Dinophysis norvegica (Dinophyceae), more a predator than a producer?

Several studies have proved that some Dinophysis species are capable of ingesting particulate organic matter besides of being photosynthetic, a form of nutrition termed mixotrophy. Phagotrophy may be an important aspect of the life history of the genus Dinophysis and the key to understand its ecology. We used modern techniques coupling flow cytometry and acidotropic probes to detect and score food vacuolated Dinophysis norvegica cells in natural samples. In addition, feeding experiments were conduced under controlled conditions to observe if D. norvegica would grow feeding on the cryptophyte Teleaulax amphioxeia. The results of the field observations showed a frequency of phagotrophy between 25 and 71% in a natural D. norvegica population from the Baltic Sea, which is higher than previous reports (1–20%). Although molecular methods have proved that the kleptoplastids of the D. norvegica from the Baltic Sea are from T. amphioxeia, the laboratory experiments showed that the presence of T. amphioxeia in the cultures did not enhance the survival rate of D. norvegica neither in phototrophic nor in heterotrophic conditions. We suggest that the D. norvegica Kleptoplats are obtained through a heterotrophic or mixotrophic protist, which have been feeding on cryptophytes, as it has recently been shown for Dinophysis acuminata. Our main conclusion is that D. norvegica, and probably all other species from the genus Dinophysis, is mainly phagotrophic and feeds on a larger prey than T. amphioxeia. Autotrophy through kleptoplastidy would be a secondary feature used as a complementary or short-term survival strategy.     

Growth responses of the mixotrophic dinoflagellates, Cryptoperidiniopsis sp. and Pfiesteria piscicida, to light under prey-saturated conditions

Recent research emphasis on the ecology of Pfiesteria spp. (Dinophyceae) has led to recognition of several morphologically similar heterotrophic dinoflagellates that often co-occur with Pfiesteria spp. in estuaries along the United States Atlantic coast. These include cryptoperidiniopsoid dinoflagellates, which resemble Pfiesteria spp. in having complex life cycles that include zoospores capable of kleptoplastidy. To examine and compare the role of kleptoplastidy in Cryptoperidiniopsis sp. and Pfiesteria piscicida, we tested the effects of irradiance on growth under prey-saturated (Storeatula major, Cryptophyceae) conditions. Growth of Cryptoperidiniopsis was strongly influenced by light intensity while no major effects were observed in P. piscicida. In Cryptoperidiniopsis, highest cell numbers and specific growth rates, but lowest specific cryptophyte consumption rates, were found at the highest light intensity tested (100 μmol photons m⁻² s⁻¹). A growth model was developed and used to estimate that the average half-life of chloroplasts ingested by Cryptoperidiniopsis decreased 3.4-fold from 12.6 h at high light to 3.7 h in the dark. These results show that light strongly enhances specific growth rate and growth efficiency of Cryptoperidiniopsis feeding on cryptophytes, and suggest that retained kleptochloroplasts may play a quantitatively significant role in carbon and energy metabolism of this organism. Differences in the effects of light between Cryptoperidiniopsis and P. piscicida may reflect different nutritional strategies, and allow these closely related dinoflagellates to occupy different niches and co-exist.