Dialkyl Glycerol Tetraethers Research Articles

Archaeal ammonia oxidation plays a part in late Quaternary nitrogen cycling in the South China Sea

Thaumarchaeota, as an ammonia oxidizing archaea (AOA), is crucial for modern marine nitrogen cycling; however, little is known about its history during the Quaternary climate change. Here, isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs), biomarkers of Thaumarchaeota, were used to trace the role of AOA in the South China Sea (SCS) for the past 160 kyr. The GDGT-[2]/[3] ratio was firstly argued as an indicator of contribution of shallow Thaumarchaeota cluster that is more active in ammonia oxidization (AO) than the deep cluster, and then, was used to reconstruct AO in the past. The inferred AO exhibited intensification in the interglacials, and moreover, showed strong precessional cycles with enhancements at the precessional maxima when boreal winter insolation was the highest. The AOA record varied in line with isotope record of organic nitrogen (δ15Norg) that is modulated by the strength of diazotroph N2 fixation (NF), suggesting a close coupling of increased AO with enhanced NF during periods of weak east Asian winter monsoon (EAWM) and hence increase of upper water stratification. AO intensification, a step of a series of dissolved oxygen consuming processes, is hereby hypothesized to encourage NF when the EAWM weakens. This result might be a reference for the future NF trend in the current situation of enhanced ocean deoxygenation due to global warming.

Glycerol configurations of environmental GDGTs investigated using a selective sn2 ether cleavage protocol

The glycerol configurations of glycerol dialkyl glycerol tetraethers (GDGTs) in environmental samples were investigated using a selective sn2 ether cleavage protocol. Using this procedure, GDGTs with a parallel glycerol configuration afford two types of derivatives, diols and diallylethers, whereas only one kind, monoallylethers, originate from their antiparallel isomers. Isoprenoidal GDGTs from a marine sediment are shown to be predominately parallel based on the distributions of these ether cleavage products. Crenarchaeol and its so-called regioisomer both have parallel configurations with the cyclohexane ring located on the sn3,3 ether bonded tricyclic biphytanyl moiety. A Messel shale sample containing isoprenoidal GDGTs contributed mainly by methanogenic archaea has a substantial portion with the antiparallel configuration. Branched (non-isoprenoidal) GDGTs in both the Messel shale and the marine sediment are mainly antiparallel. This selective sn2 ether cleavage approach provides a potentially powerful analytical tool to investigate not only the exact molecular structures of GDGT constitutional isomers and their biosynthetic pathways but also the heterogeneous inputs of sedimentary GDGT and their isotopic signatures, if different source species synthesize GDGTs with unique glycerol configurations. Further analyses of this type will reveal the glycerol configurations of the GDGTs of a broad range of microbial cultures and environmental samples.

In situ production of core and intact bacterial and archaeal tetraether lipids in groundwater

Groundwater ecosystems host diverse and complex microbial communities that play important roles in the biogeochemical processing of organic matter and in the maintenance of drinking water quality. Here we investigated the microbial community in suspended particulate matter (SPM) of biogeochemically distinct groundwaters (Hainich Critical Zone Exploratory) by analyzing branched and isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) from bacteria and archaea, respectively. The contributions of those lipids derived from dead and living organisms were determined by analyses of the core lipid distributions of core and intact polar GDGTs. We compared the groundwater GDGT distributions to the ones from soils of potential recharge areas and with archaeal 16S rRNA-gene based community reconstructions to estimate their origin in these terrestrial subsurface environments and thus their potential use for evaluating soil inputs into groundwater. In soils, the relative abundance of intact polar branched GDGTs (brGDGTs) was lower than that of isoprenoid GDGTs (isoGDGTs; 2% vs 5% of total GDGTs), while the opposite trend (71% vs 22% of total GDGTs) was observed in the core lipid pools. This supports previous observations that soil brGDGT-producing bacteria might be more active and thus have higher regeneration rates than the isoGDGT-producing archaea. We found similar trends in the groundwater that might indicate higher activity (i.e., cell division) of brGDGT-producing bacteria than of isoGDGT-producing archaea. The higher relative abundance of the hexamethylated brGDGT in the groundwater SPM (mean 65 ± 9%, n = 5) than in soils (mean 16 ± 7%, n = 22) indicated an in situ origin of brGDGT-producing bacteria. Higher contributions of penta- and tetra-methylated brGDGTs, which suggested some inputs from soil bacteria, was only detected in two out of seven groundwater samples. The strong correlation between core and intact polar isoGDGTs (R2 = 0.99, n = 7) in groundwater SPM indicated low disturbance (e.g., surface inputs) and suggested indigenous archaeal communities in the groundwater. This was supported by the results from a previous 16S rRNA-gene study that detected distinct archaeal groups in soils and groundwater. This first GDGT study in groundwater demonstrated that even dynamic karstic subsurface environments host an indigenous bacterial and archaeal community that is adapted to the living conditions. Furthermore, fast recharge events are likely detectable using tetraether lipids from the soil microbial community.

Distribution of glycerol ethers in Turpan soils: implications for use of GDGT-based proxies in hot and dry regions

Proxies based on glycerol dialkyl glycerol tetraethers (GDGTs), a suite of membrane lipids occurring ubiquitously in soils, have generated increasing interest in quantitative paleo-environmental reconstruction. Hot and dry climates are likely to have occurred in the geological past; however, the limitations and applicability of these proxies to hot and dry environments are still unknown. In this study, we analyzed the GDGT distribution in the Turpan (TRP) basin of China, where the highest soil temperature can be approximately 70°C, and the mean annual precipitation (MAP) is 15.3 mm. We compared GDGT-based proxies among TRP soils, Nanyang (NY) soils, and Kunming (KM) soils; these three sites exhibit similar mean annual air temperature (MAAT) albeit contrasting temperature seasonality and MAP. Archaeal isoprenoidal GDGTs (isoGDGTs) dominate over bacterial branched GDGTs (brGDGTs) in most TRP soils; this is a characteristic GDGT distribution pattern for soils from dry regions globally. Another feature is the anomalously high GDGT-0/crenarchaeol ratio, which is generally attributed to the contribution of anaerobic methanogenic archaea by previous studies; however, these anaerobic archaea are unlikely to be highly abundant in the dry TRP soils, indicating that certain uncultured halophilic Euryarchaeota are likely to produce a significant amount of GDGT-0 that finally results in a high GDGT-0/Cren ratio. The changes in the salinity of the TRP soils appear to be an important factor affecting the MBT’5ME and the relative abundance of 6- vs. 5-methyl pentamethylated brGDGTs (IRIIa’). This is likely to introduce certain scatters in the correlations between MBT’5ME and MAAT and that between IRIIa’ and pH determined at the global scale. A comparison of the MBT’5ME-inferred temperature between TRP, NY, and KM soils does not indicate a significant bias toward summer temperature, indicating that brGDGT paleothermometers in soils could reflect the MAAT.

Sources and distribution of sedimentary organic matter in the Beibu Gulf, China: Application of multiple proxies

Geochemical and biomarker methods were combined to investigate the sources of sedimentary organic matter (OM) and their spatial distribution in the Beibu Gulf, a subtropical estuary in southern China. We measured the total organic carbon (TOC) and total nitrogen (TN) content, the glycerol dialkyl glycerol tetraethers (GDGTs) concentration, as well as the isotopic composition of sedimentary OM (δ13Corg and δ15N) in surface sediments collected from the Beibu Gulf. The spatial distribution of the bulk organic parameters (TOC, TN, C/N ratio, δ13Corg, and δ15N) revealed a generally decreasing trend in terrestrial OM from the Maowei Sea to the outer Qinzhou Bay, and from the tributary rivers to the outer bay. The BIT (branched/isoprenoid tetraether) indices also decreased seaward from the Maowei Sea to the Qinzhou Bay, and from rivers to the outer bay, showing that soil OM was mainly delivered to the bay by rivers. The spatial patterns of these bulk parameters suggest that the terrigenous OC is constrained to the coast and adjacent regions. The relatively high GDGT-0/Crenarchaeol ratios and increasing δ15N values along the coast of the Maowei Sea indicate the anthropogenic nitrogen in the Beibu Gulf. Our results show how important a multiproxy approach can be to fully understand and quantify the relative contributions of terrestrial and marine OC components and the biogeochemical processes in the Beibu Gulf, especially in the context of growing anthropogenic activities.

An overview of the occurrence of ether- and ester-linked iso-diabolic acid membrane lipids in microbial cultures of the Acidobacteria: Implications for brGDGT paleoproxies for temperature and pH

13,16-Dimethyl octacosanedioic acid (iso-diabolic acid) is a major membrane-spanning lipid of subdivisions (SDs) 1, 3 and 4 of the Acidobacteria, a highly diverse phylum within the Bacteria. It has been suggested that these lipids are potential building blocks for the orphan bacterial glycerol dialkyl glycerol tetraethers (GDGT) that occur widely in a variety of environmental settings. Here, we expand the knowledge on the occurrence of iso-diabolic acid in Acidobacteria by examining the lipid composition of six strains belonging to SDs 6, 8, 10, and 23 of the Acidobacteria, not previously analyzed for these lipids. In addition, we examined 12 new strains belonging to SDs 1, 3 and 4. Acid hydrolysis of total cell material released iso-diabolic acid in substantial quantities (25–39% of all fatty acids) from the strains of SDs 1 and 3 (except “Candidatus Solibacter usitatus”), and, for the first time, strains of SD 6 (6–25%), but not from SDs 8, 10, and 23. The monoglycerol ether derivative of iso-diabolic acid was only dominantly present in SD 4 strains (17–34%), indicating that the occurrence of ether-bound iso-diabolic acid is mainly restricted to SD 4 species. Methylated iso-diabolic acid derivatives were encountered in SDs 1, 3, 4, and 6, but only SD 4 species produced 5-methyl iso-diabolic acid derivatives, whereas the other SDs formed 6-methyl iso-diabolic acids. This suggests that the position of methylation of iso-diabolic acid may be controlled by the phylogenetic affiliation within the Acidobacteria and thus may not be a direct but an indirect response environmental to environmental conditions as inferred from the bacterial GDGT distributions in soil, peat and rivers.

TEX86 in sinking particles in three eastern Atlantic upwelling regimes

Seasonal variations in fluxes of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) and the estimated temperatures based on TEX86 were examined in sinking particles collected using moored sediment traps in the eastern Atlantic upwelling regions. In the equatorial Guinea Basin, GDGT fluxes show a correlation with opal fluxes, implying that GDGTs are mainly transported via aggregation with diatoms. The flux-weighted TEX86H temperatures derived from particles collected both at 853 m and 3921 m depth correspond to the water temperature (24.1 °C) of ca. 50 m depth, where nitrate concentration starts to increase. This suggests that nutrient concentrations may affect the depth habitat of Thaumarchaeota, thereby influencing the TEX86-derived water temperatures. In the coastal upwelling off Namibia, TEX86H temperatures are similar to satellite-derived sea surface temperature (SST) during the warm season, but the record derived from the trap is delayed relative to the SST by approximately 26 days. Warm biases, however, occur during the cold season. Higher TEX86 values have been found within the oxygen minimum zones (OMZs) in the water column of coastal upwelling regions. Thus, contributions from GDGTs produced in OMZs might explain the warmer temperature estimates during the cold season in regions where OMZs are pronounced. This scenario could explain the observed warm bias off Namibia. We, therefore, suggest that in the eastern Atlantic upwelling systems, nutrient depth distribution and GDGTs produced in OMZs can be potential environmental factors influencing TEX86 in sinking particles. In paleoenvironmental records of TEX86, non-thermal signals have to be considered on regional scales.

Methanogenic potential of lignites in Poland

Microbial coal gasification is of great interest due to the necessity to develop more environmentally friendly methods of energy production. The two main lignite deposits in Poland (Bełchatów and Turów) have been studied for their natural methanogenic potential. The biological origin of gases occurring in the investigated strata was confirmed by carbon stable isotope fractionation. Methanogenesis was examined via long-term anaerobic microcosm incubations which were performed using coal extracted from upper- and lower- most parts of the deposits at 10 to 40 °C. Present-day methane formation was distinguished from physical desorption by application of 2-bromoethane sulphonate (BES), an inhibitor of terminal methane formation steps. Furthermore, glycerol dialkyl glycerol tetraethers (GDGTs), were used to reconstruct the palaeoenvironmental conditions and explain how the lignite environment affects the methanogenic consortia.The data obtained reveal that microbial degradation of lignite occurs in the uppermost layers of the coal seam at the Bełchatów lignite mine. Methane release was up to 0.064 ng CH4 g−1 day−1 at 30 °C. Biomarker analysis shows high abundance of GDGT-0 confirming the presence of methanogenic Archaea and implies that pH was an important factor regulating the development and activity of organic matter-degrading microbiota under neutral conditions as the most favourable for the methanogens.Inactivation of methanogenic pathways by BES led to stimulation of bacterial respiration, which suggests the presence of an active bacterial community, putatively involved in the decomposition of lignin. The biomarker analysis, depicting metabolic capabilities of the microbiota (especially GDGT-0 and GDGT-0/crenarchaeol ratio) and recognition of palaeoenvironmental conditions, is a useful tool to determine the methanogenic potential of coals.

Mid-late Holocene temperature and precipitation variations in the Guanting Basin, upper reaches of the Yellow River

The reconstruction of prehistoric temperature and precipitation variations in the upper reaches of the Yellow River is essential for understanding the cultural evolution of the region, but related information is sparse due to the limitations of the available proxies. Recent studies have shown that microbial glycerol dialkyl glycerol tetraethers (GDGTs) are promising tools for reconstructing mean annual temperature (MAT) and mean annual precipitation (MAP) in terrestrial deposits. In this study, we reconstructed mid-late Holocene climatic changes using GDGT distributions in a loess-paleosol sequence in the Lajia Ruins of the Neolithic Qijia Culture, Guanting Basin, in the southwestern end of the Chinese Loess Plateau. Our GDGT records show that MAP decreased from ca. 600 mm to 430 mm, while MAT decreased from 11.9 °C to 8.0 °C, during the past ca. 7000 yr, and a drastic decline in MAP (70 mm), accompanied by a 0.8 °C decline in MAT, occurred at 3800–3400 yr BP. Our results provide direct evidence supporting a hypothesis that the flourishing (4200–4000 yr BP) and decline (4000–3600 yr BP) of the Qijia culture (mainly based on millets cultivation) and subsequent rise of the Xindian/Kayue culture (3600–2600 yr BP), based on mixed agriculture of sheep husbandry and millets cultivation were triggered by climate change.

Temperature changes reconstructed from branched GDGTs on the central Loess Plateau during the past 130–5 ka

This study investigated the glycerol dialkyl glycerol tetraethers (GDGTs) in a loess-paleosol sequence (LPS) to quantitatively reconstruct the mean annual air temperature (MAT) on the central Chinese Loess Plateau (CLP) during the past 130–5 ka. The results show that: (1) MAT variations reconstructed by global MBT (methylation index of branched tetraethers) and CBT (cyclisation ratio of branched tetraethers) transfer functions at Xifeng range from 7.3 °C to 28.4 °C. The MAT variation on the central CLP (Xifeng site) is between the previously reported MAT variations on the western (Yuanbao site) and eastern CLP (Mangshan site), in agreement with the pattern that temperature increases from the northwestern CLP to the southeastern CLP. (2) MAT variations estimated by transfer functions of global MBT-CBT, global MBT′-CBT and Chinese MBT-CBT for LPS are nearly all higher than the modern MAT at Xifeng, indicating that the reconstructed MAT values are probably overestimated. (3) The maximum value of magnetic susceptibility at Xifeng lagged that of the MAT derived by MBT-CBT proxy during the last interglacial (∼9 ka lag) and since the last deglacial (∼6 ka lag).

Distribution of glycerol dialkyl glycerol tetraethers in surface soils along an altitudinal transect at cold and humid Mountain Changbai: Implications for the reconstruction of paleoaltimetry and paleoclimate

Glycerol Dialkyl Glycerol Tetraethers (GDGTs) serve as important tools for the quantitative reconstruction of paleoclimate and paleoecology in both continental and marine environments. Previous studies of GDGTs in the terrestrial environments focused primarily on the soils from the relatively warm-humid or cold-dry regions. However, it is still unclear how GDGTs respond to environmental variables in the cold-humid regions. Here, we collected soils along an altitudinal transect of Mountain (Mt.) Changbai, which has a typical cold-humid climate, to investigate the distribution of GDGTs and the response of GDGT-based proxies to changes in climate along the transect. The shift in the distribution of archaeal isoprenoidal GDGTs (isoGDGTs) revealed that the archaeal community varied significantly along the transect, which can affect the relationship between TEX86 and mean annual air temperature (MAT). In addition, the increased temperature seasonality at higher altitudes exerted a significant impact on TEX86. We proposed a global calibration of TEX86 for the growing season temperature reconstruction in the soil environments: T =85.19×TEX86− 46.30 ( R 2=0.84, p 0.55 are diagnostic for the cold-humid climate. Thus, the combination of MBT′5me and MBT′6me has the potential as a tool for the identification of different types of paleoclimate.

Late Cretaceous climate in the Canadian Arctic: Multi-proxy constraints from Devon Island

Arctic climate in the Late Cretaceous has long been recognized to have been warm and wet relative to the present, but quantitative assessments of paleoclimate have been challenging due, in part, to disagreements between proxies in marine and terrestrial environments. This study provides a first multiproxy evaluation of Late Cretaceous (~93–90 Ma to 73–72 Ma) paleoclimate and paleohydrology from Devon Island in the Canadian High Arctic (modern location: 76°17′N, 91°12′W; Late Cretaceous location: ~71°30′N, ~24°30′W). Surface temperatures are reconstructed at ~12.6 to 20.6 °C for the ocean and 11.7 to 16.9 °C over land, using glycerol dialkyl glycerol tetraether (GDGT) based proxies measured from marine (TEX86) and terrestrial samples (MBT′5ME). These proxies are likely skewed toward warm month temperatures, based on novel analysis and interpretation of biomarkers in sediment and co-occurring marine vertebrate coprolites. The hydrogen isotopic composition (δ2H) of precipitation is constrained to have varied from −123‰ to −82‰ (VSMOW) using evidence from n-alkanes likely derived from higher plants. δ18O of shelfal marine surface water is constrained to have been between −10.5‰ to −3.4‰, using phosphate oxygen isotopes of marine vertebrate teeth and coprolites. From these, marine salinity is modeled to have varied from 10 PSU and 30 PSU, indicative of periodic freshwater influx. These estimates indicate that large marine vertebrates lived and fed, at least intermittently, in near-shore brackish waters. Finally, the Arctic was similarly warm in both the Late Cretaceous and Paleocene/Eocene, but the Late Cretaceous isotopic composition of precipitation at Devon Island was enriched in the heavy isotope of hydrogen by up to +60 to +70‰ relative to Arctic Eocene sites. The combination of techniques used here reduces uncertainties related to the application of proxies to an environment without a modern analogue, providing novel paleoclimatic constraints on the Late Cretaceous Arctic region.

Temperature and Monsoon Tango in a Tropical Stalagmite: Last Glacial-Interglacial Climate Dynamics

High-resolution paleoclimate data on stable isotopes in a stalagmite were coupled to glycerol dialkyl glycerol tetraethers (GDGTs). The Indian Summer Monsoon (ISM) transitioned from limited rainfall during the Last Glacial Maximum (LGM) to intense precipitation during early Holocene (22 to 6 ka). This was associated with changes in stalagmite growth, abundance of branched (br) and isoprenoid (iso) GDGTs, as well as δ18O, δ13C, Sr/Ca and GDGT-derived signals providing both temperature and moisture information. The reconstructed mean annual air temperature (MAAT) of the most modern stalagmite sample at ~19 °C, matches the surface and cave MAAT, but was ~4 °C lower during LGM. Warming at the end of LGM occurred before ISM strengthened and indicate 6 ka lag consistent with sea surface temperature records. The isotope records during the Younger Dryas show rapid progressions to dry conditions and weak monsoons, but these shifts are not coupled to TEX86. Moreover, change to wetter and stronger ISM, along with warmer Holocene conditions are not continuous indicating a decoupling of local temperatures from ISM.

Archaeal and bacterial glycerol dialkyl glycerol tetraether (GDGT) lipids in environmental samples by high temperature-gas chromatography with flame ionisation and time-of-flight mass spectrometry detection

Archaeal isoprenoidal glycerol dibiphytanyl glycerol tetraether lipids (iGDGTs) and their non-isoprenoidal branched bacterial analogues (brGDGTs) have widespread applications in biogeochemistry and paleothermometry. Analysis of GDGTs usually involves separation using high performance liquid chromatography, typically coupled via atmospheric pressure chemical ionisation to mass spectrometric detection in selected ion-monitoring mode (HPLC–APCI-MS). However, reliable determination of ratios and, in particular, quantification by this technique, can be challenging due to differences in ionisation efficiencies of the various compounds. Quantification of GDGTs also relies on external calibration of the relative response to an internal standard with authenticated GDGTs, which are often not readily accessible. Here, we tested the suitability of high temperature gas chromatography with flame ionisation detection (HTGC-FID) for the determination of concentrations and tetraether lipid-based ratios in marine and terrestrial samples. For this, we identified GDGTs in environmental samples using HTGC coupled to time-of-flight mass spectrometry (HTGC–MS). Using a purified GDGT standard, we show we can quantify GDGT-0 in environmental samples by GC-FID. Some GDGT-based ratios measured by HTGC-FID exhibited a linear correlation (1:1) with ratios derived from HPLC–MS and weight-based ratios of mixtures of purified standards. However, ratios relying on minor isomers, such as TEX86 and MBT/CBT have many unresolved challenges for determination by HTGC. Detection limits were higher than for HPLC–MS. However, the advantages of employing HTGC-based methods include: (1) the independence from MS tuning-related differences in ionisation energies; (2) the potential for direct comparison with other, non-GDGT based biomarkers; and (3) a more complete insight into biomarker distributions in environmental samples by the extension of the temperature range. Quantitative elution of GDGTs from a HTGC column as demonstrated herein, will also enable their analysis by compound-specific isotope ratio mass spectrometry.

Sources and distribution of tetraether lipids in sediments from the Zhejiang–Fujian coastal mud area, China, over the past 160 years: Implications for paleoclimate change

Proxies based on glycerol dialkyl glycerol tetraether (GDGT) lipids in surface sediments have been successfully used to reconstruct sea surface temperature (SST), mean annual air temperature (MAAT), and soil pH in the East China Sea and its adjacent ocean margins. In this study, two cores from the Zhejiang–Fujian coastal mud area (ZFCMA) were studied in detail to investigate temporal changes in the source and distribution of tetraether lipids in the area, and the implications of such data for paleoclimate change. The results indicate that isoprenoid GDGTs (isoGDGTs) in the ZFCMA were derived mainly from Thaumarchaeota, with a minor contribution from in situ and allochthonous methanogenic archaea in upwelling areas, whereas branched GDGTs (brGDGTs) originated mainly from terrigenous bacteria and in situ organisms. However, the minor methanogen-derived and allochthonous isoGDGTs were considered to have had no effect on inferred SSTs in this study. The presence of brGDGTs produced in situ might corrupt the relationship between the methylation index of branched tetraethers or the cyclization index of branched tetraethers (MBT/CBT) and MAAT or pH, particularly in hypoxic areas. However, #ringtetra data, used here as an index to evaluate the sources of the brGDGTs, indicated that the brGDGTs buried in the sediments are predominantly derived from soil erosion. Furthermore, temporal changes in the GDGT-based proxy records indicate that climate changes and events (e.g., relating to the East Asian Monsoon, Pacific Decadal Oscillation (PDO)/Kuroshio Current (KC) intensity, and flood events), as well as anthropogenic influences (e.g., dam construction, soil conservation, fertilizer usage, and discharge of industrial waste water and domestic sewage) affect the signal in the core sediments. SSTs fluctuated over the past 160 years in the ZFCMA, with higher values in the 20th century than in the 19th century. In addition, the humidity, as inferred from the relationship between precipitation/soil water content and pH, and continental temperature in the mid–lower Yangtze River basin switched between dry–hot/dry–cool and wet–hot/wet–cool over the last 160 years. The results also indicate that PDO/KC intensity plays a key role in temperature changes and has a larger impact on terrestrial ecosystems than on ocean ecosystems in the ZFCMA and the adjacent continent.

Archaeal and bacterial H-GDGTs are abundant in peat and their relative abundance is positively correlated with temperature

Glycerol monoalkyl glycerol tetraether lipids (GMGTs; also called ‘H-GDGTs’) differ from the more commonly studied glycerol dialkyl glycerol tetraether (GDGTs) in that they have an additional covalent bond that links the two alkyl chains. Six different archaeal isoprenoidal H-GDGTs (H-isoGDGTs) and one branched H-GDGT (H-brGDGT), presumably produced by bacteria, have previously been found. However, the function of H-GDGTs in both domains of life is unknown. It is thought that the formation of this additional covalent bond results in enhanced membrane stability, accounting for the high abundance of H-GDGTs in extreme environments such as geothermal settings, but so far there has been little evidence to support this hypothesis.Here we report the distribution of H-GDGTs in a global peat database (n = 471) with a broad range in mean annual air temperature (MAAT) and pH. This is the first finding of H-GDGTs in soils (specifically, peat), highlighting that H-GDGTs are widespread in mesophilic settings. In addition, we report the presence of two new H-brGDGTs with one (H-1034) and two (H-1048) additional methyl groups, respectively. Our results suggest that the relative abundance of both bacterial and archaeal H-GDGTs compared to regular GDGTs is related to temperature with the highest relative abundance of H-GDGTs in tropical peats. Although other factors besides temperature likely also play a role, these results do support the hypothesis that H-GDGTs are an adaptation to temperature to maintain membrane stability. The observation that both bacterial and archaeal membrane lipids respond to temperature indicates the same adaption across the lipid divide between these two domains of life, suggesting parallel or convergent evolution (potentially facilitated by lateral gene transfer).

The importance of mass accuracy in selected ion monitoring analysis of branched and isoprenoid tetraethers

Among the new proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs), the BIT index (Branched and Isoprenoid Tetraether index) is one of the most difficult to determine accurately, as shown by two round-robin GDGT studies. Sensitivity to mass spectrometer settings and tuning, and a diversity of mass spectrometry techniques may explain the relatively large observed interlaboratory scatter. However, the mass defect difference between crenarchaeol and branched GDGTs (brGDGTs) has never been specifically scrutinized. In this study, we analyzed five sediment samples with contrasting BIT values using about 60 m/z values to assess the shape of GDGT peaks using selected ion monitoring. We then assessed the biases in relative GDGT signals and mass spectrometry-derived BIT values under two scenarios which ignore the systematic mass defect difference between crenarchaeol and brGDGTs. Our results show that approximate mass selection for GDGT analysis using selected ion monitoring generates losses of relative GDGT signals of up to 36%. The observed effects on BIT values are maximal for intermediate BIT values, with shifts of BIT values of ±0.1 unit. The shifts of BIT values due to approximate mass selection are thus not negligible compared to the interlaboratory scatter evidenced by the latest round-robin GDGT study.

Rhizospheric effects on the microbial community of e-waste-contaminated soils using phospholipid fatty acid and isoprenoid glycerol dialkyl glycerol tetraether analyses.

We performed the study of rhizospheric effects on soil microbial community structure, including bacteria, fungi, actinomycete, and archaea, at an electronic waste (e-waste) recycling site by analyzing the phospholipid fatty acid (PLFA) and isoprenoid glycerol dialkyl glycerol tetraether (GDGT) contents. By comparing PLFA and isoprenoid GDGT profiles of rhizospheric and surrounding bulk soils of 11 crop species, we observed distinct microbial community structures. The total PLFA concentration was significantly higher in rhizospheric soils than in non-rhizospheric soils, whereas no obvious difference was found in the total isoprenoid GDGT concentrations. The microbial community structure was also different, with higher ratios of fungal-to-bacterial PLFAs (F/B) and lower relative abundance of Gram-positive bacteria in rhizospheric soils. The extent of rhizospheric effects varied among plant species, and Colocasia esculenta L. had the greatest positive effects on the total microbial biomass. Dissolved organic carbon and pH were the main environmental factors affecting the microbial community represented by PLFAs, while the archaeal community was influenced by copper and zinc in all soils. These results offer a comprehensive view of rhizospheric effects on microbes in heavy metal and persistent organic pollutant co-contaminated soil, and provide fundamental knowledge regarding microbial ecology in e-waste-contaminated soils.

Evaluating Production of Cyclopentyl Tetraethers by Marine Group II Euryarchaeota in the Pearl River Estuary and Coastal South China Sea: Potential Impact on the TEX86 Paleothermometer.

TEX86 [TetraEther indeX of glycerol dialkyl glycerol tetraethers (GDGTs) with 86 carbon atoms] has been widely applied to reconstruct (paleo-) sea surface temperature. Marine Group I (MG-I) Thaumarchaeota were thought to be the primary source of GDGTs constituting the TEX86 formula; however, recent research has suggested that Marine Group II (MG-II) Euryarchaeota may also contribute significantly to the GDGT pool in the ocean. Little is known regarding the potential impact of MG-II Euryarchaeota-derived GDGTs on TEX86 values recorded in marine sediments. In this study, we assessed the relationship between distributions of GDGTs and MG-II Euryarchaeota and evaluated its potential effect on the TEX86 proxy. Lipid and DNA analyses were performed on suspended particulate matter and surface sediments collected along a salinity gradient from the lower Pearl River (river water) and its estuary (mixing water) to the coastal South China Sea (SCS, seawater). TEX86-derived temperatures from the water column and surface sediments were significantly correlated and both were lower than satellite-based temperatures. The ring index (RI) values in these environments were higher than predicted from the calculated TEX86-RI correlation, indicating that the GDGT pool in the water column of the PR estuary and coastal SCS comprises relatively more cyclopentane rings, which thereby altered TEX86 values. Furthermore, the abundance of MG-II Euryarchaeota 16S rRNA gene in the mixing water was two to three orders of magnitude higher than those observed in the river or seawater. Significant linear correlations were observed between the gene abundance ratio of MG-II Euryarchaeota to total archaea and the fractional abundance of GDGTs with cyclopentane rings. Collectively, these results suggest that MG-II Euryarchaeota likely produce a large proportion of GDGTs with 1–4 cyclopentane moieties, which may bias TEX86 values in the water column and sediments. As such, valid interpretation of TEX86 values in the sediment record, particularly in coastal oceans, should consider the contribution from MG-II Euryarchaeota.

Tropical and high latitude forcing of enhanced megadroughts in Northern China during the last four terminations

Understanding the origin and evolutionary history of drought events is of great significance, providing critical insight into future hydrological conditions under the changing climate. Due to the scarcity of drought proxies from northern China, the occurrence and underlying mechanisms of the drought events remains enigmatic on longer timescales. Here we utilize microbial lipid proxies to reconstruct significant drought events over the last four ice age terminations in the southernmost section (Weinan section) of the Chinese Loess Plateau. The abundance of archaeal isoprenoid GDGTs (glycerol dialkyl glycerol tetraethers) relative to bacterial branched GDGTs, measured by Ri/b and BIT indices, is diagnostic of enhanced drought conditions. The Ri/b (and BIT) indices are stable and low (high) throughout most of the loess section spanning the last 350 thousand years, but they do exhibit sharp transient peaks (valleys) during the intervals associated with the four ice age terminations, and especially Terminations II and IV. These enhanced drought events are, non-intuitively, associated with a significant decrease in the relative abundance of C4 plants, inferred by a decrease in the carbon isotope composition of bulk organic matter. Although the microbial records show some consistency with the Weinan grain size profiles, indicative of Eastern Asian winter monsoon variability, they also show some apparent difference. In fact, some features of the microbial records exhibit strong similarities with marine sediment planktonic foraminiferal δ13C records from the western Pacific warm pool, which reflect ENSO-like changes during glacial terminations. Therefore, enhanced droughts immediately before the interglacial warming in northern China could be explained, at least in part, by teleconnections in tropical ocean–atmosphere circulation via shifts in the Intertropical Convergence Zone (ITCZ) and associated Jet Stream over the Asian continent. According to our microbial biomarker data, these enhanced megadroughts are apparently different, both in terms of severity and causal mechanism, from the more commonly discussed dry conditions observed during glacial periods.