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Haslea ostrearia is a marine diatom known to produce marennine, a water-soluble blue-green pigment responsible for the greening of oysters in ponds along the French Atlantic coast. This phenomenon occurs seasonally when H. ostrearia blooms in oyster ponds, and it increases the economic value of cultured oysters. From an ecological perspective, H. ostrearia blooms are accompanied by a decrease in the abundance of other microalgae, suggesting that this diatom produces allelochemicals. Recent studies showed that purified marennine has other biological activities, for instance antioxidant, antibacterial, and antiviral activities, which could be used in aquaculture to promote this pigment as a natural antipathogen agent. One important issue regarding the possible use of H. ostrearia in aquaculture as a mixed algal diet, however, is the importance of marennine allelopathy. In this study, we investigated the allelopathic effect of H. ostrearia on the growth of five microalgal species relevant to aquaculture: Chaetoceros calcitrans, Skeletonema costatum, Phaeodactylum tricornutum, Tetraselmis suecica, and Tisochrysis lutea. Allelopathic tests were realized by co-culturing these microalgae with H. ostrearia in batch and in semi-continuous mode, based on initial biovolume ratios. Our findings showed that inhibition of the growth of microalgae due to the presence of H. ostrearia and marennine was species dependent. Skeletonema costatum, C. calcitrans, and T. lutea were significantly more sensitive, whereas T. suecica and P. tricornutum appeared to be more resistant. Growth irradiance significantly influenced the allelopathic effect against the sensitive species S. costatum, and the H. ostrearia production of marennine increases with irradiance. Data presented in this study partly support the hypothesis that marennine released into the culture medium possibly acts as an allelochemical compound, thus explaining the dominance of H. ostrearia and the loss of sensitive algae in oyster ponds, but also that some species are insensitive, which allows co-culturing and use in a mixed algal diet in aquaculture.

The suitability of two red algae species, the introduced Grateloupia turuturu and the native Palmaria palmata, as feed for the culture of the European abalone Haliotis tuberculata, was compared over a 5-month period. Three experimental diets were tested: (1) P. palmata, (2) G. turuturu, and (3) a mixed diet of P. palmata and G. turuturu (1:1). Biochemical composition (proteins, carbohydrates, lipids, ashes) of algae was measured. No mortality was observed during the time of the experiment in any of the treatments. Growth in length and weight was higher for abalone fed with P. palmata and the mixed diet. Abalone exhibited a preference for P. palmata and showed the highest weight gain with this macroalgae (107.8 ± 7.2 %). Grateloupia turuturu disintegrated faster in abalone rearing conditions and was not suitable for significant growth of H. tuberculata in a monospecific diet. However, when it is provided in a mixed diet, abalone seem to show a higher lipid content than when fed a P. palmata monospecific diet. The invasive G. turuturu could be an interesting food supplement for European abalone.