Light and dark carbon uptake by Dinophysis species in comparison to other photosynthetic and heterotrophic dinoflagellates

Abstract

The marine dinoflagellate genus Dinophysis includes species that are the causative agents of diarrhetic shellfish poisoning (DSP). Recent findings indicate that some Dinophysis species are rnixotrophlc, i.e. capable of both autotrophic and heterotrophic nutrition. We investigated inorganic (and organic) carbon uptake by several species of Dinophysis in the hght and dark using the 'single-cell I4C method', and compared uptake rates with those of photosynthetic Ceratium species and heterotrophic dinoflagellates in the genus Protoperidinium. Experiments were conducted with water from the Gullmar Fjord and from the Koster Strait (Swedish west coast). Nutrient-enriched phytoplankton from surface water samples were concentrated (20 to 70 pm) and incubated at i n situ temperature under artificial light conditions with high concentrations of inorganic I4C (1 pCi ml-l). Individual cells of each desired species were manually isolated under a microscope and transferred to scintillation vlals. C, tripos showed net 14C uptake only during light periods, whereas both C. lineatum and C. furca showed I4C uptake in the light as well as uptake (and sometunes losses) in the dark. Dinophysis species had similar carbon fixation rates in hght compared to Ceratium species. For D. acuminata and D. norvegica, net carbon uptake occurred in both light and dark penods. D. acuta showed a loss of carbon in the dark in one experiment, but in another, dark C uptake was sigruficantly higher than uptake in light. When exposed to light, C. furca, D. norvegica and D, acuta had high specific carbon uptake rates. Growth rates for the different species were calculated from I4C uptake by the cells during the first hours of incubation in light. D. acuminata and D. norvegica had similar maximum growth rates, 0.59 and 0.63 d-' (p); the maxlmum growth rate of D acuta was lower (0.41 d-l). The positive dark carbon uptake by Dinophysls may suggest a rnixotrophic mode of nutrition. In one expenment, both D. norveyica and D, acuta showed a significantly higher carbon uptake in a dark bottle than in a light bottle, which would be consistent with uptake of 14C-labeled organic matter by D. norvegica and D. acuta. Demonstration of direct uptake of dssolved and particulate organic matter would provide conclusive evidence of rnixotrophy and this will require the development of new protocols for measunng organic matter uptake applicable to Dinophysis in the natural assemblages.