Bioassay-guided discovery of ichthyotoxic algal compounds using in vivo fish assays is labor intensive,costly, and highly regulated. Since the mode of action of most known algal-mediated fish-killing toxinsis damage to the cell membranes in the gills, various types of cell-based bioassays are often used forbioassay guided purification of new ichthyotoxins. Here we tested the hypothesis that allelopathy isrelated to ichthyotoxicity and thus that a microalgal bioassay can be used as a proxy for ichthyotoxicityby comparing the toxicity of five strains of Prymnesium parvum toward rainbow trout (Oncorhynchusmykiss, 10 g) and the microalga Teleaulax acuta. No relationship between median effective concentrations(EC50s) on fish and median lethal concentrations (LC50s) on algae was observed in the 5 strains showingthat a microalgal bioassay cannot be used as a proxy for ichthyotoxicity. Fish were more sensitive to P.parvum with EC50s ranging from 6 × 103to 40 × 103cells ml−1, compared to the test alga where LC50sranged from 30 × 103to nearly non-toxic at 500 × 103cells ml−1. In addition, the cellular concentrationsof two recently suggested ichthyotoxins produced by P. parvum, the “golden algae toxins”, GAT 512 anda novel GAT 510, did not show any relationship to either ichthyotoxicity or allelopathy, and are not thebiologically relevant toxins, but are simply lipids found in algal chloroplasts. Finally, we demonstratethat the recently suggested ichthyotoxin, oleamide, could not be detected in any of the five P. parvum strains above the limit of detection, nor was it found in a13C-labeled strain. Instead we document thatoleamide can easily be extracted from plastic materials, which may have been the source of oleamidereported previously.
- Prymnesium parvum
- Golden algae toxinsa