Identification of free amino acids (FAA) that are important as major intracellular osmolytes in the estuarine Hong Kong oyster, Crassostrea hongkongensis

Abstract

Crassostrea hongkongensis (Hong Kong oyster), a commercially valuable bivalve thriving in the estuaries along the northern coast of the South China Sea, is threatened by the increase of salinity during the dry season. It is essential to improve its hyper-salinity tolerance, which requires a clear understanding of the mechanism to regulate its osmotic balance. For this purpose, 25 free amino acids (FAA), the most critical intracellular osmolyte in bivalves, were quantified in gill, adductor muscle, and hemolymph of C. hongkongensis during abrupt salinity changes (18–6, 12, 24 and 30‰) by high-performance liquid chromatography-tandem mass spectrometry. Taurine, alanine and glycine were dominant FAA, contributing to total FAA contents that varied with salinities, no matter in gill, muscle or hemolymph. Under hypo-osmotic stress, total FAA contents decreased in gill and adductor muscle but increased in hemolymph, probably resulting from the extrusion of some FAA from tissues to prevent swelling of cells. And taurine contributed most significantly to the hypo-salinity response, which was mainly reflected by the remarkable increase in hemolymph at 8 h after the exposure to 6‰. By contrast, total FAA contents increased in gill, muscle, and hemolymph when transferred to hyper-salinity, suggesting the assumption that it increases the osmolality and subsequently prevents the cell shrinking. Alanine and glycine contributed most significantly to hyper-salinity response. Of the two, the former was more sensitive to hyper-osmotic stress for its rapid increase in gill within 8 h after the exposure to 24 and 30‰. An inverse pattern with most FAA was presented by ornithine, the intermediate of the urea cycle, which was probably associated with the FAA metabolism. This study identified the major FAA as intracellular osmolytes in C. hongkongensis and would benefit the further understanding of the regulatory mechanisms of oyster salinity acclimation.