Dialkyl Glycerol Tetraethers Research Articles

Disentangling seasonal and annual precipitation signals in the tropics over the Holocene: Insights from δD, alkanes and GDGTs

Rainfall seasonality in the tropics has a substantial impact on both ecosystems and human livelihoods. Yet, reconstructions of past rainfall variability have so far generally been unable to differentiate between annual and seasonal precipitation changes. Past variations in seasonality are therefore largely unknown. Here, we disentangle hydrogen isotopic (δD) signals from terrestrial leaf waxes and algae in an 8000-year peat core from Sumatra, which reflect annual versus wet season rainfall signals, respectively. We validate these results using lipid biomarkers by reconstructing vegetation dynamics via n-alkane distributions and peatland hydrological conditions using glycerol dialkyl glycerol tetraethers (GDGTs), as well as biomass burning using levoglucosan concentrations in the core. Finally, we compare our proxy results to a transient climate model simulation (MPI-ESM1.2) to identify the mechanism for seasonality changes. We find that algal δD indicates stronger Indonesian-Australian Summer Monsoon (IASM) precipitation in the Mid-Holocene, between 8 and 4.2 cal ka BP. A period of alternating flooding, droughts and wildfires is reconstructed between 6 and 4.2 cal ka BP, implicating very strong monsoonal precipitation and drying out and burning during a longer and intensified dry season. We attribute this strong rainfall seasonality in the Mid-Holocene mainly to orbitally forced insolation seasonality and a strengthened IASM, consistent with the modeling results. In terms of annual rainfall, terrestrial plant δD, vegetation composition and GDGTs all indicate wetter conditions peaking between 3 and 4.5 cal ka BP, preceded by drier conditions, followed by drastic and rapid drying in the late Holocene from around 2.8 cal ka BP. Our multiproxy annual precipitation reconstruction thereby indicates the wettest overall conditions approximately 1500–2000 years later than a nearby speleothem δ18O record, which instead follows the seasonally biased algal δD in our record. We, therefore, hypothesize that speleothem reconstructions over the Holocene in parts of the tropics with low but significant seasonality may carry a stronger seasonal component than previously suggested. The data presented here contribute with new insights on how isotopic rainfall proxies in the tropics can be interpreted. Our findings resolve the seasonal versus annual components of Holocene rainfall variability in the Indo-Pacific Warm Pool region, highlighting the importance of considering seasonality in rainfall reconstructions.

Extreme drought events and land surface temperature variations on the northeastern Tibetan Plateau over the last 350 ka

The northeastern Tibetan Plateau (NETP) is located in the ecologically vulnerable northwestern China, with semi-humid and semi-arid conditions. Long-term paleo-hydrological and paleo-temperature variations and their mechanisms over the NETP remain ambiguous, primarily due to the limited availability of well-dated, long-term depositional archives and suitable proxies. In this paper, we investigate climate variations and their underlying mechanisms in the region using Glycerol Dialkyl Glycerol Tetraethers (GDGTs) spanning the last 350 ka. Our results suggest that the NETP underwent substantial drying during glacial periods, with the driest period occurring during the last glacial period. The intensity of drought during glacial periods is comparable to that in the Westerlies-dominated region but differs from that in the East Asian monsoon region. This suggests that the influence of the Westerlies surpassed that of the East Asian summer monsoon (EASM), becoming the major driver of warm-season drought events on the NETP during glacial periods. Additionally, the peaks of aridity events coincide with summer insolation maxima during glacial periods, which may be related to the direct influence of local insolation on regulating evaporation. The reconstructed land surface temperatures were about 6.7 °C during interglacial periods and 2.6 °C during glacial periods, which may be linked to a combination of variations in insolation, CO2 levels, and other internal feedback mechanisms. However, an unusual warming occurred during the last glaciation, with an average temperature of about 4.8 °C. This warming may be related to variations in soil moisture and vegetation resulting from extreme drought. Our study highlights the sensitivity of hydrological variations over the NETP to insolation and Westerlies, as well as the critical roles of soil moisture and vegetation in land surface temperature variations.

Decoupled terrestrial temperature and hydroclimate during the Plio-Pleistocene in the East Asian monsoonal region

The independent reconstructions of temperature and hydroclimate records are crucial to understanding past changes in Earth's climate. However, the temperature and hydroclimate history of East Asia during the late Cenozoic remains largely unknown. Here, we reconstructed terrestrial temperature and hydroclimate patterns in the East Asian monsoonal regions using GDGTs (glycerol dialkyl glycerol tetraethers) in a sediment core from North China spanning 5.5 million years (Ma). Our data show a decoupled terrestrial temperature and hydroclimate evolution over the past 5.5 Ma. The methylation index of branched GDGTs (brGDGTs), i.e. MBT'5ME-inferred temperature, significantly decreases at ∼2.6 Ma and likely reflects a response to Northern Hemisphere glaciation (NHG), in agreement with multiple marine temperature records. The MBT'5ME temperature record shows a decline of approximately 6.5 °C at around 2.6 Ma, which is larger than the decline recorded in marine sequences. Interestingly, this temperature decline occurs much later than the large GDGT-inferred (i.e., Ri/b and GDGT-0/Cren) increase in rainfall at ∼4.0 Ma, which aligns with the strengthening of Pacific zonal and meridional sea surface temperature (SST) gradients as well as the increased marine-land temperature gradients. Our study thus suggests differing mechanisms driving temperature and hydroclimate evolution in East Asia during the Plio-Pleistocene.

A sedimentary lipid biomarker record of Holocene temperature variations and drought events in northern China

Holocene hydroclimate changes are crucial not only for predicting future climate changes, but also for understanding their impacts on ancient societies. However, our knowledge about severe drought events in East Asia during the Holocene is largely fragmentary and the pattern of temperature changes is still uncertain. Here, we, for the first time, present high-resolution quantitative records of temperature and drought events in northern China throughout the Holocene from a well-dated loess-paleosol sequence (Weinan) on the southern Chinese Loess Plateau (CLP). Our inferred temperatures, derived from glycerol dialkyl glycerol tetraethers (GDGTs), indicate that the early-to-mid Holocene experienced warm conditions, with temperatures ranging from ∼17.7 to 20.2 °C. This was followed by a cooler late Holocene, which contrasts with the results of climate model simulations. In addition, our hydroclimate record reveals that severe drought conditions occurred during both the early and late Holocene. These intensified drought events coincide with the demise of several Chinese dynasties. We propose that the colder climate during the late Holocene led to a strengthening of the East Asian winter monsoon and a weakening of the East Asian summer monsoon over northern China, as well as the southward shift of the Intertropical Convergence Zone (ITCZ), causing more severe drought conditions during the late Holocene compared to the early Holocene. Our results extend the East Asian Holocene climate records, highlighting the importance of understanding how hydroclimate extremes may have impacted human civilization in East Asia.

Interlaboratory Comparison of Branched GDGT Temperature and pH Proxies Using Soils and Lipid Extracts

AbstractRatios of glycerol dialkyl glycerol tetraethers (GDGT), which are membrane lipids of bacteria and archaea, are at the base of several paleoenvironmental proxies. They are frequently applied to soils as well as lake‐ and marine sediments to generate records of past temperature and soil pH. To derive meaningful environmental information from these reconstructions, high analytical reproducibility is required. Based on submitted results by 39 laboratories from across the world, which employ a diverse range of analytical and quantification methods, we explored the reproducibility of brGDGT‐based proxies (MBT′5ME, IR, and #ringstetra) measured on four soil samples and four soil lipid extracts. Correct identification and integration of 5‐ and 6‐methyl brGDGTs is a prerequisite for the robust calculation of proxy values, but this can be challenging as indicated by the large inter‐interlaboratory variation. The exclusion of statistical outliers improves the reproducibility, where the remaining uncertainty translates into a temperature offset from median proxy values of 0.3–0.9°C and a pH offset of 0.05–0.3. There is no apparent systematic impact of the extraction method and sample preparation steps on the brGDGT ratios. Although reported GDGT concentrations are generally consistent within laboratories, they vary greatly between laboratories. This large variability in brGDGT quantification may relate to variations in ionization efficiency or specific mass spectrometer settings possibly impacting the response of brGDGTs masses relative to that of the internal standard used. While ratio values of GDGT are generally comparable, quantities can currently not be compared between laboratories.

Biosynthesis of GMGT lipids by a radical SAM enzyme associated with anaerobic archaea and oxygen-deficient environments

Archaea possess characteristic membrane-spanning lipids that are thought to contribute to the adaptation to extreme environments. However, the biosynthesis of these lipids is poorly understood. Here, we identify a radical S-adenosyl-L-methionine (SAM) enzyme that synthesizes glycerol monoalkyl glycerol tetraethers (GMGTs). The enzyme, which we name GMGT synthase (Gms), catalyzes the formation of a C(sp3)–C(sp3) linkage between the two isoprenoid chains of glycerol dialkyl glycerol tetraethers (GDGTs). This conclusion is supported by heterologous expression of gene gms from a GMGT-producing species in a methanogen, as well as demonstration of in vitro activity using purified Gms enzyme. Additionally, we show that genes encoding putative Gms homologs are present in obligate anaerobic archaea and in metagenomes obtained from oxygen-deficient environments, and appear to be absent in metagenomes from oxic settings.

Structural insights linking H-bridging of archaeal GDGTs to high temperature

Isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), characteristic membrane lipids of archaea, are widely used in ecological and geochemical studies, especially for paleoenvironmental reconstruction. Glycerol monoalkyl glycerol tetraethers (GMGTs, also known as H-GDGTs), a unique variant of GDGTs, have covalent bonds linking the two alkyl chains. Despite some studies suggesting a link between GMGTs and high temperatures, the reliability and mechanisms remain unclear. Using molecular dynamics simulations, we elucidated the mechanism connecting GMGTs to high temperatures. Our findings show that H-bridging linkages reduce the distance between alkyl chains, leading to thicker and denser membranes with lower fluidity and permeability. The diffusion coefficient of GMGTs decreased by approximately 35 % compared to GDGTs, indicating their role as a archaeal high-temperature adaptation. This study provides a mechanistic basis for using archaeal GMGTs in geochemical studies and enhances confidence in their use for paleotemperature reconstruction.

Identification of two archaeal GDGT lipid–modifying proteins reveals diverse microbes capable of GMGT biosynthesis and modification

Archaea produce unique membrane-spanning lipids (MSLs), termed glycerol dialkyl glycerol tetraethers (GDGTs), which aid in adaptive responses to various environmental challenges. GDGTs can be modified through cyclization, cross-linking, methylation, hydroxylation, and desaturation, resulting in structurally distinct GDGT lipids. Here, we report the identification of radical SAM proteins responsible for two of these modifications-a glycerol monoalkyl glycerol tetraether (GMGT) synthase (Gms), responsible for covalently cross-linking the two hydrocarbon tails of a GDGT to produce GMGTs, and a GMGT methylase (Gmm), capable of methylating the core hydrocarbon tail. Heterologous expression of Gms proteins from various archaea in Thermococcus kodakarensis results in the production of GMGTs in two isomeric forms. Further, coexpression of Gms and Gmm produces mono- and dimethylated GMGTs and minor amounts of trimethylated GMGTs with only trace GDGT methylation. Phylogenetic analyses reveal the presence of Gms homologs in diverse archaeal genomes spanning all four archaeal superphyla and in multiple bacterial phyla with the genetic potential to synthesize fatty acid-based MSLs, demonstrating that GMGT production may be more widespread than previously appreciated. We demonstrate GMGT production in three Gms-encoding archaea, identifying an increase in GMGTs in response to elevated temperature in two Archaeoglobus species and the production of GMGTs with up to six rings in Vulcanisaeta distributa. The occurrence of such highly cyclized GMGTs has been limited to environmental samples and their detection in culture demonstrates the utility of combining genetic, bioinformatic, and lipid analyses to identify producers of distinct archaeal membrane lipids.

Disentangling influences of climate variability and lake-system evolution on climate proxies derived from isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs): the 250 kyr Lake Chala record

Abstract. High-resolution paleoclimate records from tropical continental settings are greatly needed to advance understanding of global climate dynamics. The International Continental Scientific Drilling Program (ICDP) project DeepCHALLA recovered a 214.8 m long sediment sequence from Lake Chala, a deep and permanently stratified (meromictic) crater lake in eastern equatorial Africa, covering the past ca. 250 000 years (250 kyr) of continuous lacustrine deposition since the earliest phase of lake-basin development. Lipid biomarker analyses on the sediments of Lake Chala can provide quantitative records of past variation in temperature and moisture balance from this poorly documented region. However, the degree to which climate proxies derived from aquatically produced biomarkers are affected by aspects of lake developmental history is rarely considered, even though it may critically influence their ability to consistently register a particular climate variable through time. Modern-system studies of Lake Chala revealed crucial information about the mechanisms underpinning relationships between proxies based on isoprenoid (iso-) and branched (br-) glycerol dialkyl glycerol tetraethers (GDGTs) and the targeted climate variables, but the persistence of these relationships in the past remains unclear. Here we assess the reliability of long-term climate signals registered in the sediments of Lake Chala by comparing downcore variations in GDGT distributions with major phases in lake-system evolution as reflected by independent proxies of lake depth, mixing regime and nutrient dynamics: seismic reflection data, lithology and fossil diatom assemblages. Together, these records suggest that during early lake history (before ca. 180–200 ka) the distinct mixing-related depth zones with which specific GDGT producers are associated in the modern-day lake were not yet formed, likely due to more open lake hydrology and absence of chemical water-column stratification. Consequently absolute GDGT concentrations dating to this period are relatively low, proxies sensitive to water-column stratification (e.g., branched versus isoprenoid tetraether (BIT) index) display highly irregular temporal variability, and correlations between proxies are dissimilar to expectations based on modern-system understanding. A sequence of lake-system changes between ca. 180–200 and ca. 80 ka first established and then strengthened the chemical density gradient, promoting meromictic conditions despite the overall decrease in lake depth due to the basin gradually being filled up with sediments. From ca. 180 ka onward some GDGTs and derived proxies (e.g., crenarchaeol concentration, BIT index and IR6Me) display strong ∼ 23 kyr periodicity, likely reflecting the predominantly precession-driven insolation forcing of Quaternary climate variability in low-latitude regions. Our results suggest that GDGT-based temperature and moisture-balance proxies in Lake Chala sediments reflect the climate history of eastern equatorial Africa from at least ca. 160 ka onwards, i.e., covering the complete last glacial–interglacial cycle and the penultimate glacial maximum. This work confirms the potential of lacustrine GDGTs for elucidating the climate history of tropical regions at Quaternary timescales, provided they are applied to suitably high-quality sediment archives. Additionally, their interpretation should incorporate a broader understanding of the extent to which lake-system evolution limits the extrapolation back in time of proxy-climate relationships established in the modern system.

The global distribution of Isoprenoidal Glycerol Dialkyl Diethers (isoGDDs) is consistent with a predominant degradation origin

Glycerol dialkyl diethers (GDDs) are membrane lipids and a variation of the more commonly known glycerol dialkyl glycerol tetraethers (GDGTs). GDGTs include both archaeal and bacterial membrane lipids that are both frequently used for paleoclimate reconstruction in a range of terrestrial and aquatic environments. In contrast to GDGTs, GDDs lack one of the terminal glycerol moieties. Although both isoprenoidal (iso) and branched (br) GDDs have been found, this study focuses on isoGDDs. These lipids have been found in a few terrestrial and aquatic environments. However, the origin of isoGDDs is debated and the extent of their distribution across the surface of the Earth is poorly constrained. Based on a few single site studies, previous authors hypothesised that isoGDDs are degradation products of isoGDGTs, but more recent studies that isolated isoGDDs from cultured nitrososphaerota (formerly thaumarchaeota) proposed a biological source through direct archaeal biosynthesis. Here we compiled a global dataset of isoGDD and isoGDGT abundance in environmental samples to thoroughly investigate the distribution of isoGDDs and the correlation with isoGDGTs on a global scale and across a variety of environments (peat, mineral soils, lake sediments, and marine sediments). We find that isoGDDs are present in most samples that we analysed. Their abundance is strongly proportional to isoGDGT abundance (r2 = 0.85), dominated by the GDGT-crenarchaeol/GDD-crenarchaeol ratio (r2 = 0.94) and supported by individual compound isoGDGT/isoGDD ratios (r2 = 0.56–0.94). In addition, the degree of cyclisation of isoGDDs, reflected in the ring index, is positively correlated (r2 = 0.84) with that of isoGDGTs across all environments. We conclude that isoGDDs are abundant on the surface of the Earth and predominantly originate from the degradation of isoGDGTs.

Holocene Paleoclimate Records in Equatorial West Africa: Insights Based on the Characterization of Glycerol Dialkyl Glycerol Tetraethers

One gravity core retrieved from the Niger Delta was used to explore the origin of deposited organic matter (OM) and the paleo-climatic and environmental conditions over the Holocene in equatorial West Africa. The geochemical properties of sediments including glycerol dialkyl glycerol tetraethers (GDGTs) and elemental (%OC, %N, C/N) and isotopic (δ13Corg, δ15N) signatures were determined. The determination constrained the age of the column and revealed that the sediment OM was mainly derived from a marine source. The isoprenoid (iso)GDGTs were the dominant GDGTs, with a small amount of branched (br)GDGTs, which led to a low-branched and isoprenoid tetraether index (BIT, 0.02–0.21) and represented a low terrestrial input. Most isoGDGTs and OH-GDGTs were produced in situ by Marine Group I (MG-I) Thaumarchaeota, while the brGDGTs were mainly transported from land. A two-endmember model quantified the contribution of terrestrial OM, as 0.9–19.9% by BIT and 1.1–32.6% by δ13C. Accordingly, the millennium-scale sea surface temperatures (SSTs) were reconstructed based on the cyclopentane ring distribution (TEX86H) and the ring index of OH-GDGTs (RI-OH). The top core SSTs were lower than the modern mean annual SST due to the growth season and habitat depth of Thaumarchaeota. The reconstructed SSTs clearly revealed the four stages of paleoclimate change, in particular, the drought episode of 8.2 kyr and the following humid period. The above research has enhanced our understanding of the paleoclimate change in river outflow during the Holocene at the millennium scale.

Archaea membranes in response to extreme acidic environments

Bipolar tetraether lipids (BTL), such as glycerol dialkyl calditol tetraether (GDNT) and glycerol dialkyl glycerol tetraether (GDGT), are the dominating lipid species in thermoacidophiles that inhabit at pH ≤ 4 and temperatures ≥65°C. BTL containing archaea membranes respond to environmental pH changes by varying the number of cyclopentane rings in the isoprenoids, the amount of GDNT relative to GDGT, the ratio of tetraethers to diethers, and the level of glycosylation in polar headgroups. These structural and compositional adjustments can alter the hydrogen bond networks in the membrane polar headgroup regions and the packing tightness and rigidity in the membrane hydrophobic core. It is likely that these changes in non-covalent interactions among archaea lipids are made to retain low membrane volume fluctuations and their low sensitivity to temperature, as illustrated in the case of liposomes made of the polar lipid fraction E (PLFE) of Sulfolobus acidocaldarius. As such, a low passive proton permeability and a near neutral intracellular pH can be maintained, and, as a result, optimal activities of soluble and membrane-bound proteins in thermoacidophiles can be retained in acidic growth conditions at elevated growth temperatures.

Structure-function relationships in pure archaeal bipolar tetraether lipids.

Archaeal bipolar tetraether lipids (BTLs) are among the most unusual lipids occurring in nature because of their presumed ability to span the entire membrane to form a monolayer structure. It is believed that because of their unique structural organization and chemical stability, BTLs offer extraordinary adaptation to archaea to thrive in the most extreme milieus. BTLs have also received considerable attention for development of novel membrane-based materials. Despite their fundamental biological significance and biotechnological interests, prior studies on pure BTLs are limited because of the difficulty to extract them in pure form from natural sources or to synthesize them chemically. Here we have utilized chemical synthesis to enable in-depth biophysical investigations on a series of chemically pure glycerol dialkyl glycerol tetraether (GDGT) lipids. The lipids self-assemble to form membrane-bound vesicles encapsulating polar molecules in aqueous media, and reconstitute a functional integral membrane protein. Structural properties of the membranes were characterized via small-angle X-ray scattering (SAXS) and cryogenic electron microscopy (cryo-EM). SAXS studies on bulk aqueous dispersions of GDGT lipids over 10-90 °C revealed lamellar and non-lamellar phases and their transitions. Next we asked whether vesicles overwhelmingly composed of a single GDGT species can undergo fusion as it is difficult to conceptualize such behavior with the assumption that such membranes have a monolayer structure. Interestingly, we observed that GDGT vesicles undergo fusion with influenza virus with lipid mixing kinetics comparable to that with vesicles composed of monopolar phospholipids. Our results suggest that GDGT membranes may consist of regions with a bilayer structure or form bilayer structures transiently which facilitate fusion and thus offer insight into how archaea may perform important physiological functions that require dynamical membrane behavior.

Glycerol dialkyl glycerol tetraethers in surficial marine sediments across shelf-to-basin depth transects in the East China Sea: Their sources and potential as sea water temperature proxies

To better understand the compositions and sources of glycerol dialkyl glycerol tetraethers (GDGTs) and their use as proxies in the East China Sea (ECS), we analysed GDGTs in surficial sediments from the ECS outer shelf, slope and Okinawa Trough (OT) in open marine settings between 126°E and 129°E at water depths of either <100 or > 1000 m. The GDGT-0/Cren. values (0.35 ± 0.04) and methane index (MI) (0.17 ± 0.02) were generally low, indicating a negligible contribution of methanogenic Euryarchaeota GDGTs to the open marine settings in the ECS and that isoprenoid GDGTs (isoGDGTs) were mainly derived from marine planktonic Thaumarchaeota. Comprehensive analysis of the degree of cyclisation (DC) (0.34–0.53), #ringstetra (0.53–0.94), and IIIa/IIa (0.54–3.24) indices indicated that branched GDGTs (brGDGTs) may be predominantly produced in situ, with minimal soil contribution in this setting. The TEXH86-derived sea surface temperatures (SST) were closer to the annual SST than the seasonal SST from the World Ocean Atlas (WOA) 2013. The methylation of the branched tetraethers (MBT) index correlated well with the mean annual bottom water temperature (BWT) from WOA 2013. Therefore, we propose that the MBT index from open marine settings in the ECS may reflect BWT changes in marine bottom environments.

Molecular dynamics simulations reveal methylation in Me-GDGTs as a microbial low-temperature adaptation

Tetraether membrane lipid glycerol dialkyl glycerol tetraether (GDGT) is a characteristic lipid of archaea that contributes to adaptation to a variety of environments and has been widely used in ecological and geochemical studies. Alkyl-chain methyl-modified GDGTs (Me-GDGTs) as one important component have been detected in both environmental and culture samples. However, the effect of methyl modification on archaeal membrane lipids remains unclear, hampering the development of environmental proxies based on the exact understanding of the underlying mechanism. Here, we reveal the perturbation of molecular order by methyl modification through molecular dynamics simulations, showing that methyl modification can increase membrane fluidity, explaining that this is a form of low-temperature acclimation. This hypothesis was also validated in environmental samples. This study reveals the role of methyl modifications in low-temperature adaptation in archaea, expanding that hydrophobic carbon chain methylation has unified physiological significance in three domain life. Our study highlights the role of molecular dynamics simulations in the development of lipid biomarkers, provides new insights into the mechanisms of temperature adaptation in microorganisms, and lays the groundwork for the development of new environmental proxies.

The impact of soil chemistry, moisture and temperature on branched and isoprenoid GDGTs in soils: A study using six globally distributed elevation transects

Glycerol dialkyl glycerol tetraethers (GDGTs) are microbial membrane-spanning lipids that are produced in a variety of environments. To better understand the potentially confounding effect of soil chemistry on the temperature relationship of branched GDGTs (brGDGTs), isoprenoid GDGTs (isoGDGTs) and GDGT-based proxies MBT’5ME and TEX86, soils from 6 elevation transects (mean annual air temperature 0 – 26 ℃, n = 74) were analyzed. Corroborating earlier work, the MBT’5ME index correlates well with mean annual air temperature in the low pH (pH < 7), non-arid soils under study (r = 0.87, p < 0.001). However, a clear over-estimation of reconstructed temperature in the lowest pH (<3.5) soils is observed, explained by the correlation between brGDGT Ia and free acidity. TEX86 also shows a significant correlation with mean annual air temperature (r = 0.45, p < 0.001), driven by temperature dependent concentration changes of isoGDGTs 3 and cren’. However, an overarching correlation with P/E values dominates concentration changes of all supposed Thaumarchaeotal isoGDGTs lipids (GDGT1-3, cren and cren’), implying a potential impact of soil moisture on TEX86 values. In addition to identifying the impact of these confounding factors on the temperature proxy, GDGT ratios that can be used to constrain changes in soil chemistry, specifically exchangeable Ca2+, sum of basic cations, exchangeable Fe3+ and sum of soil metals are proposed (0.53 < r2 < 0.68), while existing ratios for soil moisture availability are tested for the first time in a dataset of non-arid soils. While the impact of soil chemistry on GDGTs may complicate the interpretation of their temperature proxies, our proposed GDGT ratios can potentially be used to constrain a subset of soil chemistry changes through time.

Lipidomic diversity and proxy implications of archaea from cold seep sediments of the South China Sea

Cold seeps on the continental margins are characterized by intense microbial activities that consume a large portion of methane by anaerobic methanotrophic archaea (ANME) through anaerobic oxidation of methane (AOM). Although ANMEs are known to contain unique ether lipids that may have an important function in marine carbon cycling, their full lipidomic profiles and functional distribution in particular cold-seep settings are still poorly characterized. Here, we combined the 16S rRNA gene sequencing and lipidomic approaches to analyze archaeal communities and their lipids in cold seep sediments with distinct methane supplies from the South China Sea. The archaeal community was dominated by ANME-1 in the moderate seepage area with strong methane emission. Low seepage area presented higher archaeal diversity covering Lokiarchaeia, Bathyarchaeia, and Thermoplasmata. A total of 55 core lipids (CLs) and intact polar lipids (IPLs) of archaea were identified, which included glycerol dialkyl glycerol tetraethers (GDGTs), hydroxy-GDGTs (OH-GDGTs), archaeol (AR), hydroxyarchaeol (OH-AR), and dihydroxyarchaeol (2OH-AR). Diverse polar headgroups constituted the archaeal IPLs. High concentrations of dissolved inorganic carbon (DIC) with depleted δ13CDIC and high methane index (MI) values based on both CLs (MICL) and IPLs (MIIPL) indicate that ANMEs were active in the moderate seepage area. The ANME-2 and ANME-3 clades were characterized by enhanced glycosidic and phosphoric diether lipids production, indicating their potential role in coupling carbon and phosphurus cycling in cold seep ecosystems. ANME-1, though representing a smaller proportion of total archaea than ANME-2 and ANME-3 in the low seepage area, showed a positive correlation with MIIPL, indicating a different mechanism contributing to the IPL-GDGT pool. This also suggests that MIIPL could be a sensitive index to trace AOM activities performed by ANME-1. Overall, our study expands the understanding of the archaeal lipid composition in the cold seep and improves the application of MI using intact polar lipids that potentially link to extent ANME activities.

Multi-molecular 14C evidence for mineral control on terrestrial carbon storage andexport.

Compound- and compound class-specific radiocarbon analysis of source-diagnostic 'biomarker' molecules has emerged as a powerful tool to gain insights into terrestrial carbon cycling. While most studies thus far have focused on higher plant biomarkers (i.e. plant leaf-wax n-alkanoic acids and n-alkanes, lignin-derived phenols), tracing paedogenic carbon is crucial given the pivotal role of soils in modulating ecosystem carbon turnover and organic carbon (OC) export. Here, we determine the radiocarbon (14C) ages of glycerol dialkyl glycerol tetraethers (GDGTs) in riverine sediments and compare them to those of higher plant biomarkers as well as markers of pyrogenic (fire-derived) carbon (benzene polycarboxylic acids, BPCAs) to assess their potential as tracers of soil turnover and export. GDGT Δ14C follows similar relationships with basin properties as vegetation-derived lignin phenols and leaf-wax n-alkanoic acids, suggesting that the radiocarbon ages of these compounds are significantly impacted by intermittent soil storage. Systematic radiocarbon age offsets are observable between the studied biomarkers, which are likely caused by different mobilization pathways and/or stabilization by mineral association. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Environmental controls on the distribution of GDGT molecules in Lake Höglwörth, Southern Germany

Tetraether lipids, such as glycerol dialkyl glycerol tetraethers (GDGTs), are an increasingly used proxy for reconstructing paleoclimatic and paleoenvironmental dynamics. On a global scale, environmental drivers controlling the distribution of GDGTs in lake systems have been described extensively. However, the drivers that control down-core sedimentary GDGT composition within single lake systems are still poorly constrained. Here, we analysed GDGTs in a sediment core collected from Lake Höglwörth (Bavaria, Germany) covering the last ∼1100 years. The increase in ratio ΣIIIa/ΣIIa values (a ratio which captures the variation of 3 structural isomers of penta- and hexamethylated brGDGTs respectively) reflects a higher contribution of branched (br) GDGTs from aquatic sources after 1800 CE. This is synchronous with a high nutrient input as a result of increased local anthropogenic activities. The climate proxy methylation of branched tetraethers (MBT'5ME) reveals a pattern that deviates from the anticipated temperature changes during the last millennium. Instead, high MBT'5ME values from ∼880 to 1120 CE coincide with a period of increased soil input, resulting from lake damming and/or construction of the Höglwörth monastery. Decreased values after 1800 CE until present date correspond to a higher contribution of aquatic brGDGTs, with changes in the dependency between Ia and IIa suggesting a direct influence of bacterial community changes on the MBT'5ME. The proxy cyclisation of branched tetraethers (CBT') shows a linear increase with time, and a stepwise increase at 1700 CE, when a creek stream inflow channel was rerouted. Over time, MBT’5ME decreases gradually, while CBT’ and ΣIIIa/ΣIIa increase. This is hypothesized to be potentially caused by decreasing water depth over time and/or post- or syn-sedimentary production of GDGTs. The degree of cyclization (DC') shows variations on shorter timescales, possibly driven by the lake mixing regime and water column redox conditions. Our results indicate that caution must be taken when interpreting GDGTs for paleoclimate and paleoenvironmental reconstruction considering the possible influence of shifts in the provenance of brGDGTs. Further studies focusing on both sedimentary and intact polar lipids are highly recommended to constrain the source (water column, or sedimentary production) of GDGTs in lake sediments.

Nitrosopumilus as main source of isoprenoid glycerol dialkyl glycerol tetraether lipids in the central Baltic Sea.

Nitrososphaeria in the phylum Crenarchaeota, is a widespread archaeal class in the oceanic realm, playing an important role in the marine carbon and nitrogen cycle. Nitrososphaeria-derived membrane lipids, i.e., isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), are commonly employed to reconstruct past water temperatures using the TetraEther indeX of 86 carbon atoms (TEX86). This index is of particular importance for the brackish Baltic Sea as to date it appears to be the only applicable organic temperature proxy. In this study, we investigated the distribution of intact and core GDGTs and their potential source organisms in the water column of three deep basins located in the central Baltic Sea to evaluate the application of TEX86. A lipidomic approach on suspended particulate matter was combined with the molecular techniques 16S rRNA gene amplicon sequencing and CARD-FISH. The archaeal community was dominated by Nitrosopumilus (~83-100% of the total archaeal sequences). As other detected taxa known to produce GDGTs each represented less than 2% of the total archaeal sequences, Nitrosopumilus is likely the most dominant GDGT producer in the central Baltic Sea. However, the occurrence of phosphohexose (PH), instead of hexose-phosphohexose (HPH) headgroups, suggested that Nitrosopumilus in the Baltic Sea may differ physiologically from representatives of marine settings and other marginal seas, such as the Black Sea. In the Baltic Sea, Nitrosopumilus is most abundant in the suboxic zone, where intact cells peak according to both CARD-FISH data and intact polar lipid concentrations. The presented data therefore suggest that TEX86 reflects subsurface rather than surface temperature in the central Baltic Sea.