Charlotte Falaise PhD defense

Valorization of the biological activities of the diatom Haslea ostrearia


Keywords:  aquaculture; diatom; Haslea; marennine; natural bioactive compound


Abstract: Blooms of the diatom Haslea ostrearia are recorded worldwide in marine environments and occur frequently in oyster ponds in the Western French coast. This diatom produces a blue-green water soluble pigment named marennine that is known to turn oyster gills green, which provides a higher market value to the bivalves. Although H. ostrearia has been identified centuries ago, little is known about the ecological significance of the blue pigment. Marennine displays allelopathic, antioxidant or antibacterial activities in vitro and its use in aquaculture was considered as some prophylactic effects were demonstrated on farmed shellfish.  The aims of this thesis were thus 1) to further identify the effects of this pigment on the growth of bacteria from the genus Vibrio that are frequently involved in bivalve mass mortality and 2) to ensure its safety on marine organisms (e.g. molluscs, crustaceans, echinoderms).

Our results on bacterial growth demonstrated that the dose-response curves did not systematically present a linear pattern (e.g. “U shape”, hormetic responses) and that marennine could either totally inhibit the growth of Vibrio or stimulate it depending on the strain tested. Also, ecologically relevant doses could impair the survival and the development of the marine organisms tested. Such noxious effects seemed to only target early and fragile developmental stages as embryos and larvae, while adults appeared unaffected. Finally, our results question the exploitation of H. ostrearia in aquaculture and highlight the need to set exposure threshold doses to prevent any adverse effects but to benefit from the prophylactic strategy resulting from the use of marennine.


Jury members:

  • Claire Hellio (president)
  • Lucie Beaulieu (reviewer)
  • Laurent Picot (reviewer)
  • Patrick Cormier
  • Juliette Fauchot
  • Marie-Agnes Travers
  • Vincent Leignel (co-supervisor)
  • Jean-Luc Mouget (supervisor)