LAUSR

For decades the microphytobenthos assemblage in the coastal Mediterranean Sea has been regularly colonized by the toxic benthic dinoflagellate Ostreopsis cf. ovata. This harmful algal species is a toxin producer and occupies the same ecological niche as various diatoms. Surprisingly, there are only few insights reported on the physiological responses of diatoms to blooms of O. cf. ovata The chemical interactions of O. cf. ovata with the co-occurring diatom Licmophora paradoxa was studied using a bioassay (measuring impact of cell-free culture filtrate) and a co-culture approach (separate by a membrane) to investigate the effects of the exometabolome and its mode of action. Bioassays highlighted a toxic effect of the exometabolome of O. cf. ovata on the diatom photosynthetic activity. However, the co-cultures revealed that these toxic effects do not occur through remote allelopathy. Contact or close interactions between cells of the two species is most likely needed to impair the diatom growth. Ovatoxins are suspected to be the toxic metabolites secreted by O. cf. ovata although the current set of data did not give confirmation of this assumption. Interestingly, the exometabolome of L. paradoxa impaired the growth and the photochemistry of O. cf. ovata in both bioassays and co-cultures. Some biomarkers possibly involved for the effect were identified using a metabolomic approach and may correspond to oxylipins, however a bacterial source of the bioactive metabolites is also considered.