Apparent digestibility of nutrients, energy, essential amino acids and fatty acids of juvenile Atlantic salmon (Salmo salar L.) diets containing whole-cell or cell-ruptured Chlorella vulgaris meals at five dietary inclusion levels

Abstract

Chlorella vulgaris, one of the most studied microalga for industrial applications, has never before been assessed as a potential ‘low-trophic’ ingredient for Atlantic salmon (Salmo salar L.). The effects on apparent nutrient digestibility coefficients (ADCs) by dietary inclusion of whole-cell or cell-ruptured C. vulgaris meals at five levels were determined. Integrity of nutrients, energy, essential amino acids (EAAs) and fatty acids were well-preserved after cell-rupture processing. Based on microscopy and protein solubility, two Microfluidizer® passes were sufficient for complete cell-rupture as no improvement in solubility (P=0.998) was achieved with a third pass. Whole-cell C. vulgaris meal reduced ADCs for dry matter, protein, lipid and energy at inclusion as low as 6–12% (P≤0.035), whereas carbohydrate ADC was not affected up to 24% (P≥0.980) and was significantly improved at 30% (P=0.028). Similarly, starch ADC was not affected by inclusion of whole-cell C. vulgaris meal at any level (P=0.256). Inclusion of cell-ruptured C. vulgaris meal did not affect ADCs for dry matter when included up to 30% (P≥0.900), protein up to 24% (P≥0.092) or lipid up to 18% (P≥0.124). Energy ADC was not affected up to 12% (P≥0.530) but reduced at higher levels (P≤0.009). Inclusion of cell-ruptured C. vulgaris meal at all levels improved carbohydrate ADC (P≤0.002), due to increased starch digestibility. In fact, starch ADCs of diets containing 18–30% was significantly higher than that of the algae-free control diet (P≤0.009). Similarly, phosphorous ADC was higher in diets containing 18–30% C. vulgaris meals than the algae-free control diet. Dietary ADCs were not affected by moderate inclusion (up to 18%) of whole-cell C. vulgaris meal for most EAAs (P≥0.116), while ADCs for leucine and phenylalanine were reduced when over 12%. Dietary ADCs were not affected by high inclusion (24–30%) of cell-ruptured C. vulgaris meal for any EAA (P≥0.076) and tryptophan and lysine were confirmed as the most limiting EAAs in C. vulgaris meals for Atlantic salmon. Reduced energy digestibility of diets containing C. vulgaris meals was associated with significantly lower ADCs for palmitic, oleic, linoleic and linolenic acids with whole-cell C. vulgaris meal at >6–12% (P≤0.022) and palmitic and oleic acids with cell-ruptured C. vulgaris meal at >12–18%. Nevertheless, inclusion of either C. vulgaris meal up to 30% did not affect dietary ADCs for palmitoleic acid, eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) (P≥0.126). Single ingredient ADCs of nutrients, energy, EAAs and major fatty acids are reported for the first time for juvenile Atlantic salmon fed diets containing whole-cell and cell-ruptured C. vulgaris meals.