Competition for phosphorus between two dinoflagellates: A toxic Alexandrium minutum and a non-toxic Heterocapsa triquetra

Abstract

The understanding of the dominance of one species with respect to others is a pertinent challenge in HAB growth dynamics studies and the nutrient supply mode is one of the factors potentially involved. The competition for phosphorus (P) between a toxic species, Alexandrium minutum, and a non-toxic species, Heterocapsa triquetra, was studied (1) along a gradient of P depletion, (2) testing different P depletion degrees before a single PO 4 supply and (3) experimenting different PO 4 supply frequencies. In conditions of PO 4 depletion, H. triquetra stopped growing after two days both in monospecific and mixed batch cultures whereas A. minutum grew progressively from day 2 until the end of the experiment. This time-lag growth of A. minutum is associated to its ability to store P intracellularly and then mobilize it for cell division when P depletion becomes severe. Heterocapsa triquetra outcompeted A. minutum when it was submitted to less than three days of P depletion before the pulse. In contrast, A. minutum outcompeted H. triquetra after more than three days of depletion. This transition was related to the capacity for A. minutum to increase its cell PO 4 uptake rate in a higher proportion to face potential PO 4 supply. As a result of this physiological acclimatation to P starvation, A. minutum consumed the whole PO 4 pulse supplied after 3 to 10 days of P depletion. This resulted in a reduction of H. triquetra growth. These two acclimatations were confirmed in a P limited semi-continuous culture experiment testing several PO 4 supply frequencies (1, 2, 4, 6 day intervals). These experiments revealed that A. minutum is a “storage specialist” species for P, which uptakes PO 4 pulses for luxury consumption, survives depletion periods and, then, utilizes P for cell growth. In contrast, H. triquetra is more a “velocity adapted” species, which utilizes PO 4 just after supply to increase their cell division rate.