Salinity stress enhances protein content and amino acid profile in Gracilaria cornea (Rhodophyta)

Marine macroalga are frequently exposed to environmental stresses impairing their overall physiology and growth potential. Among these, Gracilaria cornea (Rhodophyta) is a valuable red seaweed rich in protein and polysaccharides. To investigate its physiological responses under controlled conditions, we cultivated Gracilaria cornea in an indoor culture system at three different salinity levels (30, 40 and 50 ppt), employing continuous aeration, blue and white LED illumination (12:12 light: dark cycle), and exogenous addition of nitrogen and phosphorus. Physiological changes associated with protein content accumulation and amino acid composition were determined using in-situ reflectance spectroscopy (VIS-NIR range 560-674 nm), AI algorithm and GC-MS analysis. We developed novel tools to accurately predict amino acid composition and total protein yield, identified the environmental factors inducing trait accumulation and determined the optimal harvesting day. Hypersaline stress and cultivation day significantly influenced protein content with optimal protein content (> 35% dry weight) achieved on day 14. This peak was not correlated with the specific growth rate (SGR), indicating SGR may not reliably indicate protein yield in this context. The dry weight to fresh weight ratio (DW: FW) was higher under hypersaline conditions, leading to a greater dried biomass and higher protein content, despite a reduced overall growth rate. Protein content was maximal under high ambient pH and high salinity. Day 14 was optimal for the highest yield of essential amino acids (EAA), exceeding 40% of the total amino acids. The algorithmic model accurately predicted specific amino acid proportions.