2018 - Marc Long

LEMAR, IUEM, Plouzané // School of chemistry, University of Wollongong (Australie)

Sujet : « Allelochemical interactions between the dinoflagellate Alexandrium minutum and the diatom Chaetoceros muelleri »

Directrices de thèse : H. Hégaret, D. Jolley. Encadrants scientifiques : G. Sarthou, P. Soudant.

Soutenue en Novembre 2018

Mots clés : Alexandrium ; Allélopathie ; Cytotoxicité ; Toxine ; Physiologie ; Caractérisation ; Stress métallique

Understanding the factors that favor the establishment and the persistance of harmful algal blooms (HABs) is essential for predicting them, and subsequently mitigating their negative consequences on the marine environment, human health and the economy. Allelochemical potency, i.e. the release of compounds that inhibit competitors, is hypothesized to favor the organisms that produce them and indeed shapes plankton community. The toxic dinoflagellates from the genus Alexandrium produce allelochemicals, however, associated interactions are poorly understood. The objective of this thesis is to contribute to our understanding by studying the mechanisms behind the allelochemical interactions between the dinoflagellate A. minutum and the diatom Chaetoceros muelleri. The results of this work have highlighted that A. minutum allelochemicals disrupted membrane functioning of the diatoms within minutes. The cascade of physiological events following the allelochemical interaction that was described in this PhD included permeabilisation of cell membranes, inhibition of photosynthesis, production of reactive oxygen species and modifications of the biochemical composition of membranes. Additionally, this project has highlighted that the allelochemical potency of A. minutum can be significantly modulated by environmental parameters, as allelochemical potency of A. minutum increased 4-5 times when exposed to toxic concentrations of Cu. Finally, a bioassay developed during this PhD eased the partial isolation of the allelochemicals through guided-fractionation. Although we did not totally purify the allelochemical(s) of A. minutum, we isolated a group of hydrophobic candidates whose activity has to be further investigated following their isolation. These results provide a better understanding of the physiological and cellular mechanisms underlying these allelochemical interactions. This PhD clearly highlighted the complexity of allelochemical interactions, but also offers new tools and raises new perspectives to study the ecological consequences of these interactions on marine ecosystems.

Télécharger Marc LONG_CV_Phycotox_2015.pdf
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