Effects of light on photosynthesis, grazing, and population dynamics of the heterotrophic dinoflagellate Pfiesteria piscicida (Dinophyceae)

In studying how environmental factors control the population dynamics of Pfiesteria piscicida Steidinger et Burkholder, we examined the influence of light regime on kleptoplastidic photosynthesis, growth, and grazing. Prey (Rhodomonas sp.)-saturated growth rate of P. piscicida increased (0.67 +/- 0.03 d(-1) to 0.91 +/- 0.11 d(-1)) with light intensity varying from 0 to 200 mumol photons-m(-2).s(-1). No significant effect was observed on grazing, excluding the possibility that light enhanced P. piscicida growth through stimulating grazing. Lightgrown P. piscicida exhibited a higher gross growth efficiency (0.78 +/- 0.10) than P. piscicida incubated in the dark (0.32 +/- 0.16), and photosynthetic inhibitors significantly decreased growth of recently fed populations. These results demonstrate a role of kleptoplastidic photosynthesis in enhancing growth in P. piscicida. However, when the prey alga X sp. was depleted, light's stimulating effect on P. piscicida growth diminished quickly, coinciding with rapid disappearance of Rhodomanas-derived pigments and RUBISCO from P. piscicida cells. Furthermore, the effect of light on growth was reversed after extended starvation, and starved light-grown P. piscicida declined at a rate significantly greater than dark-incubated cultures. The observed difference in rates of decline appeared to be attributable to light-dependent cannibalism. Using a 5-chloromethylfluorescein diacetate staining technique, cannibalistic grazing was observed after 7 days of starvation, at a rate four times greater under illumination than in the dark. The results from this study suggest that kleptoplastidy enhances growth of P. Piscicida only in the presence of algal prey. When prey is absent, P. piscicida populations may become vulnerable to light-stimulated cannibalism.