Effect of water-soluble polysaccharide extract from the green alga Ulva rigida on growth performance, antioxidant enzyme activity, and immune stimulation of grey mullet Mugil cephalus

Abstract

This study was carried out to investigate and compare the effect of water-soluble polysaccharides extract of the green alga, Ulva rigida (WPU), as a dietary supplement on growth performance, antioxidant enzyme activity, non-specific immune responses, and disease resistance of juvenile grey mullet, Mugil cephalus, subjected to bacterial infection with Photobacterium damselae. Three replicate groups of fish (initial mean weight, 15 ± 0.1 g) were fed with four isonitrogenous and isolipidic diets containing four levels, 0 or control, 5, 10, and 15 mg kg−1 of WPU (WPU0 or control, WPU5, WPU10, and WPU15) for 8 weeks over the growth trial. Thereafter, ten fish from each dietary treatment were infected with bacteria P. damselae to evaluate disease resistance of infected fish. Feeding grey mullet at 10 mg kg−1 WPU level (WPU10 diets) produced higher weight gain (211.36 ± 6%), specific growth rate (1.88 ± 0.03%) and protein efficiency ratio (4.26 ± 0.1%) compare to the control group. The diets supplemented with three levels of WPU-stimulated lysozyme, phagocytic, and respiratory burst activity of fish, but, the stimulatory capacities of the extracts varied greatly, depending on the concentrations used and diet; WPU10 was higher than WPU 5 and WPU15 diets. The best responses in antioxidant enzyme activities of superoxide dismutase (4.8 U mL−1), glutathione (3.6 U mL−1), and malondialdehyde (25.0 U mL−1) were induced by the extracts obtained from WPU10 diet. In addition, using WPU extract in all diets decreased mortality in M. cephalus in a dose-dependent manner after challenge with P. damselae. These results suggest that the use of water-soluble polysaccharides from U. rigida at 10 mg kg−1 dose improve growth, immune responses, and antioxidant activity in M. cephalus.