Glycerol Tetraether Lipids Research Articles

Distribution and export of isoprenoid tetraether lipids in suspended particulate matter from the water column of the Western Atlantic Ocean

The TEX86 paleotemperature proxy is based on the distribution of archaeal glycerol dibiphytanyl glycerol tetraether (GDGT) lipids preserved in marine sediments, yet both the influence of different physiological factors on the structural distribution of GDGTs and the mechanism(s) by which GDGTs is(are) exported to marine sediments remain(s) unresolved. We investigated the abundance and structural distribution of GDGTs in the South-west and Equatorial Atlantic Ocean in four water column profiles spanning 48 degrees of latitude. The depth distribution was consistent with production by ammonia-oxidizing Thaumarchaeota; maximum GDGT concentration occurred at the base of the NO2− maximum, core GDGTs dominated the structural distribution in surface waters above the NO2− maximum, and intact polar GDGTs – potentially indicating live cells – were more abundant below the NO2− maximum. Between 0 and1000 m, > 98% of the integrated GDGT inventory was present in waters at and below the NO2− maximum. Depth profiles of TEX86 temperature values displayed local minima at the NO2− maximum, while the ratio of GDGT-2:GDGT-3 increased with depth. A model based on the results predicts an average depth of origin for GDGTs exported to sediments between ca. 80–250 m. In the model, exported TEX86 values are remarkably insensitive to change in the average depth of origin of GDGTs. However, TEX86 values exported from the water column appear to reflect euphotic zone productivity, possibly due to the correlative intensity of organic matter remineralization providing substrates for ammonia oxidation. Predicting the influence of these regional controls on sedimentary TEX86 records requires a better understanding of the interaction between GDGT production, particle dynamics, and the depth of origin for exported organic matter.

Nitrosopumilus maritimus gen. nov., sp. nov., Nitrosopumilus cobalaminigenes sp. nov., Nitrosopumilus oxyclinae sp. nov., and Nitrosopumilus ureiphilus sp. nov., four marine ammonia-oxidizing archaea of the phylum Thaumarchaeota.

Four mesophilic, neutrophilic, and aerobic marine ammonia-oxidizing archaea, designated strains SCM1T, HCA1T, HCE1T and PS0T, were isolated from a tropical marine fish tank, dimly lit deep coastal waters, the lower euphotic zone of coastal waters, and near-surface sediment in the Puget Sound estuary, respectively. Cells are straight or slightly curved small rods, 0.15-0.26 µm in diameter and 0.50-1.59 µm in length. Motility was not observed, although strain PS0T possesses genes associated with archaeal flagella and chemotaxis, suggesting it may be motile under some conditions. Cell membranes consist of glycerol dibiphytanyl glycerol tetraether (GDGT) lipids, with crenarchaeol as the major component. Strain SCM1T displays a single surface layer (S-layer) with p6 symmetry, distinct from the p3-S-layer reported for the soil ammonia-oxidizing archaeon Nitrososphaera viennensis EN76T. Respiratory quinones consist of fully saturated and monounsaturated menaquinones with 6 isoprenoid units in the side chain. Cells obtain energy from ammonia oxidation and use carbon dioxide as carbon source; addition of an α-keto acid (α-ketoglutaric acid) was necessary to sustain growth of strains HCA1T, HCE1T, and PS0T. Strain PS0T uses urea as a source of ammonia for energy production and growth. All strains synthesize vitamin B1 (thiamine), B2 (riboflavin), B6 (pyridoxine), and B12 (cobalamin). Optimal growth occurs between 25 and 32 °C, between pH 6.8 and 7.3, and between 25 and 37 ‰ salinity. All strains have a low mol% G+C content of 33.0-34.2. Strains are related by 98 % or greater 16S rRNA gene sequence identity, sharing ~85 % 16S rRNA gene sequence identity with Nitrososphaera viennensis EN76T. All four isolates are well separated by phenotypic and genotypic characteristics and are here assigned to distinct species within the genus Nitrosopumilus gen. nov. Isolates SCM1T (=ATCC TSD-97T =NCIMB 15022T), HCA1T (=ATCC TSD-96T), HCE1T (=ATCC TSD-98T), and PS0T (=ATCC TSD-99T) are type strains of the species Nitrosopumilusmaritimus sp. nov., Nitrosopumilus cobalaminigenes sp. nov., Nitrosopumilus oxyclinae sp. nov., and Nitrosopumilus ureiphilus sp. nov., respectively. In addition, we propose the family Nitrosopumilaceae fam. nov. and the order Nitrosopumilales ord. nov. within the class Nitrososphaeria.

Isoprenoid tetraether lipids in suspended particulate matter from the East China Sea and implication for sedimentary records

A total of 90 samples of suspended particulate matter (SPM) collected in surface waters of the East China Sea (ECS) in summer and winter were studied for archaeal isoprenoid glycerol dialkyl glycerol tetraether lipids (iGDGTs) and the derived sea surface temperature (SST) proxy, i.e. TEX86H. Our data showed that in summer SPM TEX86H correlated well with in-situ measured SST, but in winter the relationship became much poorer. We found that the bottom water depth (BWD) of 70 m was a critical depth when studying the TEX86H-SST relationship in winter SPM. In locations with BWD < 70 m, concentrations of iGDGTs in SPM were > 5x higher in winter than in summer, but TEX86H showed a much weaker correlation with in-situ measured SST. This was speculated to be caused by lateral transport of SPM and resuspension of surface sediments in winter due to strong winds. Whereas in locations with BWD > 70 m, TEX86H correlated well with in-situ measured SST, suggesting a minor influence of SPM lateral transport and sediment resuspension. Moreover, although only core iGDGTs were analyzed, we found that both summer and winter TEX86H values correlated better with in-situ measured SST than with seasonal and annual mean SST, suggesting that SPM iGDGTs were dominantly produced around the sampling time rather than encompassing longer time input. The linear relationships of SPM TEX86H with in-situ measured SST were different between winter (BWD > 70 m) and summer, likely due to seasonal archaeal community change. Published sedimentary TEX86H data in the ECS were reexamined here, and a BWD of 70 m was also found to be a critical depth. At sites with BWD > 70 m, TEX86H in sediments correlated well with annual mean SST, whereas the correlation became poor at BWD < 70 m, suggesting that sedimentary GDGTs were also greatly influenced by resuspension and lateral transport, and hence complicating the application of the proxy to shallow locations in the ECS.

Thermoplasmatales and Methanogens: Potential Association with the Crenarchaeol Production in Chinese Soils.

Crenarchaeol is a unique isoprenoid glycerol dibiphytanyl glycerol tetraether (iGDGT) lipid, which is only identified in cultures of ammonia-oxidizing Thaumarchaeota. However, the taxonomic origins of crenarchaeol have been debated recently. The archaeal populations, other than Thaumarchaeota, may have associations with the production of crenarchaeol in ecosystems characterized by non-thaumarchaeotal microorganisms. To this end, we investigated 47 surface soils from upland and wetland soils and rice fields and another three surface sediments from river banks. The goal was to examine the archaeal community compositions in comparison with patterns of iGDGTs in four fractional forms (intact polar-, core-, monoglycosidic- and diglycosidic-lipid fractions) along gradients of environments. The DistLM analysis identified that Group I.1b Thaumarchaeota were mainly responsible for changes in crenarchaeol in the overall soil samples; however, Thermoplasmatales may also contribute to it. This is further supported by the comparison of crenarchaeol between samples characterized by methanogens, Thermoplasmatales or Group I.1b Thaumarchaeota, which suggests that the former two may contribute to the crenarchaeol pool. Last, when samples containing enhanced abundance of Thermoplasmatales and methanogens were considered, crenarchaeol was observed to correlate positively with Thermoplasmatales and archaeol, respectively. Collectively, our data suggest that the crenarchaeol production is mainly derived from Thaumarchaeota and partly associated with uncultured representatives of Thermoplasmatales and archaeol-producing methanogens in soil environments that may be in favor of their growth. Our finding supports the notion that Thaumarchaeota may not be the sole source of crenarchaeol in the natural environment, which may have implication for the evolution of lipid synthesis among different types of archaea.

Historical changes in organic matter input to the muddy sediments along the Zhejiang-Fujian Coast, China over the past 160 years

The burial of sedimentary organic matter (SOM) in large river-influenced estuarine-coastal regions is affected by hydrodynamic sorting, diagenesis and human activities. Typically, the inner shelf region of the East China Sea is a major carbon sink for the Yangtze River-derived fine-grained sediments. Most of the previous work concentrated on studies of surface sediments or used a single-proxy in this region. In this study, two cores from the Zhejiang-Fujian Coast were analyzed using bulk (TOC, TN and δ13CTOC) and molecular biomarker (n-alkane, brassicasterol, dinosterol and glycerol dialkyl glycerol tetraether lipids) techniques to clarify the sources, spatiotemporal distribution and fate of SOM in the Yangtze River Estuary and adjacent shelf. Results indicated that the effects of diagenesis and diffusion on different sedimentary biomarkers resulted in overestimation of the relative contribution of terrestrial organic matter (%OMterr), compared with those based on δ13CTOC. The amount of terrestrial plant organic matter (OMplant) and %OMterr in sediments decreased offshore. In contrast, the amount of marine organic matter (OMmarine) increased offshore, but closer to the Yangtze River mouth, the amount of soil organic matter (OMsoil) increased. Moreover, the amounts of TOC, OMplant and OMmarine biomarkers increased, but OMsoil and %OMterr decreased over time in recent decades. Our study suggests that spatial OM distribution patterns in marine shelf sediments were controlled primarily by hydrodynamic sorting and nutrient concentration, and temporally diverse patterns were controlled predominantly by anthropogenic influence (e.g. dam construction and soil conservation, reclamation and agricultural plantations, anthropogenic nutrient input, dust storms, eutrophication, etc.) and climate events (e.g. interdecadal climatic jump and heavy rain events) in the geological period.

Tentative identification of diagenetic products of cyclic biphytanes in sedimentary rocks from the uppermost Permian and Lower Triassic

Biphytanes, which are derived from glycerol dialkyl glycerol tetraether (GDGT) lipids, are biomarkers for the presence of Archaea in geological samples. They (especially cyclic structures) are rarely reported from sediments or oils that are older than the Triassic period, where their first record may be traced back to the Late Archean. There have been only two studies that have reported diagenetic products of cyclic biphytanes using two dimensional gas chromatography. In this study, we used full scan and multiple reaction monitoring (MRM) mass spectrometry (MS) to tentatively assign a series of diagenetic products of cyclic biphytanes within uppermost Permian and Lower Triassic sedimentary rocks. We observed three distinct groups derived from mono-, bi-, or tricyclic biphytanes, echoing previous studies. Each group could be further divided into three discrete series according to structure and composition. Using MRM also revealed that numerous sedimentary rocks contained such diagenetic products, but at trace concentration below the limit of detection of full scan MS. This indicates that the presence of Archaea-containing GDGT lipids has been underestimated within paleoenvironments. Episodic occurrences of biphytanes in the Permian-Triassic transition also provide additional evidence for the distribution of Archaea in the geological past.

A 15 ka pH record from an alpine lake in north China derived from the cyclization ratio index of aquatic brGDGTs and its paleoclimatic significance

A well-dated sediment core from an alpine lake in north China (Gonghai Lake) was used for analysis of branched glycerol dialkyl glycerol tetraether lipids (brGDGTs). Based on the cyclization ratio index (CBT) of the brGDGTs, and using different terrestrial and lacustrine CBT-pH empirical relationships, the reconstructed pH records over the past ca. 15ka exhibited similar trends of variation, although the absolute pH values were different. In order to better understand the reconstructed pH record, samples of surface soils from the dust source area and the catchment of Gonghai Lake, as well as surface sediments from the lake, were collected for instrumental pH measurements and modern brGDGT analysis. The instrumental pH results demonstrated that the surface soils from the dust source area were alkaline, while those from the catchment of Gonghai Lake were acidic. In addition, the instrumental pH results for the surface sediments from the lake were slightly alkaline and the corresponding pH values were intermediate between those of the two surface soil source groups. This may reflect the effect of the input of material from different sources on the sedimentary pH record. However, the results from modern brGDGTs in some of the surface soils and surface sediment samples demonstrate that brGDGTs in the core sediments from Gonghai Lake are predominantly of aquatic origin. Therefore, the CBT-derived pH record over the past ca. 15ka from the lake can be interpreted as a record of the lake water pH history, which is consistent with the carbonate content record from a parallel core from the same lake. In addition, using the modern lacustrine calibration, the CBT-derived pH values for surface sediments of Gonghai Lake are consistent with instrumental modern lake water pH values for the lake. Further comparison indicates that the lake water pH is controlled mainly by the degree of humidity of the local climate, which is closely related to the intensity of the East Asian summer monsoon (EASM); therefore, the pH record can be used as an indicator of EASM intensity. Our results demonstrate that, on the basis of careful analysis of its origins, the CBT index of aquatic brGDGTs can be used for lake water pH reconstruction and subsequent paleoclimate reconstruction, and that in north China, the lake water pH is controlled mainly by the degree of humidity of the local climate.

Organic geochemical and palynological evidence for Holocene natural and anthropogenic environmental change at Lake Dojran (Macedonia/Greece)

In this study, we present lipid biomarker and palynological data for a sediment core from Lake Dojran (Macedonia/Greece), which covers the entire Holocene period. We analyzed vascular plant-derived n-alkanes, combustion-derived polycyclic aromatic hydrocarbons (PAHs), fecal steroids, and bacterial and archaeal glycerol dialkyl glycerol tetraether (GDGT) lipids in concert with microcharcoal and pollen assemblages to reconstruct climatic, environmental, and human impact in the Dojran catchment and the greater Dojran area. Overall, our results suggest a relationship between anthropogenic activity and environmental/climatic change since increased human impact corresponds to phases of higher humidity and high lake levels at Lake Dojran. During the early Holocene, the record reveals increasing temperatures and humidity and concurrent increasing vegetation cover and runoff/soil erosion, respectively. Following a thermal maximum during the middle early Holocene, temperatures decrease gradually until present. The middle-Holocene at Lake Dojran is characterized by relatively stable environmental conditions followed by greater climatic instability and strong anthropogenic overprint during the late-Holocene. The fecal stanol record reveals phases of increased human impact during the early Bronze Age, the late Bonze/early Iron Age, and the Middle Ages. A phase of low stanol and PAH concentrations from the late Iron Age until the early Middle Ages is either related to ecosystem changes and/or changes in settlement pattern since concurrent pollen data indicate intensified land use. Human impact re-intensified during the Middle Ages with some variability probably related to climatic variations of the ‘Medieval Warm Period’ and the ‘Little Ice Age’.

Production rates of bacterial tetraether lipids and fatty acids in peatland under varying oxygen concentrations

Interpretations of the abundance and distribution of branched glycerol dialkyl glycerol tetraether (brGDGT) lipids have been increasingly applied to infer changes in paleoenvironment and to estimate terrigenous organic matter inputs into estuarine and marine sediments. However, only preliminary information is known regarding the ecology and physiology of the source organisms of these biomarkers. We assessed the production rates of brGDGTs under different redox conditions in peat, where these lipids are found in high concentrations, particularly at greater depths below the fluctuating water table. The incorporation of hydrogen relative to carbon into lipids observed in our dual stable isotope probing assay indicates that brGDGTs were produced by heterotrophic bacteria. Unexpectedly, incubations with stable isotope tracers of the surface horizon (5–20cm) initiated under oxic conditions before turning suboxic and eventually anoxic exhibited up to one order of magnitude higher rates of brGDGT production (16–87ngcm−3y−1) relative to the deeper, anoxic zone (20–35cm; ca. 7ngcm−3y−1), and anoxic incubations of the surface horizon (<3ngcm−3y−1). Turnover times of brGDGTs in the surface horizon ranged between 8 and 41years in the incubations initiated under oxic conditions, in contrast to 123–742years in anoxic incubations. As brGDGTs were actively produced during both anoxic incubations and those exposed to oxygen, we conclude that their source organisms are likely facultative aerobic heterotrophs that are particularly active in the peat acrotelm. Production rates of bacterial fatty acids (ca. 2μgcm−3y−1) were roughly two orders of magnitude higher than those of brGDGTs, suggesting that brGDGT producers are a minor constituent of the microbial community or that brGDGTs are a small component of the microbial cell membrane in comparison to fatty acids, despite the typically high brGDGT concentrations observed in peat. Multivariate analysis identified two branched fatty acids that shared a similar production pattern as brGDGTs among the experimental treatments and may be associated with brGDGT biosynthesis.

Biogeochemistry of intertidal microbial mats from Qatar: New insights from organic matter characterisation

Understanding early organic matter (OM) alteration and preservation in marine carbonate-evaporite systems could improve understanding of carbon cycling and hydrocarbon source rock prediction in such environments. It is possible that organic-rich microbial mats are important contributors to preserved hydrocarbons and, to explore this, we examined changes in lipid composition in such a mat from a mesohaline intertidal lagoon, eastern Qatar. The mat reaches>5cm thickness over a carbonate mud substrate, rich in seagrass, gastropods and other small bioclasts. Clear lamination, with distinct downward colour changes from green to pink to brown, reflects different microbial mat communities. The layers contain spheroids of probable dolomite, the precipitation of which was plausibly bacterially-mediated. Lipids [n-alkanes, fatty acids (FAs), hopanoids, isoprenoid hydrocarbons, dialkyl glycerol diethers (DAGEs), and isoprenoid (0–4) and branched (Ia–IIIa) GDGTs] reflected the diverse mat-building phototrophic, heterotrophic and chemoorganotrophic microorganisms, as well as some likely allochthonous material (i.e. steroids, n-alkanols, high molecular weight n-alkanes). The lipids clearly documented the change in microbial community, with phytene(s) being the predominant hydrocarbon in the phototrophic surface layer. O2 and pH dropped significantly 0.2cm below the mat surface, coincident with the predominance of Deltaproteobacteria and increased concentrations of archaeal and bacterial glycerol dialkyl glycerol tetraether lipids and C15/C16 and C16/C17 dialkyl glycerol ether lipids in the deeper layers. Archaeol, likely of methanogen origin, was most abundant in the deepest layer. Allochthonous inputs occurred throughout the mat, including abundant steroids, especially dinosterol and dinostanol, possibly related to periodic algal (dinoflagellate) blooms in the Arabian Gulf. Both n-alkanes and n-alkanols appeared to be derived from seagrass. OM content decreased in the deepest mat; this suggests an overall low preservation potential of OM in intertidal mesohaline-hypersaline mats of the Arabian Gulf, which in turn suggests that these are not the major source of hydrocarbons in microbially-dominated carbonate-evaporite systems.

Influence of Growth Phase, pH, and Temperature on the Abundance and Composition of Tetraether Lipids in the Thermoacidophile Picrophilus torridus.

The abundance and composition of glycerol dibiphytanyl glycerol tetraether (GDGT) and glycerol tribiphytanyl glycerol tetraether (GTGT) lipids were determined as a function of growth phase as a proxy for nutrient availability, the pH of growth medium, and incubation temperature in cultures of the thermoacidophile Picrophilus torridus. Regardless of the cultivation condition, the abundance of GDGTs and GTGTs was greater in the polar than core fraction, with a marked decrease in core GDGTs in cultures harvested during log phase growth. These data are consistent with previous suggestions indicating that core GDGTs are re-functionalized during polar lipid synthesis. Under all conditions examined, polar lipids were enriched in a GDGT with 2 cyclopentyl rings (GDGT-2), indicating GDGT-2 is the preferred lipid in this taxon. However, lag or stationary phase grown cells or cells subjected to pH or thermal stress were enriched in GDGTs with 4, 5, or 6 rings and depleted in GDGTs with 1, 2, 3, rings relative to log phase cells grown under optimal conditions. Variation in the composition of polar GDGT lipids in cells harvested during various growth phases tended to be greater than in cells cultivated over a pH range of 0.3–1.1 and a temperature range of 53–63°C. These results suggest that the growth phase, the pH of growth medium, and incubation temperature are all important factors that shape the composition of tetraether lipids in Picrophilus. The similarity in enrichment of GDGTs with more rings in cultures undergoing nutrient, pH, and thermal stress points to GDGT cyclization as a generalized physiological response to stress in this taxon.

A diagnostic GDGT signature for the impact of hydrothermal activity on surface deposits at the Southwest Indian Ridge

The impact of hydrothermal activity on wider ocean geochemistry and microbial ecology remains a topic of much interest. To explore whether hydrothermal microbial signatures are exported to surrounding marine sediments or if such organisms serve as an important source of sedimentary organic matter, we determined the distributions of glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in surficial normal marine sediments, metalliferous sediments and low-temperature hydrothermal deposits at newly discovered hydrothermal fields and adjacent areas at the Southwest Indian Ridge (SWIR). The GDGTs in those samples varied significantly, evidently representing a variable influence of the hydrothermal activity. GDGT compositions of surficial background sediments in SWIR were similar to those commonly observed in marine sediments, dominated by GDGTs associated with marine planktonic archaea and especially GDGT−0 and crenarchaeol. In contrast, the GDGTs of metalliferous sediments strongly impacted by hydrothermal activity and low-temperature hydrothermal deposits were markedly different, characterized by high relative abundances of isoprenoid GDGTs (iGDGTs) bearing multiple rings (yielding a higher ring index), low relative abundances of crenarchaeol, and the presence of glycerol monoalkyl glycerol tetraether lipids (GMGTs; so called ‘H-tetraethers’) that were absent in the normal marine sediments. Sources for these hydrothermal-specific tetraether lipids likely include methanogens and anaerobic methanotrophic archaea (GDGT−0 and GDGT−1–3, respectively), Thermoprotei and Thermoplasmatales (elevated GDGT−3 and 4), and other thermophilic archaea including Methanobacteriales (GMGTs). Deposits influenced by low-temperature hydrothermal activity also contained higher abundances of branched GDGTs (brGDGTs) typically attributed to soil bacteria. The more distal metalliferous sediments influenced by the neutrally buoyant plume did not contain putative hydrothermal GDGTs, having the same GDGT distribution as the background sediments. This suggests that the neutrally buoyant plume has a limited potential to directly influence the organic matter inputs to surrounding sediments, due to a rapidly waning chemosynthetic microbial contribution relative to normal marine contributions as the plume dispersed and was diluted.

Distribution of glycerol dialkyl glycerol tetraether (GDGT) lipids in a hypersaline lake system

Isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) of archaeal origin and branched (br)GDGTs of bacterial origin occur in a diverse range of lacustrine sedimentary environments. They have attracted attention as potential temperature proxies, providing high resolution (palaeo)environmental reconstruction from continental interiors. For this study, the distribution of GDGTs in, and application of GDGT-based proxies to, surface samples from Chaka Salt Lake (China), as well as soils and in-flow river sediments, were investigated to assess whether GDGT-based proxies were applicable to this hypersaline lake system. We show that iso- and brGDGTs were present in all sediments and soils from the Chaka Salt Lake system. GDGT-0 and crenarchaeol were generally the two most abundant isoGDGTs, suggesting Thaumarchaeota as a major biological source of isoGDGTs. The low ratio of crenarchaeol/crenarchaeol regioisomer suggests that Thaumarchaeota of the lake sediments is likely Thaumarchaeota group I.1b derived from the surrounding alkaline soils, arguing against the use of the TEX86 proxy in this system. Because alkaline soils generally have high isoGDGT concentrations, it is likely that a large allochthonous input of isoGDGTs would be a pervasive challenge to palaeoclimate applications in such settings. On the other hand, the brGDGT distributions in the lake and river sediments differed markedly from those in the surrounding soils, suggesting that, instead of deriving from the surrounding soils, at least part of the brGDGTs is synthesized in situ or delivered from more distal upland soils. Taken together, our results indicate that the mixed sources of GDGTs in Chaka Salt Lake complicate the application of GDGT-based proxies, and it will be challenging to use such proxies in this system.

Effect of Headgroups on Small-Ion Permeability across Archaea-Inspired Tetraether Lipid Membranes.

This paper examines the effects of four different polar headgroups on small-ion membrane permeability from liposomes comprised of Archaea-inspired glycerolmonoalkyl glycerol tetraether (GMGT) lipids. We found that the membrane-leakage rate across GMGT lipid membranes varied by a factor of ≤1.6 as a function of headgroup structure. However, the leakage rates of small ions across membranes comprised of commercial bilayer-forming 1-palmitoyl-2-oleoyl-sn-glycerol (PO) lipids varied by as much as 32-fold within the same series of headgroups. These results demonstrate that membrane leakage from GMGT lipids is less influenced by headgroup structure, making it possible to tailor the structure of the polar headgroups on GMGT lipids while retaining predictable leakage properties of membranes comprised of these tethered lipids.

Stable carbon isotope ratios of intact GDGTs indicate heterogeneous sources to marine sediments

Thaumarchaeota, the major sources of marine glycerol dibiphytanyl glycerol tetraether lipids (GDGTs), are believed to fix the majority of their carbon directly from dissolved inorganic carbon (DIC). The δ13C values of GDGTs (δ13CGDGT) may be powerful tools for reconstructing variations in the ocean carbon cycle, including paleoproductivity and water mass circulation, if they can be related to values of δ13CDIC. To date, isotope measurements primarily are made on the C40 biphytane skeletons of GDGTs, rather than on complete tetraether structures. This approach erases information revealed by the isotopic heterogeneity of GDGTs within a sample and may impart an isotopic fractionation associated with the ether cleavage. To circumvent these issues, we present δ13C values for GDGTs from twelve recent sediments representing ten continental margin locations. Samples are purified by orthogonal dimensions of HPLC, followed by measurement of δ13C values by Spooling Wire Microcombustion (SWiM)-isotope ratio mass spectrometry (IRMS) with 1σ precision and accuracy of ±0.25‰. Using this approach, we confirm that GDGTs, generally around −19‰, are isotopically “heavy” compared to other marine lipids. However, measured δ13CGDGT values are inconsistent with predicted values based on the 13C content of DIC in the overlying water column and the previously-published biosynthetic isotope fractionation for a pure culture of an autotrophic marine thaumarchaeon. In some sediments, the isotopic composition of individual GDGTs differs, indicating multiple source inputs. The data appear to confirm that crenarchaeol primarily is a biomarker for Thaumarchaeota, but its δ13C values still cannot be explained solely by autotrophic carbon fixation. Overall the complexity of the results suggests that both organic carbon assimilation (ca. 25% of total carbon) and multiple source(s) of exogenous GDGTs (contributing generally <30% of input to sediments) are necessary to explain the observed δ13CGDGT values. The results suggest caution when interpreting the total inputs of GDGTs to sedimentary records. Biogenic or open-slope sediments, rather than clastic basinal or shallow shelf sediments, are preferred locations for generating minimally-biased GDGT proxy records.

Isoprenoid and branched GDGT-based proxies for surface sediments from marine, fjord and lake environments in Chile

Proxies based on glycerol dialkyl glycerol tetraether (GDGT) lipids from archaea [isoprenoid GDGTs] and bacteria [branched (br) GDGTs] in 33 surface sediments from marine, fjord and lake systems between 25°S and 50°S in Chile were analyzed. The regional TEXH86 calibration obtained from the marine and fjord sediments and mean annual surface temperature (T=59.6×TEXH86+33.0; r2 0.9; n=23) is statistically identical to the global ocean calibration based on suspended particulate material in terms of slope, but not in terms of intercept. The regional surface and subsurface TEXH86 calibrations were statistically different from the existing global ocean core top calibrations. The TEX86 calibration model based on most of the relatively large lakes studied here (T=50.7×TEX86−11.8; r2 0.9; n=5) is statistically identical to the global lake calibration. The relatively high TEX86 values from smaller lakes suggested an additional source for isoprenoid GDGTs, likely terrestrial or aquatic methanogenic archaea. Application of the soil-calibrated MBT′/CBT (methylation and cyclization of br GDGTs, respectively) temperature proxy to the marine and fjord sediments resulted in an overestimation of continental mean annual air temperature (MAAT), suggesting in situ production of certain br GDGTs in the water column or surface sediment. For the lakes, MBT′/CBT-based surface air temperature estimates were 3–6°C below MAAT. However, temperature estimates from the lake-specific MBT/CBT global calibration were in good agreement with mean annual surface temperature for all the lakes. The results highlight the need for testing local vs. global calibrations of GDGT-based proxies before their application for palaeoenvironmental reconstruction.

Influence of temperature, pH, and salinity on membrane lipid composition and TEX86 of marine planktonic thaumarchaeal isolates

Marine ammonia-oxidizing archaea of the phylum Thaumarchaeota are a cosmopolitan group of microorganisms representing a major fraction of the picoplankton in the ocean. The cytoplasmic membranes of Thaumarchaeota consist predominantly of intact polar isoprenoid glycerol dibiphytanyl glycerol tetraether (GDGT) lipids, which may be used as biomarkers for living Thaumarchaeota. Fossil thaumarchaeal GDGT core lipids accumulate in marine sediments and serve as the basis for geochemical proxies such as the TEX86 paleothermometer. Here, we demonstrate that the responses of membrane lipid compositions and resulting TEX86 values to growth temperature strongly diverge in three closely related thaumarchaeal pure cultures, i.e., Nitrosopumilus maritimus and two novel strains isolated from South Atlantic surface water, although the inventories of intact polar lipids and core lipids were overall similar in the three strains. N. maritimus and its closely related strain NAOA6 showed linear relationships of TEX86 and growth temperature but no correlation of TEX86 and temperature was observed in the more distantly related strain NAOA2. In contrast, the weighted average number of cycloalkyl moieties (ring index) was linearly correlated with growth temperature in all strains. This disparate relationship of TEX86 to growth temperature among closely related Thaumarchaeota suggests that the ring index but not the TEX86 ratio represents a universal response to growth temperature in marine planktonic Thaumarchaeota. Furthermore, the distinct TEX86-temperature relationships in the cultivated strains indicate that environmental GDGT signals may include an ecological component, which has important implications for ocean temperature reconstructions using the TEX86 proxy. In contrast, different growth medium salinities in the range 27–51‰ tested for N. maritimus showed no systematic effect on intact polar GDGT composition and TEX86. Similarly, N. maritimus showed only small changes in intact polar GDGT composition and TEX86 when grown at different medium pH in the range 7.3–7.9. Overall, our pure culture studies suggest that the TEX86 paleotemperature proxy is not solely dependent on growth temperature, but may amalgamate physiological, environmental, and ecological factors.

Sulfolobus Spindle-Shaped Virus 1 Contains Glycosylated Capsid Proteins, a Cellular Chromatin Protein, and Host-Derived Lipids

Geothermal and hypersaline environments are rich in virus-like particles, among which spindle-shaped morphotypes dominate. Currently, viruses with spindle- or lemon-shaped virions are exclusive to Archaea and belong to two distinct viral families. The larger of the two families, the Fuselloviridae, comprises tail-less, spindle-shaped viruses, which infect hosts from phylogenetically distant archaeal lineages. Sulfolobus spindle-shaped virus 1 (SSV1) is the best known member of the family and was one of the first hyperthermophilic archaeal viruses to be isolated. SSV1 is an attractive model for understanding virus-host interactions in Archaea; however, the constituents and architecture of SSV1 particles remain only partially characterized. Here, we have conducted an extensive biochemical characterization of highly purified SSV1 virions and identified four virus-encoded structural proteins, VP1 to VP4, as well as one DNA-binding protein of cellular origin. The virion proteins VP1, VP3, and VP4 undergo posttranslational modification by glycosylation, seemingly at multiple sites. VP1 is also proteolytically processed. In addition to the viral DNA-binding protein VP2, we show that viral particles contain the Sulfolobus solfataricus chromatin protein Sso7d. Finally, we provide evidence indicating that SSV1 virions contain glycerol dibiphytanyl glycerol tetraether (GDGT) lipids, resolving a long-standing debate on the presence of lipids within SSV1 virions. A comparison of the contents of lipids isolated from the virus and its host cell suggests that GDGTs are acquired by the virus in a selective manner from the host cytoplasmic membrane, likely during progeny egress. Although spindle-shaped viruses represent one of the most prominent viral groups in Archaea, structural data on their virion constituents and architecture still are scarce. The comprehensive biochemical characterization of the hyperthermophilic virus SSV1 presented here brings novel and significant insights into the organization and architecture of spindle-shaped virions. The obtained data permit the comparison between spindle-shaped viruses residing in widely different ecological niches, improving our understanding of the adaptation of viruses with unusual morphotypes to extreme environmental conditions.

A multiproxy analysis of sedimentary organic carbon in the Changjiang Estuary and adjacent shelf

AbstractSurface sediments from the Changjiang Estuary and adjacent shelf were analyzed using a variety of bulk and molecular techniques, including grain size composition, sediment surface area (SSA), elemental composition (C, N), stable carbon isotopic composition (δ13C), n‐alkanes, lignin phenols, and glycerol dialkyl glycerol tetraether lipids to obtain a more comprehensive understanding of the sources and fate of sedimentary organic carbon (SOC) in this dynamic region. Bulk N/C ratios of 0.09 to 0.15, δ13C of −24.4‰ to −21.1‰, branched/isoprenoid tetraether index of 0 to 0.74, n‐alkane content of 0.02 to 0.37 mg g−1 organic carbon (OC), and lignin content (Λ8) of 0.10 to 1.46 mg/100 mg OC and other related molecular indices in these samples indicate a mixed source of marine, soil, and terrestrial plant‐derived OC in the study area. A three‐end‐member mixing model using principal component analysis (PCA) factors as source markers and based on Monte Carlo (MC) simulation was constructed to estimate the relative contributions of OC from different sources. Compared with traditional mixing models, commonly based on a few variables, this newly developed PCA‐MC model supported bulk and biomarker data and yielded a higher‐resolution OC inputs to different subregions of this system. In particular, the results showed that the average contributions of marine, soil, and terrestrial OC in the study area were 35.3%, 47.0%, and 17.6%, and the highest contribution from each OC source was mainly observed in the shelf, inner estuary, and coastal region, respectively.

Descent toward the Icehouse: Eocene sea surface cooling inferred from GDGT distributions

AbstractThe TEX86 proxy, based on the distribution of marine isoprenoidal glycerol dialkyl glycerol tetraether lipids (GDGTs), is increasingly used to reconstruct sea surface temperature (SST) during the Eocene epoch (56.0–33.9 Ma). Here we compile published TEX86 records, critically reevaluate them in light of new understandings in TEX86 palaeothermometry, and supplement them with new data in order to evaluate long‐term temperature trends in the Eocene. We investigate the effect of archaea other than marine Thaumarchaeota upon TEX86 values using the branched‐to‐isoprenoid tetraether index (BIT), the abundance of GDGT‐0 relative to crenarchaeol (%GDGT‐0), and the Methane Index (MI). We also introduce a new ratio, %GDGTRS, which may help identify Red Sea‐type GDGT distributions in the geological record. Using the offset between TEX86H and TEX86L (ΔH‐L) and the ratio between GDGT‐2 and GDGT‐3 ([2]/[3]), we evaluate different TEX86 calibrations and present the first integrated SST compilation for the Eocene (55 to 34 Ma). Although the available data are still sparse some geographic trends can now be resolved. In the high latitudes (&gt;55°), there was substantial cooling during the Eocene (~6°C). Our compiled record also indicates tropical cooling of ~2.5°C during the same interval. Using an ensemble of climate model simulations that span the Eocene, our results indicate that only a small percentage (~10%) of the reconstructed temperature change can be ascribed to ocean gateway reorganization or paleogeographic change. Collectively, this indicates that atmospheric carbon dioxide (pCO2) was the likely driver of surface water cooling during the descent toward the icehouse.