A Novel <i>Coelastrella tenuitheca</i> Isolate Enhances Growth, Immunity, and Gene Expression in Whiteleg Shrimp

In this study, the freshwater microalga <i>Coelastrella terrestris</i> NIOF17/005 was utilized as a feed additive for the Pacific whiteleg shrimp (<i>Litopenaeus vannamei</i>). <i>Coelastrella terrestris</i> was characterized morphologically, phylogenetically (using 18s rRNA), and biochemically. Shrimp diets were supplemented with <i>C. terrestris</i> at 0, 1, 2.5, and 5 g/kg. After a 56-day feeding trial, the biochemical analysis of <i>C. terrestris</i> revealed a composition of 38.12% protein, 37.82% lipids, and 15.75% carbohydrates, with valuable bioactive compounds including polyunsaturated fatty acids and antioxidant substrates. The supplementation of 2.5 g/kg <i>C. terrestris</i> significantly improved the growth performance of <i>L. vannamei</i> (10.23 g final weight), survival rate (92.67%), and feed conversion ratio (1.71). The whole-body composition analysis of <i>L. vannamei</i> indicated enhanced protein content (59.74%) and lipid content (7.92%) in the algal-supplemented groups in comparison to the control. Digestive enzyme activities of amylase and lipase increased significantly, with peak activities observed at 2.5 g/kg supplementation (49.55 and 60.06 IU/L, respectively). Nonspecific immunity parameters, including lysozyme (4.47 µg/mL), superoxide dismutase (SOD; 10.77 IU/g), and catalase (CAT; 10.47 IU/g) activities, were substantially elevated in shrimp-fed <i>C. terrestris</i>-supplemented diets, with optimal levels at 2.5 g/kg. The gene expression analysis showed that both studied growth-related genes and immunity-related genes were upregulated. These genes reached their maximum expression at a supplementation level of 2.5 g/kg, with the expression levels being approximately 2-3 times higher in the supplemented group in comparison to the control group. In conclusion, these findings suggested that the inclusion of <i>C. terrestris</i> NIOF17/005 (2.5 g/kg) into shrimp feed formulations could enhance productivity, improve shrimp health, and potentially increase the sustainability of shrimp farming operations.