Microalgae production is one of the major limiting factors for bivalve aquaculture due to its high production costs, risks of contamination, and nutritional variability between algal species. Alternative diets to live microalgae have been tested in bivalves, but no satisfactory products have been developed that complete their nutritional requirements. The baker's yeast (Saccharomyces cerevisiae) represents a potential candidate, however, its poor digestibility, partly associated to the high mannan content in the cell wall, limits its use in aquaculture. This study evaluated the use of the S. cerevisiae mnn9 mutant (Δmnn9), whose mutation results in a truncation of the mannan structure, to substitute microalgal diets for feeding Pacific oyster (Crassostrea gigas) spat. First, the optimal feeding ration of a bi-algal diet (Chaetoceros muelleri and Tisochrysis lutea, 50:50 based on dry weight (DW)) was determined (trial 1). Then, the replacement of the microalgal diet with Δmnn9 was evaluated at five substitution levels (0, 25, 50, 75, and 100%, based on DW; trial 2). Finally, the nutritional value of Δmnn9 was compared with the wild-type S. cerevisiae by replacing 50% (DW) of the algal diet with either Δmnn9 or the wild type (trial 3). Gene expressions of elongation of very-long-chain fatty acids (Cg-ELOVL) and β-glucan-binding protein (Cg-βGBP) were measured in trial 2 and trial 3, respectively. Trial 1: the lowest daily feeding ration in attaining the highest significant specific growth rate (SGR) was 2% (SGR: 7.6% day−1). Trial 2: Δmnn9 could substitute 50% (DW) of the microalgal diet without significantly affecting the general performance of the spat. Up-regulation of Cg-ELOVL mRNA was observed in oysters fed the 75% substitution diet compared with those fed 100% algae. Trial 3: significant growth improvement was observed in oysters fed Δmnn9 compared with those fed wild type. Higher gene expression of Cg-βGBP was observed with the Δmnn9-containing diet than with the wild-type diet, suggesting immunostimulatory properties of Δmnn9 cells. These results indicate that Δmnn9 cells are efficiently ingested and digested by C. gigas spat, and represents a suitable alternative to a microalgal diet in this species. Fatty acid elongases seem to participate in the biosynthetic pathways of polyunsaturated fatty acids (PUFAs) in C. gigas and their activity could increase as a response to PUFA deficiencies in the diet. |
