Comparison of lipid biomarkers and their hydrogen isotopic values in submerged vascular macrophytes and charophytes

Submerged macrophytes incorporate lake water directly during lipid synthesis, making the δ²H values of their lipids a crucial tool for reconstructing past hydrology and lake states. Despite this significance, the lipid biomarkers of aquatic plants have received less attention compared to terrestrial plants. In particular, in the context of organic geochemical applications, submerged aquatic plants are typically considered as a single group, ignoring potential differences in lipid distributions and hydrogen isotope composition between vascular macrophytes and macroalgae such as charophytes. This gap limits the use of lipid biomarkers in lake sediments to understand past lake water isotopes and vegetation dynamics.

In this study, we analyzed the lipid contents and δ²H values of fatty acids, n-alkanes, and the chlorophyll side-chain phytol from paired vascular macrophytes and charophytes collected from 12 oligo-mesotrophic hardwater lakes in northeast Germany. We aim to assess differences between macrophyte groups and their relation to environmental factors and lake water properties, such as lake water δ²H values and pH levels.

Our preliminary results reveal a notable predominance of fatty acids over n-alkanes in both macrophyte groups. Vascular macrophytes tended to exhibit a higher, albeit variable, abundance of n-alkanes, fatty acids, and phytol concentrations compared to charophytes. The n-alkanes profiles were mainly comprised of mid to long-chain hydrocarbons (n-C₂₃ to n-C₂₇) and exhibited striking variability among macrophytes. Nonetheless, charophytes were notably characterized by a prominent dominance of n-C₂₇. While the C_16:0 fatty acid was the most abundant hydrocarbon in both macrophyte types, vascular macrophytes exhibit a greater abundance of long-chain (C₂₄ toC₃₀) fatty acids. However, our data revealed marked differences in the relative abundance of these long-chained compounds. The overall disparities in the lipid profiles point to distinct lipid biosynthesis pathways or environmental adaptations among the studied aquatic plants. Despite the differences in lipid distributions, no systematic differences were observed in the δ²H values for any studied compound class between the two macrophyte groups, Our results suggest that reconstructions of lake water isotopes based on δ²H values of aquatic plant lipids are unlikely to be influenced by changes in the relative contributions from vascular macrophytes and charophytes.