Lipid compound classes display diverging hydrogen isotope responses in lakes along a nutrient gradient

Abstract

Compound specific hydrogen isotope ratios (2 H/ 1 H) of lipid biomarkers 36! preserved in sediments are used as a paleohydrologic proxies. However, several 37! variables, including contributions from different source organisms and their growth 38! rates, can influence 2 H/ 1 H fractionation between lipids and source water. Significant 39! uncertainties remain about how these factors combine to produce the net 2 H/ 1 H signal 40! exported to sediments. 41! To assess the influence of phosphorus availability on 2 H/ 1 H ratios of lipids 42! accumulating in lake sediments, we analyzed surface sediments and sediment traps 43! from ten central Swiss lakes, representing a wide range of trophic states. In agreement 44! with results from laboratory cultures, 2 H/ 1 H fractionation for the diatom biomarker 45! brassicasterol (24-methyl cholest-5,22-dien-3β-ol) increased in more productive lakes 46! (0.6 ± 0.1 ‰ per µg/L total P in sediment traps and surface sediments). In contrast, 47! 2 H/ 1 H fractionation for phytol, the isoprenoid side-chain moiety of chlorophyll, 48! decreased with increasing total P (-0.4 ± 0.1 ‰ per µg/L total P in sediment traps), 49! suggesting that different biochemical mechanisms are responsible for changes in 2 H/ 1 H 50! fractionation for each class of isoprenoidal lipids. Opposing changes in 2 H-51! fractionation for sterols and phytol cause their 2 H/ 1 H ratios to converge as total P 52! increases. This response may be a new tracer for phytoplankton growth conditions and 53! is not influenced by the source water isotope value. 54! Interpreting the 2 H/ 1 H ratios of short to long chain (C 14-C 30) n-alkanoic acids 55! and n-alkanols is complicated by likely contributions from heterotrophs and/or 56! vascular plants. These values generally did not correlate with lake water isotopes, nor 57! did their fractionation factors correlate with total P. For most lipids there was no 58! significant difference between sediment trap and surface sediment 2 H/ 1 H ratios. 59! However, n-C 14n-C 18 fatty acids were 2 H-enriched in the surface sediments, most 60! likely due to degradation in the water column. Our results indicate that interpretations 61! of short-chain fatty acids as a water isotope signal likely require supporting 62! information about ecological conditions and community structure, but that paired H 63! isotope measurements of phytoplankton-derived sterols and phytol may be developed 64! as a proxy for phytoplankton growth.

Metabolic exchange between pathways for isoprenoid synthesis and implications for biosynthetic hydrogen isotope fractionation

“…Low ε 2 H sitosterol-phytol values in P. aquatica Phytosterols are highly enriched in 2 H relative to phytol, with a mean enrichment from published values of 135 ‰ (Sessions et al, 1999;Sessions, 2006;Sachs et al, 2016Sachs et al, , 2017Ladd et al, 2017Ladd et al, , 2018) (Figure 5b). This 2 H-enrichment is typically attributed to incorporation of relatively more 2 H-depleted photosynthetic NADPH into MEP-derived intermediates in comparison to those derived from the MVA pathway, which should have more 2 H-enriched NAD(P)H from sugar metabolisms such as the oxidative pentose phosphate pathway (Sessions et al, 1999;Schmidt et al, 2003;Sachs et al, 2017) (Figure 6).…”

“…All rights reserved A recovery standard containing 5α-androstane, 3-eicosanone, n-C 19 -alkanol, and n-C 19alkanoic acid was quantitatively added to freeze-dried, homogenized leaves prior to extraction in 30 mL of 9:1 dichloromethane/methanol (DCM/MeOH) in a SolvPro Microwave Reaction System (Anton Parr, Graz, Austria) following the extraction scheme of Ladd et al (2018). Total lipid extracts were base saponified and separated into neutral and acidic fractions, and the neutral fraction was dissolved in hexane and purified by Si gel column chromatography as in Ladd et al (2018). Alkanes eluted in 4 mL of hexane (Hex), squalene in 4 mL of 2:1 Hex/DCM, and phytol and phytosterols in 4 mL of 19:1 DCM/MeOH.…”

Leaf Wax Hydrogen Isotopes as a Hydroclimate Proxy in the Tropical Pacific

“…Purity and concentrations of n-alkanes from mangrove samples were assessed by gas chromatography -flame ionization detection (GC-FID) using the GC program and instrumentation described in Ladd and Sachs (2017). For lake samples, nalkane and n-acid homologues were quantified using the same GC program and instrumentation described in Ladd et al (2018).…”

Contrasting sensitivity of lake sediment n-alkanoic acids and n-alkanes to basin-scale vegetation and regional-scale precipitation δ2H in the Adirondack Mountains, NY (USA)

“…It is therefore possible that the relationship between ε C29-C30 and C/N is linked to lipids sourced from emergent or submerged aquatic plants and their use of lake water. Long-chain n-alkanoic acids can be sourced from lacustrine microalgae (Volkman et al, 1998;Ladd et al, 2018;van Bree et al, 2018), which could potentially account for lower (aquatic-derived) C/N sediments having the largest ε C29-C30 values, reflecting contrasting (terrestrial or aquatic) water sources for n-alkanes and n-alkanoic acids, respectively.…”