Dry season dominance of salinity's impact on hydrogen isotope fractionation in Aegiceras corniculatum mangrove lipids

The hydrogen isotope ratios (δ²H) of mangrove leaf waxes are influenced by both precipitation and water salinity, making them promising proxies for paleohydrologic and paleosalinity reconstructions. However, the mechanism by which salinity affects ²H/¹H fractionation remain unclear. While previous studies have shown that fractionation between source and leaf water pools is not the primary driver, it is still uncertain how biosynthetic isotope fractionation contributes to this process and how these effects vary across different seasons. To our knowledge, no studies have directly compared the seasonal variations in isotope fractionation between the dry and wet seasons, which are characteristic of tropical and subtropical regions. To address these questions, we measured δ²H values of n-alkanes and n-fatty acids in the leaves of Aegiceras corniculatum collected from the Zhanjiang estuary during both the dry and wet seasons. We compared these data with δ²H and δ¹⁸O values from leaf water, xylem water, estuary surface water, and sediment pore water to discern potential differences in isotopic fractionation mechanisms. Our findings indicate that net ²H/¹H fractionation increases with salinity for both C₃₁ n-alkanes (2.5 ± 0.9 ‰ ppt⁻¹) and C_16:0 n-fatty acids (1.0 ± 0.2 ‰ ppt⁻¹) during the dry season, whereas no similar such trends were observed in the wet season. These seasonal variations highlight the dominant impact of salinity on hydrogen isotope fractionation in A. corniculatum lipids during the dry season. We also found that salinity-driven fractionation is not solely related to water uptake but rather to physiological responses to high salinity. This finding aligns with previous studies, which indicate that salinity-induced effects on hydrogen isotopic fractionation are primarily driven by physiological adaptations, rather than by salinity-dependent fractionation mechanisms in leaf and xylem water. Building upon this understanding, we propose novel hypotheses: heightened salinity in the dry season reduces photosynthetic efficiency in A. corniculatum due to limited CO₂ availability, which in turn triggers increased production of compatible solutes. This may reduce cellular water availability and limit isotopic exchange. Additionally, elevated salinity could intensify carbon metabolism, affecting the residence time of intermediates in the TCA cycle and influencing isotopic water exchange. While we propose these as potential mechanisms, further studies are needed to confirm its role in biosynthetic fractionation and its relationship with water isotopes in mangrove plants. In contrast, during the wet season, increased rainfall dilutes salinity, normalizing photosynthetic and metabolic activity in the mangrove. These findings provide new insights into the mechanisms of isotopic fractionation in mangrove lipids and the role of seasonality in fractionation patterns, which are important for improving paleohydrologic and paleosalinity reconstructions.