on the environmental factors that may trigger P. piscicida blooms has made them difficult to anticipate. Earlier studies for the development ...
