Blooms of the marine microalga Fibrocapsa japonica (Raphidophyceae) are suggested to lead to the diminution of fish stocks, but the exact mechanism(s) involved in ichthyotoxicity is still unclear. In the present study fish tests were performed, using larvae of the common flat fish sole (Solea solea), to further investigate properties and variability of F. japonica toxicity. Sole larvae were directly and indirectly exposed to a density range of a Dutch Wadden Sea F. japonica strain, as well as F. japonica cell extracts, for a maximum of 7 days, during which larval mortality was monitored. Most experiments were done at 16 degrees C, however some experiments were performed at 20 degrees C. Furthermore, larvae were exposed to concentration ranges of the alleged toxic compounds, e.g. the brevetoxins PbTx-2 and PbTx-3, and three poly-unsaturated fatty acids (PUFAs): M, EPA and OTA.
The larvae showed highly variable responses to the F. japonica treatments. At the late exponential growth phase of F. japonica highest sole mortality was found: 100% mortality after 7 days of direct exposure. Indirect and cell extract exposures as well as direct exposure to other growth phases caused lower larval mortality (up to 40%) after the same incubation period. Temperature had a distinct effect on sole mortality, showing significantly enhanced toxicity at 20 degrees C, compared with 16 degrees C. Exposure to PbTx-2 showed an LC50 of 28.7 ng mL(-1) after 48 h, while exposure to PbTx-3 caused no 100% mortality up to 10 ng mL(-1). Exposure to pure PUFAs showed the fastest mortality observed in all experiments, with an LC50 of
Our research suggests pronounced ichthyotoxicity in late exponential F. japonica cells, related primarily with the combination of endo- and exotoxins, with most probably haemolytic PUFAs as the main endotoxins, and other haemolysins and/or ROS as the main exotoxins. Implications are given for present and future risks in the already warming Dutch coastal waters. (C) 2012 Elsevier B.V. All rights reserved.