Microalgae production is one of the major limiting factors for bivalve aquaculture due to high production costs, risks of contamination and nutritional variability of algae species. Alternative diets to live microalgae have been tested in bivalves, but no satisfactory products have been developed that fulfill completely their nutritional requirements. The presence of polyunsaturated fatty acids (PUFAs), amongst others, is proven to be essential for the optimal development of marine bivalves.
This study evaluated the use of the baker’s yeast Saccharomyces cerevisiae mutant, Δmnn9, to substitute microalgae diets in Pacific oyster (Crassostrea gigas) spat . The Δmnn9 yeast is defective in the synthesis of the α -1,6-mannose arm of N-glycans, resulting in a cell wall that is characterized by low levels of mannan and elevated levels of chitin and β-glucan. Mutant yeasts exhibiti ng these characteristics have been shown to be digested more efficiently by marine species than the wild type . We replaced a microalgal diet ( Chaetoceros muelleri:Tisochrysis lutea ; 50/50 based on dry weight (DW) ) with Δmnn9 at five substitution levels (0, 25, 50, 75 and 100%, based on DW ; trial 1). Secondly, we compared the nutritional value of Δmnn9 with the wild-type S. cerevisiae for oyster spat by replacing 50% of the mixed algal diet with either Δ mnn9 or the wild type ( trial 2). It was observed that Δmnn9 could substitute 50% of the microalgae diet without significantly affecting the general performance of the spat (figure 1). Also, growth rate was significantly higher in oysters fed partly on Δ mnn9 than those fed partially on the wild type baker’s yeast, emphasizing again the role of the cell wall for yeast digestion. Gene expressions of fatty acid elongase (enzyme that participates in the elongation of long-chain fatty acids) and β-glucan-binding protein (involves in innate immune response) were also measured in the trials , and are discussed.
These results highlight that Δmnn9 cells are efficiently ingested and digested by C. gigas spat, representing a suitable alternative for microalgae in bivalve aquaculture . A higher gene expression of fatty acid elongase, which participates in the biosynthetic pathways of PUFAs, was observed in the diet with a substitution level of 75%, possibly as a response to the deficiency of PUFA.