de Boer MK †, Tyl MR, Gieskes WWC, Vrieling EG, van Rijssel M
† Department
of Marine Biology, Centre for Ecological and Evolutionary Studies, University
of Groningen, Biological Centre, P.O. Box, 14 9750 AA Haren, the Netherlands
Massive fish kills caused by raphidophytes have been reported to occur in European coastal waters since 1998. One frequently observed raphidophyte in Dutch coastal waters is Fibrocapsa japonica, which most probably has been introduced recently. F. japonica from the Dutch Wadden Sea was reported to be ichtyotoxic, which was attributed to neurotoxins. In this study the toxicity of F. japonica strains isolated worldwide has been examined, with a Wadden Sea strain tested for a broad variety of environmental conditions. Only minor (non-ichtyotoxic) amounts of relevant neurotoxins could be demonstrated in samples with living F. japonica cells. Allelopathic effects were observed using a bioassay with fluorescent bacteria. Furthermore, F. japonica extracts exhibited strong haemolytic activity. Based on these bioassays, three polyunsaturated fatty acids (C18:4n3; C20:5n3; C20:4n6) were characterised as the active toxic constituents. F. japonica is known to form large sticky aggregates by extrusion of mucus threads from its characteristic mucocysts. This mechanical defence mechanism could also be triggered by specific environmental conditions. The harmful effects studied here mainly act at a microenvironmental level and probably serve as a competition strategy. The exact mechanism of F. japonica ichtyotoxicity and the role of the different bioactive components is still the subject of scientific debate.
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