Influence of inorganic nutrition on growth and PSP toxin production of Alexandrium minutum (Dinophyceae) from Cork Harbour, Ireland

The physiological response of the PSP toxin producing dinoflagellate Alexandrium minutum isolated from the Irish coast was assessed after modulating the initial concentrations of nitrate and phosphate in batch cultures. The cell growth in cultures of strain CK.A02 was primarily controlled by nitrate availability. In all experiments, only gonyautoxins 2 and 3 (GTX2 and 3) were synthesized along the different growth phases, with GTX3 dominating (≈80%) at all stages, making the GTX2-3 toxin profile a possible population marker of A. minutum in Cork Harbour. The cellular toxin quotas remained low and relatively stable at around 2 pg cell^-1, except when high N:P ratios were initially used for culture inoculations; in these conditions PSP toxins accumulated up to 14 pg cell^-1. Due to the composition of the toxin profile, the toxicity of strain CK.A02 was generally relatively low (from 1.1 to 1.7 pg STX eq cell^-1) in comparison with strains from other geographic areas except when phosphate limiting culture conditions were applied (maximum of 12.5 pg STX eq cell^-1). Results showed that sufficient soluble protein quotas were necessary to observe the intra-cellular accumulation of PSP toxins in phosphate limiting conditions, highlighting also the requirement of adequate nitrogen supplies. The possible existence of localized toxicity hot spots in the field, linked to the accumulation of PSP toxins within A. minutum cells as a metabolic response to adverse environmental conditions, could potentially increase risks for shellfish farming operations.