Effect of two types of microbial biomass added at low dietary levels on growth, assimilation and hepatic protection in Nile tilapia (Oreochromis niloticus)

  Bacteria, cyanobacteria, and other single cell proteins have been considered as reliable ingredients when used to manufacture diets for aquatic organisms. Several nutritional advantages indicate a positive nutritional performance of the microbial biomass.  The present study aimed to evaluate the dietary effect of a bacterial biomass and a cyanobacteria (Arthrospira platensis) on growth, nutrient assimilation and hepatic protection in juvenile Nile tilapia (Oreochromis niloticus). Five experimental diets (isoproteic and isoenergetic) were formulated with additive levels of (3 and 6%) of each microbial ingredient. 320 fish were distributed in quadruplicate tanks and fed for 28 days. At the end of the feeding assay, there were no significant differences in weight gain and feed conversion ratios among treatments. In order to estimate the proportion assimilation of nutrients supplied by the experimental ingredients, the isotopic values of nitrogen were analyzed in ingredients and fish muscle tissue. The isotopic values of diets were transferred to muscle tissue fast, and an isotopic equilibrium was achieved. Results indicated that diets containing 3% of microbial biomass supplied structural nitrogen to muscle biosynthesis. With the objective of estimating the degree of hepatic protection promoted by the ingredients, a diesel exposition was conducted. Results indicated that there was a significant increase in the enzymatic activity of carboxylesterase in all dietary treatments. Glutathione S-tranferase activity increased in fish that were previously fed Arthrospira biomass. Overall results indicate positive physiological responses elicited by the dietary addition of micriobial biomass.