Intraspecific differences in the effects of salinity and light on growth and prymnesin cell quota in Prymnesium parvum

The haptophyte Prymnesium parvum produces large polyether compounds (prymnesins) with lytic properties that accumulate in the cell or may be released into the surrounding environment. At high cell abundances the prymnesins cause devastating damage to ecosystems. Blooms of P. parvum are frequently reported in inland water bodies and brackish coastal areas, and thus salinity has been suggested as a main regulatory factor in bloom control. However, toxicity has mainly been assessed through bioassay and the effect of abiotic factors on prymnesin production is lacking. Here, we explore the effect of salinity and light on prymnesin production in three strains of P. parvum. We find that P. parvum can grow over a wide range of salinities, but that net production of cellular prymnesin is, contrary to previous suggestions, inhibited at low salinities, presumably due to resources being diverted to deal with hypoosmotic stress. In two of the strains, increased cellular prymnesin quotas during salinity-induced stress were instead due to accumulation of toxins in the cells because of low cell division rates. We further observed intraspecific differences in cell physiology in response to light. We found general negative relationships between growth rate and cellular prymnesin quotas, both within and among strains. Overall, we find large intraspecific variation in cell physiology in response to salinity and light but demonstrate several general patterns. Such broad intraspecific variation may partly explain the success of various strains of P. parvum in a wide range of environments.