Abstract
AbstractHydrogen isotopes of plant‐derived biomarkers can vary by >100‰ at a single location. Isotope fractionation associated with the movement of water in plant leaves cannot account for this variability alone. Biochemical processes therefore must play a fundamental role in controlling hydrogen isotope fractionation during secondary compound biosynthesis. Different biosynthetic pathways utilize discrete hydrogen pools and occur within distinct cell compartments. We analyzed hydrogen isotope compositions of C16 and C18 fatty acids and phytol from seven salt marsh plants and compared these data with (i) leaf water and n‐alkane δ2H, (ii) leaf carbon and nitrogen contents, and (iii) nitrogen isotopes of bulk tissue, to evaluate the relationship between biochemical processes, cellular compartmentalization, and hydrogen isotope fractionation. Interspecies variation in chloroplastic fatty acids and phytol δ2H exceeds leaf water δ2H, indicating that different commitments of metabolites among species at branching points in chloroplast metabolic processes may be important determinants of lipid δ2H values. Dominant osmoregulatory strategies, in particular, show strong correlation with leaf wax n‐alkane δ2H. Species that preferentially produce nitrogenous compounds (dicots/shrubs) as protective solutes have 2H‐enriched n‐alkanes relative to species that produce mainly carbohydrates (monocots). n‐Alkane δ2H values, in combination with δ15N data and elemental (C, N) composition, together provide information about biochemical environmental adaptations exhibited by different higher plant species in response to environmental stresses. Thus, while spatial and temporal integration of biomarkers may produce an isotopic record of ecosystem function, biomarkers from individual plant or microbial remains may hold additional details into biologic function and adaptation to ancient environments.
Highlights
The hydrogen isotope composition of n-alkyl lipids from terrestrial plants are widely interpreted to reflect source water δ2H values, with modification by a range of processes within the plant such as root uptake, transpiration, and leaf water evaporative enrichment (Kahmen et al, 2013; McInerney et al, 2011; Sachse et al, 2010, 2012)
In contrast to leaf wax n-alkane hydrogen isotope data (Eley et al, 2014), the most 2H-depleted phytol value is observed for the evergreen dicot A. portulacoides (À361 ± 1‰), while the most 2H-enriched is found in the C4 monocot grass S. anglica (À298 ± 1‰)
Statistical comparison of the δ2H of phytol and leaf water (Table S1) reveals a positive relationship, this is only significant at an 80% confidence interval (r = 0.5, P = 0.2, n = 7, Pearson’s product moment correlation, Minitab v. 17)
Summary
The hydrogen isotope composition of n-alkyl lipids from terrestrial plants are widely interpreted to reflect source water δ2H values, with modification by a range of processes within the plant such as root uptake, transpiration, and leaf water evaporative enrichment (Kahmen et al, 2013; McInerney et al, 2011; Sachse et al, 2010, 2012). Recent studies of extant plants growing at a single geographical location have revealed that variation in δ2Hwax can exceed 100‰, which cannot be fully accounted for by processes controlling the isotopic composition of soil, xylem, or leaf water (Eley et al, 2014; Oakes & Hren, 2016). These studies highlight the significant influence that biochemical mechanisms may exert over the δ2H of leaf wax biomarkers. A more detailed appreciation of the biological mechanisms that control leaf wax lipid hydrogen isotope values (and the way these perturbations are recorded in the n-alkane hydrogen isotope signal) has the potential to expand the organic molecular toolkit for paleoenvironmental investigations, enabling reconstruction of factors such as plant energy budgets and metabolism (Cormier et al, 2018)
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