Optimization of Salinity and Nutrient Level for Indoor Nursing of Sea Lettuce (Ulva Lactuca) in Bangladesh

Abstract

Indoor-based cultivation of sea lettuce (Ulva lactuca) provides a sustainable, year-round source of high-quality seedlings for large-scale, nature-based cultivation to ensure food security, nutritional demands, and income generation for coastal people. This study aims to assess the feasibility of indoor-based seaweed (U. lactuca) cultivation, focusing on optimizing salinity and nutrient concentrations for enhanced growth performance and biochemical composition. The experiment was conducted from October 2023 to November 2024 at the Coastal Biodiversity Marine Fisheries and Wildlife Research Centre, Doriyanagor, Cox's Bazar, Bangladesh. Six different culture media (PES, BF, VS, PES+VS, PES+BF, and VS+BF) were initially used to compare and found VS medium was the most suitable medium for promoting growth rate (p < 0.05). The most effective nutrient concentration for maximum growth rate was determined to be 1.5 mL/L (p < 0.05), selected from the doses of 1.0, 1.5, 2.5, 3.0, and 3.5 mL/L using the VS medium. This optimized media and dose were applied to evaluate the effect of six salinity levels (5, 10, 15, 20, 25, and 30 ppt) under a 12:12 h light-dark cycle. The highest growth performance (DGR: 6.20 ± 0.20% day⁻¹, p < 0.05; SGR: 6.01 ± 0.19% day⁻¹, p < 0.05) of U. lactuca was recorded in February at 30 ppt salinity. Biochemical analysis showed the highest protein (18.38 ± 0.22%, p < 0.05), fiber (23.92 ± 0.13%, p < 0.05), and fat (0.68 ± 0.02%, p < 0.05) contents in sea lettuce at 15 ppt, while the highest ash content (6.27 ± 0.07%, p < 0.05) was observed at 5.0 ppt. The elevated level of antioxidant capacity (20.74 ± 0.00 mg/100g, p < 0.05) and the maximum flavonoid content (233.96 ± 0.02 mg QE/100g, p < 0.05) were recorded at 5.0 ppt and 30 ppt, respectively. FT-IR analysis confirmed the presence of key bioactive compounds in indoor-cultured U. lactuca, including polysaccharides (C–O–C, peak at 1023 cm⁻¹), amide/carbonyl groups (C=O, peak at 1650 cm⁻¹) at 10, 15, 20, 25 and 30 ppt salinities, and hydroxyl groups (O–H, peak at 3755 cm⁻¹) at 5.0 and 10 ppt salinity. Elemental analysis revealed thirteen essential minerals (Na, Ca, K, Fe, P, Cu, Ni, Zn, Mn, Co, V, Se, Rb) and four trace heavy metals (Cd, Cr, Pb, Hg) In U.lactuca of all treatments with significant differences among them (p < 0.05), except for Ni and Se. The highest mineral content, Ca (4169 ± 37.59 mg/kg, p < 0.05), and the lowest heavy metal concentration, Hg (0.10 ± 0.01 mg/kg, p < 0.05), were both observed at 5.0 ppt. Salinity showed a strong positive correlation with potassium (K) and a strong negative correlation with calcium (Ca). These findings demonstrate the xi potential of indoor seaweed cultivation for year-round seed production, extending the cultivation cycle and supporting seaweed mariculture in Bangladesh.