Underestimated role of the specialized metabolism in microalgae species dynamics using the toxic benthic dinoflagellate Ostreopsis cf. ovata as a model.

As increasing knowledge has been gained onto the chemical interactions between marine organisms, their role in shaping communities structure is becoming obvious and cannot be further circumvented when investigating species dynamics. Allelochemicals are thought to be of particular importance for marine microalgae when they interact with predators, competitors and pathogens or even for intra-specific interplay. Among microalgae, toxic dinoflagellates are one of the most likely to use chemical interactions due to their rich and complex specialized metabolism. The toxins they produce and/or their derivatives may play various roles, including defense against predators, and competing advantage. Recurrent blooms of the toxic dinoflagellate Ostreopsis cf. ovata have been reported in the Northern Mediterranean Sea and field observations suggest that the growth of other microalgae sharing the same habitat is inhibited during toxic blooms. A series of experiments performed in replete nutrients conditions was carried out, including co-cultures between Ostreopsis and its competitors, and bioassays against these competitors with Ostreopsis’ chemical extracts as well as pure toxins. The results highlighted the species specific character of their chemical interactions and also the variability of these interactions throughout the growth of the microalgae. One diatom, Licmophora paradoxa, was particularly sensitive to the presence of Ostreopsis, and further investigation revealed that the photosynthetic electron transfer chain (but not photosystem II) was a target of the toxins. All together, these results show that the specialized metabolism can greatly influence species dynamics at short-scale and should be taken into account when considering ecosystem processes.