Formation of two types of cysts by a mixotrophic dinoflagellate, Pfiesteria piscicida

Despite the widespread occurrence of mixotrophic dinoflagellates, most research on cyst formation in dinoflagellates has focused on phototrophic organisms or on factors affecting phototrophic growth (i.e. light intensity and nutrient supply). Presumably, factors that stimulate cyst formation in mixotrophic organisms would be combinations of those factors that affect phototrophic and phagotrophic growth (such as limiting light intensity and limiting prey concentrations). The toxic dinoflagellate Pfiesteria piscicida is an interesting test case because it has a complex life history and is considered mixotrophic. Recently, a form of P. piscicida has been described that does not have the ability to produce toxins (termed 'non-inducible'). The objectives of this study were to identify which life stages are likely to be present in a mixotrophic culture of 'non-inducible' F piscicida and to determine the morphological differences based on fluorescent stain uptake. Also, we examined which combinations of adverse environmental factors (low light intensity and low prey concentrations) affect life stage transformations. In this culture, we observed 3 distinct life stages (a zoospore stage, and 2 cysts). One of the cysts (termed Cyst A) has a thick cell wall and appears to form from actively feeding zoospores regardless of light intensity. The second type of cyst (Cyst B) has a much thinner cell wall and only forms from recently fed zoospores maintained in complete darkness. During the experiment, encystment rates to either cyst were low, suggesting that encystment will not dramatically affect bloom dynamics on small timescales.