Allelopathy in the prymnesiophyte Chrysochromulina polylepis: effect of cell concentration, growth phase and pH

Seven species (8 isolates) of dinoflagellates were exposed to a dense Chrys ochromulina polylepis suspension. All species (with 1 exception) lost their motility, indicating that C. polylepis produces toxins, allelochemicals wh ich affect other algae. The role of cell concentration, growth phase, and p H in the ability of C. polylepis to immobilize the dinoflagellate Heterocap sa triquetra was studied in batch cultures. Loss of motility of H. triquetr a cells could be detected at cell concentrations of C, polylepis above 3 x 10(4) cells ml(-1). Senescent cultures of C. polylepis did not immobilize H . triquetra cells. The ability of C. polylepis to immobilize H. triquetra c ells was dependent on the pH of the growth medium. More non-motile H. triqu etra cells were obtained in alkaline growth medium than in neutral or acidi c media. Growth interactions between C. polylepis and 15 species (16 isolat es) were also studied in mixed batch cultures using a nutrient replete grow th medium. The algae selected for these experiments included diatoms, dinof lagellates, silicoflagellates, raphidophytes, euglenophytes, cryptophytes, and prasinophytes. C. polylepis had a harmful effect on all the tested alga e, except the dinoflagellate Prorocentrum minimum. The harmful effect of C. polylepis was observed as an initial decrease in growth rate of the tested algae, followed by a decline in their population numbers. The harmful effe ct of C. polylepis on the tested algae could in a few cases be ascribed to the high pH in the culture medium. In most cases, however, the harmful effe ct was observed at a pH which did not affect the growth of these species wh en they were grown in monoculture. This indicates that toxins released by C . polylepis had a harmful effect on most of the tested algae in the mixed c ultures.