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

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 (H-2/H-1) 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 H-2/H-1 ratios of lipids from different phytoplankton groups remains poorly constrained, especially when other growth parameters known to influence H-2/H-1 fractionation are held constant, such as growth rate, irradiance, and salinity. We investigated H-2/H-1 ratios responses of lipid biomarkers (fatty acids (FAs), sterols and alkenones) to temperature (12 similar to 27 degrees 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 H-2/H-1 fractionation of FAs in the dinoflagellate decreased by 2.2 +/- 0.3 parts per thousand degrees 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 parts per thousand degrees C-1 and 1.9 parts per thousand degrees C-1 (p < 0.05), respectively, as temperature increased. H-2/H-1 fractionation of the three types of lipids in the coccolithophores did not vary systematically with temperature. The underlying mechanism by which temperature alters lipid H-2/H-1 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.