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

摘要

Hydrogen isotopic compositions (delta D) of fatty acids (FAs) in lake sediments have been widely used in palaeoenvironmental reconstruction, but investigations on FA delta D values in algae and submerged plants from natural lakes are still limited, greatly hampering the application and interpretation of their delta D values in lacustrine environments. Here, we systematically investigated delta D values of short-chain (C-14-C-18), mid-chain (C-20-C-24), and long-chain (C-26-C-30) 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) C-20-C-30 FAs in each algal and submerged plant sample have similar variation patterns in delta D values, suggesting that delta D values of C-20-C-30 FAs have consistent responses to their source water delta D. In C-4-like taxa (Potamogeton, Ruppia, and Chara), the C-16 and C-26 FAs have minor differences in their delta D values, while in the C-3-like Cladophora, delta D values of C-16 FAs are significantly more negative (avg. -48 +/- 7 parts per thousand) than that of the C-26 FAs. We propose that different photosynthetic mechanisms in these algae and submerged plants account for the different inter-molecular delta D variations between C-16 and C-26 FAs; (ii) delta D values of C-26 FAs in the carbonate-coated alga Chara are significantly correlated with the delta O-18 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 (epsilon(FA-W)) between C-26 FAs of Chara and lake water; (iii) delta D values of C-26 FAs from Potamogeton, Ruppia and Cladophora are correlated well with lake water delta D values (R-2 = 0.84, p 0.001, n