The Influence of Growth Rate on (2)H/(1)H Fractionation in Continuous Cultures of the Coccolithophorid Emiliania huxleyi and the Diatom Thalassiosira pseudonana

The hydrogen isotope ((2)H/(1)H) ratio of lipids from phytoplankton is a powerful new tool for reconstructing hydroclimate variations in the geologic past from marine and lacustrine sediments. Water (2)H/(1)H changes are reflected in lipid (2)H/(1)H changes with R(2) > 0.99, and salinity variations have been shown to cause about a 1‰ change in lipid δ(2)H values per unit (ppt) change in salinity. Less understood are the effects of growth rate, nutrient limitation and light on (2)H/(1)H fractionation in phytoplankton. Here we present the first published study of growth rate effects on (2)H/(1)H fractionation in the lipids of coccolithophorids grown in continuous cultures. Emiliania huxleyi was cultivated in steady state at four growth rates and the δ(2)H value of individual alkenones (C(37:2), C(37:3), C(38:2), C(38:3)), fatty acids (C(14:0), C(16:0), C(18:0)), and 24-methyl cholest-5,22-dien-3β-ol (brassicasterol) were measured. (2)H/(1)H fractionation increased in all lipids as growth rate increased by 24‰ to 79‰ (div d(-1))(-1). We attribute this response to a proportional increase in the fraction of NADPH from Photosystem I (PS1) of photosynthesis relative to NADPH from the cytosolic oxidative pentose phosphate (OPP) pathway in the synthesis of lipids as growth rate increases. A 3-endmember model is presented in which lipid hydrogen comes from NADPH produced in PS1, NADPH produced by OPP, and intracellular water. With published values or best estimates of the fractionation factors for these sources (α(PS1) = 0.4, α(OPP) = 0.75, and α(H2O) = 0) and half of the hydrogen in a lipid derived from water the model indicates α(lipid) = 0.79. This value is within the range measured for alkenones (α(alkenone) = 0.77 to 0.81) and fatty acids (α(FA) = 0.75 to 0.82) in the chemostat cultures, but is greater than the range for brassicasterol (α(brassicasterol) = 0.68 to 0.72). The latter is attributed to a greater proportion of hydrogen from NADPH relative to water in isoprenoid lipids. The model successfully explains the increase in (2)H/(1)H fractionation in the sterol 24-methyl-cholesta-5,24(28)-dien-3β-ol from marine centric diatom T. pseudonana chemostat cultures as growth rate increases. Insensitivity of α(FA) in those same cultures may be attributable to a larger fraction of hydrogen in fatty acids sourced from intracellular water at the expense of NADPH as growth rate increases. The high sensitivity of α to growth rate in E. huxleyi lipids and a T. pseudonana sterol implies that any change in growth rate larger than ~0.15 div d(-1) can cause a change in δ(2)H(lipid) that is larger than the analytical error of the measurement (~5‰), and needs to be considered when interpreting δ(2)H(lipid) variations in sediments.