Growth inhibition of several marine diatom species induced by the shading effect and allelopathic activity of marennine, a blue-green polyphenolic pigment of the diatom Haslea ostrearia (Gaillon/Bory) Simonsen

Haslea ostrearia, a tychopelagic marine diatom distributed world-wide, characteristically settles in oyster-ponds. This diatom synthesizes and releases into the external medium a blue-green polyphenolic pigment called "marennine" (Mn), which is responsible for oyster greening. During the bloom of H. ostrearia in ponds, mainly occurring in autumn along the French Atlantic coast (Marennes-Oléron), a decrease in the abundance of other microalgae, including several diatom species, is observed. In this article, we investigate in vitro the influence of the intra- and extracellular forms of the pigment (IMn and EMn) on the development of diatoms usually coexisting with H. ostrearia in oyster-ponds: Skeletonema costatum, Nitzschia closterium, Haslea crucigera and Entomoneis pseudoduplex. We show that there is no significant difference between IMn and EMn in their ability to reduce, in a dose-dependent manner, the growth of these benthic diatoms. S. costatum, N. closterium and H. crucigera are significantly more sensitive to the inhibitory effect of marennine than E. pseudoduplex. In comparison, H. ostrearia tolerates much higher concentrations of its pigment and can even be stimulated at low marennine concentration. Using an experimental set-up to study independently the inhibitory effect of marennine due to light attenuation, we demonstrate that all the species tested are also inhibited in a dose-dependent manner by a shading effect, without a significant difference between IMn and EMn. Taken together, these results suggest that S. costatum, N. closterium and H. crucigera are inhibited by a cumulative effect of chemical mediation and light attenuation induced by marennine. E. pseudoduplex is inhibited solely by the shading effect. Finally, H. ostrearia is inhibited by the shading effect and stimulated by chemical mediation. The in vitro experimental data presented herein are discussed in relation to an ecophysiological function of marennine, leading us to assume that the form of the pigment released into the external medium could act as an allelochemical at the level of the pond as a whole. Allelopathy may therefore play an important role in interspecific competition and contribute to H. ostrearia bloom maintenance.