Effect of light on 2H/1H fractionation in lipids from continuous cultures of the diatom Thalassiosira pseudonana

Continuous cultures of the marine diatom Thalassiosira pseudonana were grown at irradiances between 6 and 47 mu mol m(-2) s(-1) in order to evaluate the effect of light on hydrogen isotope fractionation in lipids. delta(2) H values increased with irradiance in phytol by 1.1 parts per thousand (mu mol m(-2) s(-1))(-1) and by 0.3 parts per thousand (mu mol m(-2) s(-1))(-1) in the C14:0 fatty acid, but decreased by 0.8 parts per thousand (mu mol m(-2) s(-1))(-1) in the sterol 24-methyl-cholesta-5,24(28)-dien-3 beta-ol (C-28(sic)(5,24(28))). The anticorrelation between delta(2) H values in C-28(sic)(5,24(28)) and irradiance is attributed to enhanced sterol precursor synthesis via the plastidic methylerythritol phosphate (MEP) pathway at high irradiance, relative to the cytosolic mevalonic acid (MVA) pathway, and the supposition that MEP precursors are H-2-depleted compared to MVA precursors because they incorporate a greater proportion of hydrogen from photosynthetically produced NADPH. Increasing delta(2) H values of phytol and C14: 0 with irradiance is attributed to a greater proportion of pyruvate, the last common precursor to both lipids, being sourced from glycolysis in the mitochondria and cytosol, where enhanced incorporation of metabolic NADPH and further hydrogen exchange with cell water can enrich pyruvate with H-2 relative to pyruvate from the chloroplast. Irrespective of the biosynthetic mechanisms responsible for the H-2/H-1 fractionation response to light, the high sensitivity of lipid delta(2) H values in T. pseudonana continuous cultures would result in -30 parts per thousand to +40 parts per thousand variations in delta(2) H over a 40 mu mol m(-2) s(-1) range in sub-saturating irradiance if expressed in the environment, depending on the lipid. (C) 2017 Elsevier Ltd. All rights reserved.