MEPS 299:137-148 (2005)  -  doi:10.3354/meps299137

ABSTRACT: Prymnesium parvum (Haptophyceae) was grown in 3 different nutrient treatments in semi-continuous cultures, where the N:P molar ratios were modified to 80:1, 4:1 and 16:1. The nutrient concentrations in the media affected the cellular nutrient content of P. parvum. The phosphorus content showed larger flexibility in the cells and determined more the cellular nutrient ratios than nitrogen or carbon. Toxicity was measured as haemolytic activity 14 times during the culturing. All the cultures were toxic during the exponential growth, but toxicity increased in the nutrient limited cultures during semi-continuous dilution. The allelopathic effect of P. parvum was examined with low cell densities (final abundance 2 and 5 × 10³ cells ml^–1) on cryptomonad Rhodomonas salina. With lower cell density, cell structures of R. salina were damaged in one-third of the cells and the cell density decreased slightly. With higher P. parvum cell density, less than half of the R. salina cells remained in the nutrient limited cultures after 23 h. P. parvum grown in nutrient balanced cultures negatively affected R. salina cells. The cellular N:P ratio, which was scaled to the Redfield ratio, could explain 67 and 75% of the variation in the haemolytic activity and the allelopathic effect, respectively. Our results confirm the hypothesis that nutrient deficiency increases toxicity of P. parvum. The haemolytic activity varied during semi-continuous culturing, but it was significantly dependent on intracellular N:P ratios. Due to the allelopathic effect, formation of a P. parvum bloom may accelerate after a critical cell density is reached when the competing species are eliminated.

KEY WORDS: Prymnesium parvum · Toxicity · Haemolytic activity · Allelopathy · C:N:P stoichiometry · Harmful algal bloom · Phytoplankton · Nutrients