| Summary: | Dinoflagellate species of <i>Dinophysis</i> are obligate mixotrophs that require light, nutrients, and prey for sustained growth. Information about their nitrogenous nutrient preferences and their uptake kinetics are scarce. This study aimed to determine the preferred nitrogen sources in cultures of <i>D. acuminata</i> and <i>D. acuta</i> strains from the Galician Rías Baixas (NW Spain) and to compare their uptake kinetics. Well-fed versus starved cultures of <i>D. acuminata</i> and <i>D. acuta</i> were supplied with N<sup>15</sup> labeled inorganic (nitrate, ammonium) and organic (urea) nutrients. Both species showed a preference for ammonium and urea whereas uptake of nitrate was negligible. Uptake rates by well-fed cells of <i>D. acuminata</i> and <i>D. acuta</i> were 200% and 50% higher, respectively, than by starved cells. Uptake of urea by <i>D. acuminata</i> was significantly higher than that of ammonium in both nutritional conditions. In contrast, similar uptake rates of both compounds were observed in <i>D. acuta</i>. The apparent inability of <i>Dinophysis</i> to take up nitrate suggests the existence of incomplete nitrate-reducing and assimilatory pathways, in line with the paucity of nitrate transporter homologs in the <i>D. acuminata</i> reference transcriptome. Results derived from this study will contribute to understand Harmful Algal Blooms succession and differences in the spatio-temporal distribution of the two <i>Dinophysis</i> species when they co-occur in stratified scenarios.
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