Distributions of geohopanoids in peat: Implications for the use of hopanoid-based proxies in natural archives

This is the final version of the article. Available from Elsevier via the DOI in this record.Hopanoids are pentacyclic triterpenoids produced by a wide range of bacteria. Within modern settings, hopanoids mostly occur in the biological 17β,21β(H) configuration. However, in some modern peatlands, the C31 hopane is present as the ‘thermally-mature’ 17α,21β(H) stereoisomer. This has traditionally been ascribed to isomerisation at the C-17 position catalysed by the acidic environment. However, recent work has argued that temperature and/or hydrology also exert a control upon hopane isomerisation. Such findings complicate the application of geohopanoids as palaeoenvironmental proxies. However, due to the small number of peats that have been studied, as well as the lack of peatland diversity sampled, the environmental controls regulating geohopanoid isomerisation remain poorly constrained. Here, we undertake a global approach to investigate the occurrence, distribution and diagenesis of geohopanoids within peat, combining previously published and newly generated data (n = 395) from peatlands with a wide temperature (−1 to 27 °C) and pH (3–8) range. Our results indicate that peats are characterised by a wide range of geohopanoids. However, the C31 hopane and C32 hopanoic acid (and occasionally the C32 hopanol) typically dominate. C32 hopanoic acids occur as αβ- and ββ-stereoisomers, with the ββ-isomer typically dominating. In contrast, C31 hopanes occur predominantly as the αβ-stereoisomer. These two observations collectively suggest that isomerisation is not inherited from an original biological precursor (i.e. biohopanoids). Using geohopanoid ββ/(αβ + ββ) indices, we demonstrate that the abundance of αβ-hopanoids is strongly influenced by the acidic environment, and we observe a significant positive correlation between C31 hopane isomerisation and pH (n = 94, r2 = 0.64, p 1 pH unit) and longer-term (>1 kyr) variation. Overall, our findings demonstrate the potential of geohopanoids to provide unique new insights into understanding depositional environments and interpreting terrestrial organic matter sources in the geological record.This research was funded through the advanced ERC grant ‘The Greenhouse Earth System’ (T-GRES. Project reference: 340923). RDP acknowledges the Royal Society Wolfson Research Merit Award. YZ thanks the National Natural Science Foundation of China (Project reference: 41372033). ELM acknowledges the Philip Leverhulme Prize. We also thank the NERC Life Sciences Mass Spectrometry Facility (Bristol) for analytical support and D. Atkinson for help with the sample preparation. GNI thanks Janet Dehmer and Philippe Schaeffer for helpful discussions. Members of the T-GRES Peat Database collaborators are M.J. Amesbury, H. Biester, R. Bindler, J. Blewett, M.A. Burrows, D. del Castillo Torres, F.M. Chambers, A.D. Cohen, S.J. Feakins, M. Gałka, A. Gallego-Sala, L. Gandois, D.M. Gray, P.G. Hatcher, E.N. Honorio Coronado, P.D.M. Hughes, A. Huguet, M. Könönen, F. Laggoun-Défarge O. Lähteenoja, M. Lamentowicz, R. Marchant, X. Pontevedra-Pombal, C. Ponton, A. Pourmand, A.M. Rizzuti, L. Rochefort, J. Schellekens, F. De Vleeschouwer. Finally, we thank Darci Rush, Phil Meyers and an anonymous reviewer for their comments and thoughtful suggestions which greatly improved this manuscript

Effects of temperature and pH on archaeal membrane lipid distributions in freshwater wetlands

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.  Freshwater wetlands harbour diverse archaeal communities and associated membrane lipid assemblages, but the effect of environmental factors (e.g. pH and temperature) on the distribution of these lipids is relatively poorly constrained. Here we explore the effects of temperature and pH on archaeal core-lipid and intact polar lipid (IPL) derived core lipid distributions in a range of wetlands. We focus not only on the commonly studied isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs), but also widen our analyses to include more recently identified but relatively widespread archaeal lipids such as isoGDGT isomers, methylated isoGDGTs (Me-GDGTs), and butanetriol and pentanetriol tetraethers (BDGTs and PDGTs). Based on multivariate analysis and a globally distributed set of wetlands, we find that the degree of isoGDGT cyclisation does increase along with temperature and pH in wetlands; however and unlike in some other settings, this relationship is obscured in simple scatterplots due to the incorporation of isoGDGTs from highly diverse archaeal sources with multiple ring-temperature or ring-pH relationships. We further show that the relative abundance of early eluting to later eluting isoGDGT isomers increases with pH, representing a previously unknown and seemingly widespread archaeal membrane homeostasis mechanism or taxonomic signal. The distribution and abundance of crenarchaeol, a marker for Thaumarchaeota, demonstrates that in wetlands these Archaea, likely involved in ammonia oxidation, are restricted primarily to the generally dryer, soil/sediment surface and typically are more abundant in circumneutral pH settings. We identify Me-GDGTs and Me-isoGMGTs (homologs of isoGDGTs and isoGMGTs, but with additional methylation on the biphytanyl chain) as being ubiquitous in wetlands, but variation in their abundance and distribution suggests changing source communities and/or membrane adaptation. The high relative abundance of BDGTs and PDGTs in the perennially anoxic part of the peat profile (catotelm) as well as their elevated abundance in a circumneutral pH wetland is consistent with an important input from their only known culture source, the methanogenic Methanomassiliicoccales. Our results underline the diversity of archaeal membrane lipids preserved in wetlands and provide a baseline for the use of archaeal lipid distributions in wetlands as tracers of recent or ancient climate and biogeochemistry.NERCRoyal SocietyER

Effects of temperature and pH on archaeal membrane lipid distributions in freshwater wetlands

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.  Freshwater wetlands harbour diverse archaeal communities and associated membrane lipid assemblages, but the effect of environmental factors (e.g. pH and temperature) on the distribution of these lipids is relatively poorly constrained. Here we explore the effects of temperature and pH on archaeal core-lipid and intact polar lipid (IPL) derived core lipid distributions in a range of wetlands. We focus not only on the commonly studied isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs), but also widen our analyses to include more recently identified but relatively widespread archaeal lipids such as isoGDGT isomers, methylated isoGDGTs (Me-GDGTs), and butanetriol and pentanetriol tetraethers (BDGTs and PDGTs). Based on multivariate analysis and a globally distributed set of wetlands, we find that the degree of isoGDGT cyclisation does increase along with temperature and pH in wetlands; however and unlike in some other settings, this relationship is obscured in simple scatterplots due to the incorporation of isoGDGTs from highly diverse archaeal sources with multiple ring-temperature or ring-pH relationships. We further show that the relative abundance of early eluting to later eluting isoGDGT isomers increases with pH, representing a previously unknown and seemingly widespread archaeal membrane homeostasis mechanism or taxonomic signal. The distribution and abundance of crenarchaeol, a marker for Thaumarchaeota, demonstrates that in wetlands these Archaea, likely involved in ammonia oxidation, are restricted primarily to the generally dryer, soil/sediment surface and typically are more abundant in circumneutral pH settings. We identify Me-GDGTs and Me-isoGMGTs (homologs of isoGDGTs and isoGMGTs, but with additional methylation on the biphytanyl chain) as being ubiquitous in wetlands, but variation in their abundance and distribution suggests changing source communities and/or membrane adaptation. The high relative abundance of BDGTs and PDGTs in the perennially anoxic part of the peat profile (catotelm) as well as their elevated abundance in a circumneutral pH wetland is consistent with an important input from their only known culture source, the methanogenic Methanomassiliicoccales. Our results underline the diversity of archaeal membrane lipids preserved in wetlands and provide a baseline for the use of archaeal lipid distributions in wetlands as tracers of recent or ancient climate and biogeochemistry.NERCRoyal SocietyER

Averaged isoGDGT isomers in global database of wetlands

This is the archaeal biomarker dataset for global peatlands. The first dataset contains biomarker data for each individual sample as for many peatlands we analysed more than one sample. The second dataset contains the averaged data for isoGDGT isomers in all sites that contains both associated pH and temperature measurements

Global distribution of archaeal biomarkers in wetlands

This is the archaeal GDGT dataset for global peatlands. The first dataset contains biomarker data for each individual sample as for many peatlands we analysed more than one sample. The second dataset contains the averaged data for isoGDGT isomers in all sites that contains both associated pH and temperature measurements. Other biomarker data for these samples can be found here: https://doi.pangaea.de/10.1594/PANGAEA.88376

δ¹³C values of bacterial hopanoids and leaf waxes as tracers for methanotrophy in peatlands

Methane emissions from peatlands contribute significantly to atmospheric CH4 levels and play an essential role in the global carbon cycle. The stable carbon isotopic composition (δ13C) of bacterial and plant lipids has been used to study modern and past peatland biogeochemistry, especially methane cycling. However, the small number of recent peatlands that have been characterised and the lack of consistency between target compounds means that this approach lacks a rigorous framework. Here, we undertake a survey of bacterial and plant lipid δ13C values in peatlands from different geographic regions, spanning a wide range of temperature (−8 to 27 °C) and pH (∼3 to 8), to generate a reference dataset and probe drivers of isotopic variability. Within our dataset, the carbon fixation pathway predominantly determines leaf wax (n-alkane) δ13C values. Bacterial-derived C31 hopane δ13C values track those of leaf waxes but are relatively enriched (0 to 10‰), indicating a heterotrophic ecology and preferential consumption of 13C-enriched substrates (e.g. carbohydrates). In contrast, ≤C30 hopanoids can be strongly 13C-depleted and indicate the incorporation of isotopically light methane into the bacterial community, especially at near neutral pH (∼5–6 pH). Previous analysis of Eocene sediments has suggested isotopic decoupling between C31 and ≤C30 hopanoid δ13C values. Our work suggests a globally widespread decoupling in recent peatlands; this persists despite the profound diversity of hopanoid producing bacteria and associated controls on their δ13C values and it has significant implications for future work. Re-analysis of published data from: (1) the (mid-to-early) Holocene and late Glacial, and (2) latest Paleocene and earliest Eocene in this revised context highlights that perturbations to the peatland methane cycle occurred during the past, and we envisage that this approach could provide unique (qualitative) insights into methane cycling dynamics throughout the geological record