High Salinity Stress Affects Flavor Free Amino Acid Content and GLUL Gene Expression in Crassostrea hongkongensis

Crassostrea hongkongensis (C. hongkongensis) is an important aquacultured bivalve species inhabiting estuarine environments, where salinity fluctuations can influence its amino acid composition and associated taste attributes. In this study, the effect of hyper-salinity stress on free amino acid metabolism and glutamate–ammonia ligase (ChGLUL) expression was investigated. Tissue expression analysis showed that ChGLUL was significantly upregulated in hepatopancreas, gill, mantle, gonad, and adductor muscle following hyper-salinity stress, with the most pronounced increase observed in the gonads (7.08-fold). Hyper-salinity markedly elevated the activities of glutamine synthetase (GS) and glutamate decarboxylase (GAD), whereas ChGLUL silencing suppressed stress-induced enzyme activities. Metabolically, hyper-salinity stress increased total free amino acids (TFAAs) and total flavor free amino acids (FFAAs) in the mixed tissues, characterized by markedly increased sweet (glycine and alanine) and bitter amino acids but decreased umami amino acids (glutamic and aspartic acids). Conversely, ChGLUL silencing led to decreased total FFAAs and sweet amino acids but increased umami amino acids. Furthermore, ChGLUL knockdown significantly reduced the expression of key genes in flavor amino acid metabolism, including GAD1, PHGDH, and PAH. Collectively, these findings demonstrate that hyper-salinity stress affects both the flavor free amino acid content and ChGLUL expression in C. hongkongensis.