Molecules produced and released by the bacterial community associated with the toxic dinoflagellate Ostreopsis cf. ovata stimulate the proliferation of algal cells. Therefore, bacteria living in commensal association with the dinoflagellate applies a chemical control on the microalgal growth but does not influence the phycotoxins’ biosynthesis. The chemicals mediating this interaction are unknown compounds, some of which containing halogenated atoms. Bacteria were also shown to alter the chemical structure of the phycotoxins once they are released in seawater by the algal cells. The phycotoxins' half-lives are very short under bacterial activity (< 10 hours) and to a lesser extent under solar radiation (> 36-62 hours according to the toxin). This reduced stability of waterborne phycotoxins explains why undetectable concentrations in the water column or the sea surface microlayer were measured in the field or in a controlled system mimicking a real bloom. The controlled experiment confirmed the presence of phycotoxins in the aerosols in an enclosed system, indicating that without wind dispersion, Ostreopsis’ phycotoxins can be detected to the sea spray aerosols. However, the undetectable concentrations of the waterborne phycotoxins suggest an unusual mechanism for this transfer: rather than waterborne toxins aerosolization, the mucus or cell debris would rather be the vector for this transfer. A field campaign confirmed the presence of several of the known phycotoxins in the aerosols produced during a natural bloom of O. cf. ovata. However, very low levels, under the detection limit of the instrument (HRMS), were detected most sampling days, raising the question about the relationship between phycotoxins in sea spray aerosols and respiratory distress in Humans. To unravel this question, future work should involve assessment of the inflammatory potential of natural aerosols emitted during a bloom of O. cf. ovata. Bio-mitigation of HABs is often suggested as a sustainable solution for the control of toxic proliferation. In this context, the characterization of an algicidal compound produced by a co-occurring benthic diatom Licmophora paradoxa led to the identification of a fatty acid containing 13 carbon atoms. This simple molecule is able to induce dormant forms (encystment) in the dinoflagellate O. cf. ovata. A positive dose-effect was observed when O. cf. ovata was exposed to the algicide. In addition, the discovery of two new families of toxins produced by O. cf. ovata, that were named liguriatoxins and rivieratoxins, suggests that the chemical diversity in dinoflagellates is far from being thoroughly unraveled.
The results are mainly being disseminated within the scientific community through several peer-reviewed publications as well as through national and international conferences. However, the knowledge acquired on the unstable character of the waterborne phycotoxins and their transfer to the aerosols are of high importance to assess the toxicity of the blooms. This knowledge is currently used within a national group of experts working on the Ostreopsis hazards in the French Basque area. The output of this working group will be a review of the existing knowledge as well as recommendations to local marine stakeholders dealing with blooms of O. cf. ovata. Additionally, the newly described toxins could be added to the phycotoxins monitoring effort performed by national institutes of the whole Mediterranean basin.
