Sensitivity of the 2H/1H fractionation factor, α, to intracellular hydrogen source.

The fractionation factor, α, can respond to both (A) f, the fraction of lipid hydrogen derived from NADPH versus intracellular water, and (B) x the fraction of NADPH-derived hydrogen in lipids that comes from photosynthesis as opposed to the OPP pathway. δ2H values of NADPH/PS1 and NADPH/OPP are set at -600‰ and -250‰, respectively. Intracellular water δ2H is set at 0‰. The shaded areas in (A) indicate the range of α values measured for 3 lipid classes (fatty acids, FA-green; alkenones-red; brassicasterol-purple) in our E. huxleyi continuous cultures (Table 3). The slope of the relationship would increase if less than half of the NADPH-derived hydrogen in lipids came from photosynthesis (i.e., x f = 0.5). A greater fraction of NADPH from photosynthesis (higher x) results in lower α values since photosynthetically produced hydride is 2H -depleted relative to NADPH produced via OPP in the cytosol. In (B) the sensitivity of α to changes in x decreases if either δ2H NADPH/PS1 > -600‰ or δ2H NADPH/OPP