CHANGES IN DINOFLAGELLATE INTRACELLULAR AMINO-ACIDS IN RESPONSE TO DIURNAL CHANGES IN LIGHT AND N-SUPPLY

Seven species of dinoflagellates were examined with respect to their intracellular amino acid (InAA) content under different conditions of N-nutrition and light, and for release of dissolved free amino acids (DFAA). Alexandrium spp. did not contain significant amounts of a neutral nonprotein amine present in all other phototrophic dinoflagellates tested but, in contrast, it always maintained high levels of glutamine and of the ratio of intracellular glutamine/glutamate (Gln/Glu). Dinoflagellates responded to a decreased availability of N with a fall in Gln/Glu but, unlike other microalgae, arginine was always a significant component of InAA. Refeeding of N-limited cultures with nitrate during the dark phase of growth, simulating refeeding of a natural population following a diurnal migration to the nutricline, resulted in an increased Gln/Glu during the dark phase followed by a decline during the light phase. N-refeeding of N-deprived cells led a rapid rise in the ratio, except during N-refeeding of stationary phase Gymnodinium catenatum (which were of abnormal shape and motility) when Glu rather than Gln increased. The sensitivity of the analytical method would enable sampling from natural dinoflagellate populations in order to assess their physiological status; extracts from 200 to 2000 cells, depending on cell size, are sufficient. A significant quantity of an unidentified amino acid accumulated in the growth media of Gymnodinium catenatum but otherwise DFAA in dinoflagellate culture media were similar to that from other microalgae.