The present study aimed to identify the carotenoid content and evaluate the antioxidant activities of spirulina, Arthrospira platensis (in vitro), and the effect of its supplementation to a vegetarian diet on growth performance, feed utilization, body proximate composition and physiological status of Nile tilapia, Oreochromis niloticus (in vivo). The carotenoid content of spirulina was identified by UPLC-MS/MS and showed that trans and cis β-carotene were the major carotenoids (88.3%) followed by β-cryptoxanthin and zeaxanthin. The antioxidant activity of spirulina was determined in water and crude carotenoid extracts by 2,2-diphenyl-1-picryhydrazyl (DPPH) scavenging assay, both extracts showed a significant free radical scavenging capacity. In addition, Nile tilapia fry (0.83 ± 0.01 g) was fed a vegetarian diet supplemented with different levels of spirulina for 12 weeks. The results revealed that the highest growth performance and nutrient utilization were recorded with 0.5% spirulina and the peak response determined at 0.63%-0.65% using polynomial second order regression. Also, the hemoglobin content improved in a quadratic regression model with the peak at 0.67% spirulina. Plasma total protein and lipid contents increased significantly with spirulina levels over 0.50%. Moreover, the aminopeptidase activities and glucose level decreased significantly with increasing spirulina levels. The current study recommended the supplementation of the Nile tilapia vegetarian diet with spirulina at levels of 0.63-0.65% for better growth performance and physiological status.
