Investigation on the effects of nitrate and salinity stress on the antioxidant properties of green algae with special reference to the use of processed biomass as potent fish feed ingredient

Antioxidant properties of four common green algal genera viz. Cladophora glomerata, Chaetomorpha aerea, Rhizoclonium crassipellitum, and Pithophora cleveana collected from Sunderban areas of Eastern India were tested in controlled and nutrient stress conditions. Experimental algae were exposed to growth media with variations in nitrate and NaCl salt concentrations from 0 to 1500 ppm level for a period of 21 days. A dose-dependent upregulation in total phenols, flavonoids, pigment, and antioxidant property in relation to % DPPH activity and % ferrous ion chelating activity was observed. Maximum phenol content was observed for algal biomass exposed to 1500 ppm nitrate stress conditions (52.55 ± 0.01 mg GAE/g DW-16.48 ± 0.041 mg GAE/g DW) with Rhizoclonium showing the maximum contents. In the case of total flavonoid content, maximum upregulation was noted in the hypersaline conditions and Pithophora showed the highest contents amongst all (71.8 ± 0.21 mg QE/g DW). The highest carotenoid contents were observed in 750 ppm stress condition. Optimally stressed algal biomass having maximum carotenoid levels and high antioxidant levels were selected as fish feed ingredients, based on dose- and time-dependent two-way Anova and Tukeys test (750 ppm nitrate, 7 days). The processed biomass was included as fish feed ingredient to study the nutritive effects on goldfish (Carassius auratus). Algal feed (AF) was prepared from algal biomass only, which was further mixed with the commercial feed (CF) in varying proportions to formulate three different types of value-added feed (VAF). The experimental feed (25% VAF, 50% VAF, 75%VAF and AF) were tested against commercial feed (control) for 30 days in artificially aerated static aquaria system. The feed type formulated as 50% VAF was found to be most effective in terms of skin pigmentation, growth, and antioxidant activities of experimental fishes showing almost (1.44–4-folds increase) compared to the control setup. Statistically significant results were obtained regarding nutritive properties of experimental feed and fishes using PCA and Tukeys test respectively. Body weight gain and specific growth rate also changed significantly in experimental fishes.