Quantification of Pfiesteria piscicida growth and encystment parameters using a numerical model

In the past decade, there has been growing interest in understanding the physiological ecology and life cycle of toxic forms of Pfiesteria piscicida. However, transformations among non-inducible (NON-IND; formerly described as nontoxic) stages have received less attention despite the fact that NON-IND stages are found in nature and may be ecologically important as prey and predators. NON-IND stages are also mixotrophic and have the ability to retain and utilize prey chloroplasts in a process termed 'kleptoplastidic mixotrophy'. Quantifying growth, grazing and encystment rates from P. piscicida laboratory experiments is confounded by the interrelationship between mixotrophy and life stage transformations. By fitting a numerical model to a laboratory experiment on NON-IND P. piscicida, we were able to isolate the potential mechanisms that cause encystment and speculate on the interrelationship between adverse conditions (i.e. low light and limiting prey) and life stage transformations. The structure of the laboratory experiment allowed for the estimation of several growth and encystment parameters including grazing rates, gross growth and assimilation efficiencies, as well as the retention time of chloroplasts. Model results suggest a link between encystment and mixotrophic ability. Furthermore, the model results suggest that encystment rates and gross growth and assimilation efficiencies calculated from the model are lower than expected.