Effect of temperature on 2H/1H fractionation in semi-continuous cultures of a marine diatom, coccolithophore, and dinoflagellate

Abstract

Temperature regulates the metabolic rate of organisms and, together with salinity, determines seawater density, a key driver of ocean circulation and heat transport. The hydrogen isotope composition (²H/¹H) of phytoplankton lipids has been shown to vary systematically with salinity and applied as a paleosalinity proxy in marine sediments. The influence of temperature on ²H/¹H ratios of lipids from different phytoplankton groups remains poorly constrained, especially when other growth parameters known to influence ²H/¹H fractionation are held constant, such as growth rate, irradiance, and salinity. We investigated ²H/¹H ratios responses of lipid biomarkers (fatty acids (FAs), sterols and alkenones) to temperature (12 ∼ 27 °C) in three marine phytoplankton taxa, the diatom Phaeodactylum tricornutum, the dinoflagellate Prorocentrum donghaiense, and the coccolithophorid Emiliania huxleyi using semi-continuous cultures. We show that ²H/¹H fractionation of FAs in the dinoflagellate decreased by 2.2 ± 0.3 ‰ °C^–1 (p < 0.05), while that of sterols in the dinoflagellate and fatty acid C_14:0 in the diatom increased by 2.3 ‰ °C^–1 and 1.9 ‰ °C^–1 (p < 0.05), respectively, as temperature increased. ²H/¹H fractionation of the three types of lipids in the coccolithophores did not vary systematically with temperature. The underlying mechanism by which temperature alters lipid ²H/¹H fractionation likely involves its effect on the proportions of photosynthetic and metabolic NADPH incorporated into lipids, NADPH residence time in the chloroplast, the contribution of extra-plastidic pyruvate to FA synthesis, and the proportion of mevalonic acid- versus methylerythritol phosphate-derived precursors in sterols.