Changes in toxins, intracellular and dissolved free amino acids of the toxic dinoflagellate Gymnodinium catenatum in response to changes in inorganic nutrients and salinity

The paralytic shellfish poison producing dinoflagellate Gymnodinium catenatum was subjected to changes in salinity, phosphate, ammonium and nitrate using continuous culture and batch culture methods. In contrast with other algae, this species showed very slow changes in the concentration of intracellular amino acids, in the Gln:Glu ratio, and, in contrast with Alexandrium spp., only slow changes in toxin content, during such events as N-feeding of N-deprived cells or during nutrient deprivation. ?his organism was found to be very susceptible io disturbance; maximum growth rates, around 0.25-0.3 day(-1) with a minimum C:N mass ratio of 5.5, were attained when cultures were only disturbed by sampling once a day. P-deprived cells were larger (twice the usual C content of 4 ng C cell(-1) and volume of 20 pl). The content of free amino acids was always low (5% of cell-N), with low contributions made by arginine (the precursor for paralytic shellfish toxins). Cells growing using ammonium had the lowest C:N ratios and the highest proportion of intracellular amino acids as arginine. The toxin profile (equal mole ratios of dcSTX, GTX(5), dcGTX(2/3) C-1 and C-2, and half those values for C-3 and C-4) was stable and the toxin concentration varied between 0.2 and 1 mM STX equivalents (highest when ammonium was not limiting,lowest in P-deprived cells, though as the latter were larger toxin per cell was not so variable). Decreased salinity did not result in increases in toxin content. Significant amounts of amino acids (mainly serine and glycine, with a total often exceeding 4 mu M) accumulated in the growth medium during batch growth even though the cultures were not bacteria free.