Ichthyotoxic Active Compounds from Red Tides and Mechanism of Ichthyotoxicity

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Saturated and unsaturated fatty acids present in the red tides alga Chattonella antiqua showed an ichthyotoxic activity when assayd against killifish. Among the unsaturated fatty acids, stearidonic acid (C_<18 : 4>) had the strongest activity (LC_<100> = 7 ppm). Other C_<18->unsaturated fatty acids assayd were less active, although their ichthyotoxic activities increased with increase of the unsaturation degree. However, the activity of these unsaturated fatty acids disappeared by addition of anti-oxidizing reagents (alpha-tocopherol and sodium ascorbate) at pH 8.1. These facts indicate that the ichthyotoxicity is due to damage caused to the biomembrane by radical species generated form the autoxidation of the unsaturated fatty acids.Among the saturated fatty acids, myristic acid (C_<14 : 0>) and palmitic acid (C_<16 : 0>) had the strongest activity (LC_<100> = 10 ppm) at pH 8.1. The activities of myristic acid and palmitic acid decreased at pH 4.5 due to a diminished solubility in water. However, the activities of decanoic acid (C_<10 : 0>) and lauric acid (C_<12 : 0>) were stronger at pH 4.5 rather than at pH 8.1. At pH 4.5, decanoic acid and lauric acid are ionized to an extent of less than 30%. These facts indicate that the ichthyotoxicity ascribes to the hydrophobic interaction with the biomembrane by the non-ionic form of the saturated fatty acid.