Effects of hyposalinity on ion content, organic osmolytes, and lipid peroxidation in the seagrass Halodule wrightii

Critical seagrass ecosystems are predicted to be impacted by droughts and floods of increasing severity and frequency as the climate changes. Hyposalinity events alter seagrass composition and challenge resilience, yet understanding of the response mechanisms is incomplete but necessary for predicting outcomes. Hyposalinity stress response mechanisms in Halodule wrightii (shoal grass), a seagrass with wide salinity tolerance, were examined in mesocosm experiments by measuring monovalent and divalent ions, free amino acid (FAA) concentrations, and the Thiobarbituric Acid Reactive Substances (TBARS) indicator of lipid peroxidation. Two progressive, 3-step treatments, to moderate and extreme hyposalinity endpoints S15 and S5 were compared. Concentrations of Na+, K+, Ca2+, and Cl− decreased two to threefold with greater hyposalinity. Proline, the most abundant FAA initially, progressively decreased with increasing hyposalinity as asparagine became dominant. Glutamine and serine increased significantly, and all four FAA exhibited consistent patterns of change. Results are consistent with proline’s role as osmolyte and regulator, and suggest that asparagine mobilizes or stores N, possibly due to another limiting nutrient. TBARS changed transiently at intermediate salinities, but were not significantly different at hyposalinity endpoints indicating that H. wrightii may have already reached stress equilibrium. Prolonged hyposalinity events may have consequences for seagrass resilience.