Archaeal Glycerol Dialkyl Glycerol Tetraethers Research Articles

Water depth affecting thaumarchaeol production in Lake Qinghai, northeastern Qinghai–Tibetan plateau: Implications for paleo lake levels and paleoclimate

Archaeal glycerol dialkyl glycerol tetraethers (GDGTs) are increasingly popular and versatile tool for palaeolimnology studies, but their applications in paleohydrology are scarce, especially for thaumarchaeol which is specific for the newly proposed phylum Thaumarchaeota. After investigating our published GDGT data of Lake Qinghai, we found that both the concentration of thaumarchaeol and the relative abundance of thaumarchaeol to total archaeal GDGTs (%thaum) in core-top sediments increased significantly with increasing water depth (R=0.88 and 0.95, respectively), with lower concentrations of 5±5ng/g in shallow areas (water depth<5m) and higher concentrations of 121±65ng/g in deep areas (water depth>10m). This is likely because that the producers of thaumarchaeol, Thaumarchaeota, prefer living in the relative deeper zone in lacustrine systems, where probably both competition of ammonium (the substrate) from other microbes and light intensity are low. Therefore, we proposed that thaumarchaeol was mainly produced in situ and changes in %thaum might reflect water-depth variations in this closed-basin lake. The application of %thaum as a water-depth indicator in a Holocene sediment sequence of core QH-2011 provided a high-resolution relative lake-level history of Lake Qinghai which resembles that inferred from the δ13Corg value obtained in the same core. This supports the use of %thaum as an indicator of lake water depth in paleohydrology studies, especially for medium lakes. Moreover, the records of the two independent proxies in core QH-2011 confirmed a shallow Lake Qinghai in the early Holocene and a late-Holocene highstand, highlighting the importance of local temperature (and evaporation loss) in controlling effective moisture in the arid/semi-arid region.

Hydroxylated isoprenoidal GDGTs in the Nordic Seas

Archaea modify their membrane lipid composition to maintain integrity and exchange capabilities across a wide temperature range. The number of rings in the tetraether structure increases with higher ambient temperatures. Recently, novel glycerol dialkyl glycerol tetraethers (GDGTs) with an additional hydroxyl group on the alkyl chain were described (OH-GDGTs). Here we show that at a regional scale in the Nordic Seas (western Barents, Norwegian, Greenland, and Iceland Seas), the OH-GDGTs' contribution to the total archaeal GDGT pool as well as the ratio of specific OH-GDGTs versus crenarchaeol are higher in colder waters, especially in polar water masses. We also observe a shift in the number of rings of the OH-GDGTs with temperature. Thus, both the contribution of the OH-GDGTs to the total isoprenoidal GDGT pool and the number of rings in OH-GDGTs could be used as potential indicators of polar waters and might complement the currently used paleo sea surface temperature proxies in polar regions.

Occurrence of tetraether lipids in stalagmites: Implications for sources and GDGT-based proxies

Lipid biomarkers in stalagmites have potential in palaeoenvironmental reconstruction, but glycerol dialkyl glycerol tetraethers (GDGTs) in stalagmites have rarely been investigated. We report the occurrence of archaeal isoprenoid GDGTs and bacterial branched GDGTs in Heshang Cave stalagmites in central China. The archaeal component, crenarchaeol, dominated bacterial branched GDGTs in the stalagmites and on the weathered parent rock surface collected inside the cave. In contrast, the distribution of GDGTs in the overlying soil shows that branched GDGTs dominated archaeal GDGTs. Hence, the majority of archaeal GDGTs in stalagmites may be produced by Crenarchaeota inhabiting the cave rather than being inherited from the overlying soil. Bacterial branched GDGT distributions, expressed as MBT and CBT indices, showed remarkable differences between stalagmite and soil samples, implying that most branched GDGTs may also be produced in situ in the cave. The TEX 86 or TEX 86 ′ values were found to increase with more depleted δ 18O values representing much stronger monsoon and therefore usually higher temperatures, in the study region. However, the MBT/CBT index in stalagmites may be influenced by a variety of factors besides palaeotemperature.

Occurrence and distribution of non-extractable glycerol dialkyl glycerol tetraethers in temperate and tropical podzol profiles

Glycerol dialkyl glycerol tetraethers (GDGTs) are high molecular weight lipids present in the membranes of archaea and some bacteria. Isoprenoid GDGTs with acyclic or ring containing dibiphytanyl chains are known to be synthesised by archaea. In soil, another type of GDGT, which can be distinguished from tetraethers of archaeal origin by way of the branched nature of the alkyl chain, was discovered recently. Alkyl branched GDGTs were suggested to be produced by anaerobic bacteria and can be used to reconstruct past air temperatures and soil pH. Lipids in soils can take two broad forms: extractable, i.e. recoverable via solvent extraction, and non-extractable, linked to the mineral or organic matrix. The present study aimed at comparing the abundance and distribution of these two pools of GDGTs in two contrasting podzol environments: a temperate podzol 40 km north of Paris (France) and a tropical podzol from the Amazon basin (Brazil). Five samples were collected from the whole profile of the temperate podzol. Five additional samples were obtained from three profiles of the tropical soil sequence, which are representative of the transition between a latosol and a well developed podzol. For the first time, we showed that substantial amounts of non-extractable GDGTs can be released after acid hydrolysis of solvent-extracted soils, non-extractable GDGTs representing 25 ± 15% of the total (i.e. extractable + non-extractable) bacterial GDGTs and 29 ± 17% of the total archaeal GDGTs in podzol samples. This implies that extractable GDGTs can be incorporated into the organic and/or mineral matrix of soil. In addition, we observed that extractable and non-extractable GDGTs could present different distribution patterns, notably suggesting that some extractable GDGTs might be preferentially transferred to the non-extractable pool and/or might be preferentially degraded by soil microorganisms. The relative abundances of bacterial and archaeal GDGTs were compared along the temperate soil profile and the tropical soil sequence. The relative amount of bacterial vs. archaeal GDGTs was shown to be much higher in the extractable than in the non-extractable fraction in the surficial horizons of the temperate podzol and in the well-developed part of the tropical podzol, implying that extractable archaeal GDGTs could be preferentially transferred to the non-extractable lipid pool compared to extractable bacterial GDGTs in these horizons. This might be due to the fact that the types of polar head groups associated with bacterial GDGTs differ from, and are more labile than, those associated with archaeal GDGTs.

Application of bacterial glycerol dialkyl glycerol tetraethers (GDGTs) to develop modern and past temperature estimates from New Zealand lakes

Branched isoalkyl glycerol dialkyl glycerol tetraethers (GDGTs) of bacterial origin have been found in high abundance in both modern and glacial sediments of New Zealand South Island freshwater lakes covering a wide range of altitude (101–2000 m). Like isoprenoid GDGTs of archaeal origin, they provide excellent potential for temperature assessment. For this study, their distribution patterns (MBT, methylation ratio and CBT, cyclisation ratio of branched GDGTs) have been successfully used to develop an initial temperature calibration for the study area and to provide preliminary (palaeo)environmental interpretations. MBT data from modern lake sediments correlate well with measured annual air temperature ( R 2 0.74), enabling a regional calibration for reconstructing palaeotemperatures for fossil samples. MBT-derived palaeotemperatures for Alpine Lake, calibrated against mean annual temperature, were determined for the Last Glacial during an early cold phase (between 29,000 and 26,000 years BP) and for later less cold phases (between 23,000 and 18,000 years BP). Compared with the modern temperature regime, the MBT data indicate a decrease of ca. 5.6 and 2.8 °C respectively, during this time. Modern and past MBT-derived temperatures correlate with chironomid-based temperature reconstructions in the area. Archaeal GDGTs, commonly used for the TEX 86 index, are abundant in fossil sediments (Alpine Lake) but scarce in modern sediments, hindering a new local calibration for this palaeotemperature proxy.

Occurrence and distribution of extractable glycerol dialkyl glycerol tetraethers in podzols

The abundance and distribution of glycerol dialkyl glycerol tetraethers (GDGTs) were compared in two contrasting environments: a temperate podzol located 40 km north of Paris (France) and a tropical podzol from the Amazon Basin (Brazil). Five samples were collected from the profile of the temperate podzol, including the litter layer. Five others were obtained from three profiles from the tropical soil sequence, which is representative of the transition between a latosol and a well-developed podzol. GDGTs can occur as core lipids (i.e. as extractable “free” lipids) within the soil organic extract. They may also be present in ester- and/or glycosidic ether-bound form (i.e. as extractable “bound” lipids) in the extract. To date, most studies of GDGTs in soil have considered extractable “free” GDGTs but have not investigated extractable “bound” GDGTs. The extractable “free” and “bound” GDGT pools were therefore both investigated. Accelerated solvent extraction (ASE) was used to extract total extractable (i.e. “free” and “bound”) GDGTs. Half of these extracts was then submitted to acid methanolysis to release ester- and/or glycosidic ether-bound GDGTs, allowing subsequent analysis of total extractable GDGTs using high performance liquid chromatography–mass spectrometry (HPLC/MS). The other half was directly analysed for “free” GDGTs. Branched GDGTs derived from soil bacteria were present in all the samples, along with GDGTs of archaeal origin, although the latter were detected in much lower abundance than bacterial GDGTs. A higher degree of methylation was observed for the alkyl branched GDGTs from the temperate samples than for the tropical ones, in agreement with differences in air temperature. The amount of bacterial and archaeal GDGTs in the surficial sample from the well-developed part of the tropical podzol was greater than in the latosol and transition profiles. The difference could be related to a lower pH in the developed podzol than in the other two profiles. Furthermore, branched GDGTs were detected in the litter layer from the temperate podzol, which may indicate the presence of branched GDGT-producing bacteria in the litter, probably in anoxic microsites. Last, GDGT abundance and distribution were comparable in the extractable “free” and total extractable lipid fractions of most of the soil samples, suggesting that GDGTs mainly occur as “free” lipids in the extracts of all these soils and that similar data concerning past air temperature and soil pH could be derived from both extractable “free” and total extractable GDGT distributions.