Inter-molecular variations of fatty acid δD in algae and submerged plants from the north-eastern Tibetan Plateau

Hydrogen isotopic compositions (δD) of fatty acids (FAs) in lake sediments have been widely used in palaeoenvironmental reconstruction, but investigations on FA δD values in algae and submerged plants from natural lakes are still limited, greatly hampering the application and interpretation of their δD values in lacustrine environments. Here, we systematically investigated δD values of short-chain (C14-C18), mid-chain (C20-C24), and long-chain (C26-C30) FAs in four algae and submerged macrophytes, two algal genera Chara and Cladophora and two angiosperm genera Potamogeton and Ruppia, that commonly occur in six lakes on the north-eastern Tibetan Plateau. We obtained the following results: (i) C20-C30 FAs in each algal and submerged plant sample have similar variation patterns in δD values, suggesting that δD values of C20-C30 FAs have consistent responses to their source water δD. In C4-like taxa (Potamogeton, Ruppia, and Chara), the C16 and C26 FAs have minor differences in their δD values, while in the C3-like Cladophora, δD values of C16 FAs are significantly more negative (avg. -48 ± 7‰) than that of the C26 FAs. We propose that different photosynthetic mechanisms in these algae and submerged plants account for the different inter-molecular δD variations between C16 and C26 FAs; (ii) δD values of C26 FAs in the carbonate-coated alga Chara are significantly correlated with the δ18O values of carbonate encrustations - such a co-variance suggests that in addition to lake water, other factors may also influence both lipid synthesis and carbonate encrustation building up in Chara, leading to a significantly positive apparent hydrogen isotope fractionation factors (εFA-W) between C26 FAs of Chara and lake water; (iii) δD values of C26 FAs from Potamogeton, Ruppia and Cladophora are correlated well with lake water δD values (R2 = 0.84, p < 0.001, n = 19), suggesting that the δD values of long-chain FAs potentially record δD variations of lake water with the apparent hydrogen isotope fractionation factors (εFA-W) of C26 FAs ranging from −153‰ to −134‰. Our results highlight the impact of different photosynthetic pathways on the inter-molecular difference of FA δD values in algae and submerged plants and illustrate both the potential and the challenge of applying their FA δD values as a palaeo-hydrology proxy.