Intraspecific differences in the effects of salinity on growth and prymnesin production in Prymnesium parvum

Prymnesium parvum is a toxin-producing haptophyte known for forming harmful blooms in brackish and inland waters. While salinity has been proposed as a key factor influencing bloom dynamics, the relationship between salinity and toxin production remains poorly understood. We investigated the effect of salinity on prymnesin production across three strains of P. parvum. Our results show that<br/>while the species can grow across a broad salinity range, prymnesin production is significantly reduced at low salinities—likely due to resource allocation toward coping with hypoosmotic stress. In two strains, elevated toxin quotas under salinity stress were attributed to intracellular accumulation caused by reduced cell division rates, rather than increased synthesis. We also observed substantial<br/>intraspecific variation in physiological responses to salinity, yet identified consistent negative correlations between growth rate and cellular prymnesin content. These findings suggest that prymnesin production incurs physiological costs and highlight the importance of strain-specific responses in understanding bloom toxicity and environmental adaptability.