LIGHT AND DARK CARBON UPTAKE BY DINOPHYSIS SPECIES IN COMPARISON TO OTHER PHOTOSYNTHETIC AND HETEROTROPHIC DINOFLAGELLATES

The marine dinoflagellate genus Dinophysis includes species that are t he causative agents of diarrhetic shellfish poisoning (DSP). Recent fi ndings indicate that some Dinophysis species are mixotrophic, i.e. cap able of both autotrophic and heterotrophic nutrition. We investigated inorganic (and organic) carbon uptake by several species of Dinophysis in the Light and dark using the 'single-cell C-14 method', and compar ed uptake rates with those of photosynthetic Ceratium species and hete rotrophic dinoflagellates in the genus Protoperidinium. Experiments we re conducted with water from the Gullmar Fjord and from the Koster Str ait (Swedish west coast). Nutrient-enriched phytoplankton from surface water samples were concentrated (20 to 70 mu m) and incubated at in s itu temperature under artificial light conditions with high concentrat ions of inorganic C-14 (1 mu Ci ml(-1)). Individual cells of each desi red species were manually isolated under a microscope and transferred to scintillation vials. C. tripes showed net C-14 uptake only during l ight periods, whereas both C. lineatum and C. furca showed C-14 uptake in the Light as well as uptake (and sometimes losses) in the dark. Di nophysis species had similar carbon fixation rates in Light compared t o Ceratium species. For D. acuminata and D. norvegica, net carbon upta ke occurred in both Light and dark periods. D. acuta showed a loss of carbon in the dark in one experiment, but in another, dark C uptake wa s significantly higher than uptake in Light. When exposed to Light, C. furca, D. norvegica and D. acuta had high specific carbon uptake rate s. Growth rates for the different species were calculated from C-14 up take by the cells during the first hours of incubation in light. D. ac uminata and D. norvegica had similar maximum growth rates, 0.59 and 0. 63 d(-1) (mu); the maximum growth rate of D. acuta was lower (0.41 d(- 1)). The positive dark carbon uptake by Dinophysis may suggest a mixot rophic mode of nutrition. In one experiment, both D. norvegica and D. acuta showed a significantly higher carbon uptake in a dark bottle tha n in a Light bottle, which would be consistent with uptake of C-14-lab eled organic matter by D. norvegica and D. acuta. Demonstration of dir ect uptake of dissolved and particulate organic matter would provide c onclusive evidence of mixotrophy and this will require the development of new protocols for measuring organic matter uptake applicable to Di nophysis in the natural assemblages.