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 Chrysochromulina polylepis suspension. All species (with 1 exception) lost their motility, indicating that C. polylepis pro- duces toxins, allelochemicals which affect other algae. The role of cell concentration, growth phase, and pH in the ability of C. polylepis to immobilize the dinoflagellate Heterocapsa triquetra was stud- ied in batch cultures. Loss of motility of H. triquetra cells could be detected at cell concentrations of C. polylepis above 3 × 10 4 cells ml -1 . Senescent cultures of C. polylepis did not immobilize H. trique- tra cells. The ability of C. polylepis to immobilize H. triquetra cells was dependent on the pH of the growth medium. More non-motile H. triquetra cells were obtained in alkaline growth medium than in neutral or acidic media. Growth interactions between C. polylepis and 15 species (16 isolates) were also studied in mixed batch cultures using a nutrient replete growth medium. The algae selected for these experiments included diatoms, dinoflagellates, silicoflagellates, raphidophytes, euglenophytes, cryptophytes, and prasinophytes. C. polylepis had a harmful effect on all the tested algae, 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 effect 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 effect was observed at a pH which did not affect the growth of these species when 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 cultures.

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