page_title: Hydrogen isotope biogeochemistry
section_title: Plant leaf waxes
hierarchical_section_title: Hydrogen isotope biogeochemistry / Applications / Paleo-reconstruction / Biomolecules / Plant leaf waxes
caption_attribution_description: English: Photo of a leaf, covered in epicuticular wax.
context_page_description: Hydrogen isotope biogeochemistry is the scientific study of biological, geological, and chemical processes in the environment using the distribution and relative abundance of hydrogen isotopes. There are two stable isotopes of hydrogen, protium ¹H and deuterium ²H, which vary in relative abundance on the order of hundreds of permil. The ratio between these two species can be considered the hydrogen isotopic fingerprint of a substance. Understanding isotopic fingerprints and the sources of fractionation that lead to variation between them can be applied to address a diverse array of questions ranging from ecology and hydrology to geochemistry and paleoclimate reconstructions. Since specialized techniques are required to measure natural hydrogen isotope abundance ratios, the field of hydrogen isotope biogeochemistry provides uniquely specialized tools to more traditional fields like ecology and geochemistry.
context_section_description: Terrestrial plants make leaf waxes to coat the surfaces of their leaves as an adaptation to minimize water loss. These waxes are largely straight-chain n-alkyl lipids. They are insoluble, non-volatile, chemically inert, and resistant to degradation, making them easily preserved in the sedimentary record, and therefore good targets as biomarkers.
  The main water source for terrestrial plants is soil water, which largely resembles the hydrogen isotope composition of rain water, but varies between environments and with enrichment by precipitation, depletion by evaporation, and exchange with atmospheric water vapor. There can be a significant offset between the δD value of source water and the δD value of leaf water at the site of lipid biosynthesis. No fractionation is associated with water uptake by roots, a process usually driven by capillary tension, with the one exception of xerophytes that burn ATP to pump water in extremely arid environments (with a roughly 10‰ depletion). However, leaf water can be substantially enriched relative to soil water due to transpiration, an evaporative process which is influenced by temperature, humidity, and the composition of surrounding water vapor. The leaf water hydrogen isotope composition can be described with a modified Craig-Gordon model, where ΔDₑ is the steady state enrichment of leaf water, εeq is the temperature-dependent equilibrium fractionation between liquid water and vapor, εₖ is the kinetic isotope effect from diffusion between leaf internal air space and the atmosphere, ΔDᵥ is the leaf/air disequilibrium, eₐ is atmospheric vapor pressure, and eᵢ is internal leaf vapor pressure.
  The Péclet effect, which describes the opposing forces of advection and diffusion can be added to the model as
  where E is the transpiration rate, L is the length scale of transport, C is the concentration of water, and D is the diffusion coefficient.
  While the role of rain water δD as the fundamental control on the final δD of lipids is well documented, the importance of fractionation effects from rain water to soil water and leaf water on εl/w is appreciated but remains poorly understood.
  Organic biomolecules are generally depleted relative to the δD of leaf water. However, differences between organisms, biosynthetic pathways, and biological roles of different molecules can lead to huge variability in fractionation; the diversity of lipid biomarkers spans a 600‰ range of δD values. Lipid biosynthesis is biochemically complex, involving multiple enzyme-dependent steps that can lead to isotope fractionations. There are three major pathways of lipid biosynthesis, known as the mevalonate pathway, the acetogenic pathway, and the 1-deoxyD-xylulose-5-phosphate/2-methylerythroyl-4-phosphate pathway. The acetogenic pathway is responsible for the production of n-alkyl lipids like leaf waxes, and is associated with a smaller δD depletion relative to source water than the other two lipid biosynthesis pathways. While leaf water is the main source of hydrogen in leaf biomolecules, relatively depleted hydrogen from acetate or NADPH is often added during biosynthesis, and contributes to the hydrogen composition of the final molecule. Secondary hydrogen exchange reactions, meaning hydrogenation and dehydrogenation reactions outside of the primary biosynthetic pathway, also contribute substantially to the variability of lipid hydrogen isotope composition
  It is important to note that biological differences in fractionation stem not only from biochemical differences between different molecules, but also from physiological differences between different organisms. For example, the δD values of multiple leaf wax molecules are enriched in shrubs (median ~ -90‰) relative to trees (median ~ -135‰), which themselves are enriched relative to both C3 (median ~ -160‰) and C4 grasses (median ~ -140‰). Between individual species, substantial variation of δD values have been documented. Other physiological factors that contribute to variable leaf wax δD values include the seasonal timing of leaf development, response to external
caption: Photograph of a leaf, with sunlight reflecting off of the waxy surface.
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