Growth and biochemical profiling of indoor nursery cultured Gracilaria sp. under different salinity conditions

Abstract

In Bangladesh, seasonal salinity variability and seed limitations constrain open-water cultivation of Gracilaria sp., while indoor land-based nursery systems can be a controlled, year-round solution for sustainable year-round source of quality seedlings to increase large-scale cultivation, which enhances food security, meet nutritional demands, and generates income for coastal communities. This study evaluated the feasibility of indoor nursery culture of Gracilaria sp. by examining salinity-driven effects on growth performance and biochemical composition. Six nutrient media (Provasoli’s Enriched Seawater (PES), Von Stosch (VS), and Bosfoliar® Aktiv (BF), PES+VS, PES+BF, and VS+BF) at six concentrations (1.0–3.5 mL/L) were tested to identify the most effective medium which was 2.5mL/L PES+VS. The optimal medium (PES+VS) and dose (2.5mL/L) were subsequently evaluated under six salinity levels (5, 10, 15, 20, 25 and 30 ppt) using a 12:12 h light–dark photoperiod. Growth performance, biochemical composition, antioxidant activity, anthocyanin content, flavonoid activity, functional groups (FTIR), mineral content, and phytochemical profiles (GC–MS) were analysed. Salinity significantly influenced growth, biochemical traits, mineral accumulation, and phytochemical composition of Gracilaria sp. under indoor nursery conditions (p < 0.001). In the aquarium system, SGR and DGR increased from (3.12 ± 0.09) % day⁻¹ and (3.17 ± 0.1) % day⁻¹ at 5 ppt to maxima of (4.44 ± 0.16) % day⁻¹ and (4.54 ± 0.17) % day⁻¹ at 30 ppt, while biomass more than doubled (3.68 ± 0.22) g L⁻¹ to (7.57 ± 0.65) g L⁻¹. Tank culture showed similar trends, with peak biomass at 30 ppt (65.67 ± 3.06 g tank⁻¹). Antioxidant capacity remained stable at 5–20 ppt (~2.1 mg 100 g⁻¹) but declined sharply at ≥25 ppt, whereas flavonoid content peaked at moderate salinity (212.32 ± 6.01) mg 100 g⁻¹ at 10 ppt. Anthocyanin content was highest under low salinity (61.12 ± 0.22 mg 100 g⁻¹ at 5 ppt) and decreased with increasing salinity. FTIR analysis confirmed the consistent presence of polysaccharides, proteins, phenolic hydroxyls, alkenes, and halogenated compounds across treatments, with stronger hydrogen-bonded OH features at higher salinities. Thirteen essential minerals and four heavy metals were detected, with iron (up to 1006.50 ± 26.16 mg kg⁻¹) and lead (253.10 ± 9.76 mg kg⁻¹) showing significant enrichment at 30 ppt. GC–MS profiling identified 62 phytochemicals, with nitrogenous metabolites (40.32%) dominating and higher salinity favouring sulphur-containing, heterocyclic nitrogen, and terpenoid compounds. Overall, optimized indoor nursing of Gracilaria sp. offers a viable year-round seed supply strategy to extend farming cycles and promote sustainable seaweed mari-culture in Bangladesh.