Mixotrophy and nitrogen uptake by Pfiesteria piscicida (Dinophyceae)

The nutritional versatility of dinoflagellates is a complicating factor in identifying potential links between nutrient enrichment and the proliferati on of harmful algal blooms. For example, although dinoflagellates associate d with harmful algal blooms (e.g. red tides) are generally considered to be phototrophic and use inorganic nutrients such as nitrate or phosphate, man y of these species also have pronounced heterotrophic capabilities either a s osmotrophs or phagotrophs, Recently, the widespread occurrence of the het erotrophic toxic dinoflagellate, Pfiesteria piscicida Steidinger et Burkhol der, has been documented in turbid estuarine waters. Pfiesteria piscicida h as a relatively proficient grazing ability, but also has an ability to func tion as a phototroph by acquiring chloroplasts from algal prey, a process t ermed kleptoplastidy. We tested the ability of kleptoplastidic P. piscicida to take up N-15-labeled NH4+, NO3-, urea, or glutamate, The photosynthetic activity of these cultures was verified, in part, by use of the fluorochro me, primulin, which indicated a positive relationship between photosyntheti c starch production and growth irradiance, All four N substrates were taken up by P, piscicida, and the highest uptake rates were in the range cited f or phytoplankton and were similar to N uptake estimates for phagotrophic P. piscicida, The demonstration of direct nutrient acquisition by kleptoplast idic P. piscicida suggests that the response of the dinoflagellate to nutri ent enrichment is complex, and that the specific pathway of nutrient stimul ation (e.g. indirect stimulation through enhancement of phytoplankton prey abundance vs. direct stimulation by saprotrophic nutrient uptake) may depen d on P. piscicida's nutritional state (phagotrophy vs. phototrophy).