Inter-Research
Aquatic Microbial Ecology

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 mixotrophic, i.e. capable 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 ¹⁴C 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 µm) and incubated at in situ temperatureunder artificial light conditions with high concentrations of inorganic ¹⁴C (1 µCi ml^-1). Individual cells of each desired species were manually isolated under a microscope and transferred to scintillation vials. C. tripos showed net ¹⁴C uptake onlyduring light periods, whereas both C. lineatum and C. furca showed ¹⁴C uptake in the light as well as uptake (and sometimes losses) in the dark. Dinophysis species had similar carbon fixation rates in light compared to Ceratium species. For D. acuminata and D. norvegica, net carbon uptake 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 was significantly 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 ¹⁴C 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^-1 (µ); themaximum growth rate of D. acuta was lower (0.41 d^-1). The positive dark carbonuptake by Dinophysis may suggest a mixotrophic mode of nutrition. In one experiment, both D. norvegica 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 dissolved and particulate organic matter would provide conclusive evidence of mixotrophy and this will require the development of new protocols for measuring organic matter uptake applicable to Dinophysis in the natural assemblages.

KEY WORDS: Dinophysis · Growth rate · Mixotrophy · Carbon uptake · Ceratium · Protoperidinium

Published in AME Vol. 13, No. 2 (1997) on August 21
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 1997