Recycling of spent seawater produced by land-based seaweed cultivation for the production of value-added products from a marine microalga Chlorella sp.

Edible seaweeds have gained significant interest among the nutraceutical industries due to their rich content of amino acids, fatty acids, and minerals. The tank cultivation of edible seaweed like Ulva sp. generates a significant amount of spent seawater, which must either be reused or treated for safe discharge into coastal waters. This study aims to utilize spent seawater from Ulva sp. grown tanks for the production of marine microalgal biomass, focusing on the extraction of high-value lutein and protein. Accordingly, three major pre-treatment methods—autoclaving, filtration, and chlorination—were performed on spent seawater, both with and without medium supplementation, followed by the cultivation of marine Chlorella sp. 1151 as a model microalga. Among the treatment methods, spent seawater either sterilized by autoclave or chlorination with medium addition resulted in higher biomass (0.91–1.03 g L⁻¹), lutein (1.10–1.88 mg g⁻¹), and protein (64.86 – 68.83 mg g⁻¹) yields, which were almost comparable to those obtained with fresh seawater supplemented with medium. Chlorella sp. 1151 efficiently utilized nitrate and phosphate by 95–97% in the spent seawater for the optimal experimental combinations as stated above. Further, analysis of heavy metals including Co, Cu, Fe, Mn, Zn, Ba, Ni, Mo, Pb, Cr, and Cd in the cultivated spent seawater were well below the permissible limits for safe discharge. This study demonstrates the novel approach of repurposing spent seawater from seaweed cultivation for the production of marine microalgal biomass- based lutein and protein.