Large D/H variations in bacterial lipids reflect central metabolic pathways

Abstract

Large hydrogen-isotopic (D/H) fractionations between lipids and growth water have been observed in most organisms studied to date. These fractionations are generally attributed to isotope effects in the biosynthesis of lipids, and are frequently assumed to be approximately constant for the purpose of reconstructing climatic variables. Here, we report D/H fractionations between lipids and water in 4 cultured members of the phylum Proteobacteria, and show that they can vary by up to 500‰ in a single organism. The variation cannot be attributed to lipid biosynthesis as there is no significant change in these pathways between cultures, nor can it be attributed to changing substrate D/H ratios. More importantly, lipid/water D/H fractionations vary systematically with metabolism: chemoautotrophic growth (approximately ؊200 to ؊400‰), photoautotrophic growth (؊150 to ؊250‰), heterotrophic growth on sugars (0 to ؊150‰), and heterotrophic growth on TCA-cycle precursors and intermediates (؊50 to ؉200‰) all yield different fractionations. We hypothesize that the D/H ratios of lipids are controlled largely by those of NADPH used for biosynthesis, rather than by isotope effects within the lipid biosynthetic pathway itself. Our results suggest that different central metabolic pathways yield NADPH-and indirectly lipids-with characteristic isotopic compositions. If so, lipid ␦D values could become an important biogeochemical tool for linking lipids to energy metabolism, and would yield information that is highly complementary to that provided by 13 C about pathways of carbon fixation.fatty acids ͉ fractionation ͉ metabolism ͉ hydrogen isotopes T he hydrogen-isotopic composition ( 2 H/ 1 H or D/H ratio, commonly expressed as a ␦D value) of lipids is being explored by scientists with diverse interests, including the origins of natural products (1, 2), biogeochemical cycles (3), petroleum systems (4), and paleoclimate (5-7). Because the D/H ratios of lipids are generally conserved over Ϸ10 6 -year time scales (8), they are a potentially useful tracer of biogeochemical pathways and processes in the environment. Most research to date has focused on higher plants, in which environmental water is the sole source of external hydrogen and consequently provides primary control over the D/H ratio of biosynthesized lipids (9). Although ␦D values for plant lipids and environmental water are generally well correlated, they are also substantially offset from each other. The biochemical basis for this lipid/water fractionation is not well understood. It is generally assumed to arise from a combination of isotope effects during photosynthesis and the biosynthesis of lipids (9-12), and is often treated as approximately constant to reconstruct isotopic compositions of environmental water as a paleoclimate proxy.There is, however, mounting evidence that the net D/H fractionation between lipids and water can vary by up to 150‰ in plants, even in the same organism (12-16). Modest fractionations associated with fatty acid elongation and desaturation ...

Isotopic remembrance of metabolism past

“…Each reaction is accompanied by a fractionation, designated by ␣. The results of Zhang et al (9) indicate that fractionation associated with lipid biosynthesis is secondary to the upstream reactions that set the deuterium content of NADPH. At the bottom: The deuterium content of lipid biomarkers is displayed, all originating from waters with similar deuterium content.…”

“…1. ) Zhang et al (9) overturn this paradigm by demonstrating that the upstream metabolic pathways generating reducing power for lipid biosynthesis exert greater control on ultimate lipid deuterium content than does net fractionation during biosynthesis. Water and organic material do supply the deuterium in lipids, but the pathway that generates biosynthetic reducing power in the form of NADPH serves as an important second level of control.…”

“…They then measured the extent of deuterium fractionation in the cells' lipids. This experimental design mathematically constrains the sources providing hydrogen to lipids for each growth substrate, and it enables graphic representation of the isotopic relationships through fractionation curves (9,10). With the aid of these curves the authors rigorously exclude lipid biosynthetic reactions as a major determinant of the observed deuterium fractionation.…”

“…Most commonly these analyses are used to track past changes of climate and aridity, enabled by the strong correlation of deuterium content between plant-derived lipids and source water (4)(5)(6)(7)(8). In this issue of PNAS, Zhang et al (9) move in a new direction and lay the groundwork for a proxy of cellular metabolism based on lipid deuterium. They present results from laboratory experiments with bacterial cultures demonstrating that the deuterium content of lipid biomarker compounds is strongly modulated by the central biochemical pathway a cell uses to generate reducing power for biosynthesis.…”

“…The cast of bacterial characters examined by Zhang et al (9) includes four strains representing three lineages known as ␣, ␤, and ␥ Proteobacteria. The heterotroph Escherichia coli, a ␥-proteobacterium, is the most (in)famous and was cultivated with oxygen on organic substrates.…”