Effect of cell wall characteristics on algae nutrient digestibility in Nile tilapia (Oreochromis niloticus) and African catfish (Clarus gariepinus)

This study aimed to assess the effect of cell wall hardness and fish species on digestibility of unicellular sources. The gross composition, and the composition and cell wall hardness of the sources were determined for four sources. These were 3 microalgae species (Chlorella vulgaris, Scenedesmus dimorphus and Nannochloropsis gaditana) and a cyanobacterium (Arthrospira maxima). Apparent digestibility coefficients (ADCs) of their nutrients were determined in Nile tilapia and African catfish, at a 30% diet inclusion level. It was hypothesized that herbivores can access and thus digest unicellular proteins better than omnivores, and that the differences in protein digestion between the fish species increase with the robustness of the cell walls. Differences in cell wall hardness were quantified as the cells' resistance to mechanical shear. A. maxima was least resistant to shear: the time needed to disrupt 50% of the cells was 2 min compared to 24-33 min for the other sources. Differences were also measured in nutrient digestibility between the sources in both fish species. Contrary to the basal diet, which was digested differently between the fish species, there was no fish species effect on nutrient ADCs of the unicellular sources. A. maxima had the highest protein ADCs in both fish species (81.4-82.5%), followed by C. vulgaris (80.7-80.9%), N. gaditana (72.4-74.7%) and S. dimorphus (67.0-68.3%). Ingredient fat ADCs ranged between 65.1 and 89.1%. Unicellular non-starch polysaccharides (NSP), comprising the unicellular cell wall fraction, was not inert in both fish species (ADC > 46.0%), which was attributed to fermentation. The digestibility data suggest that the differences in nutrient accessibility of unicellular sources are dominant over the differences in digestive systems between herbivorous and omnivorous fish. Nevertheless, nutrient digestibility of the unicellular sources did not relate to the mechanical cell wall hardness.