Membrane lipids of mesophilic anaerobic bacteria thriving in peats have typical archaeal traits

The 16S ribosomal DNA based distinction between the bacterial and archaeal domains of life is strongly supported by the membrane lipid composition of the two domains; Bacteria generally contain dialkyl glycerol diester lipids, whereas Archaea produce isoprenoid dialkyl glycerol diether and membrane-spanning glycerol dialkyl glycerol tetraether (GDGT) lipids. Here we show that a new group of ecologically abundant membrane-spanning GDGT lipids, containing branched instead of isoprenoid carbon skeletons, are of a bacterial origin. This was revealed by examining the stereochemistry of the glycerol moieties of those branched tetraether membrane lipids, which was found to be the bacterial 1,2-di-O-alkyl-sn-glycerol stereoconfiguration and not the 2,3-di-O-alkyl-sn-glycerol stereoconfiguration as in archaeal membrane lipids. In addition, unequivocal evidence for the presence of cyclopentyl moieties in these bacterial membrane lipids was obtained by NMR. The biochemical traits of biosynthesis of tetraether membrane lipids and the formation of cyclopentyl moieties through internal cyclization, which were thought to be specific for the archaeal lineage of descent, thus also occur in the bacterial domain of life.     

13,16-Dimethyl octacosanedioic acid (iso-diabolic acid), a common membrane-spanning lipid of Acidobacteria subdivisions 1 and 3

The distribution of membrane lipids of 17 different strains representing 13 species of subdivisions 1 and 3 of the phylum Acidobacteria, a highly diverse phylum of the Bacteria, were examined by hydrolysis and gas chromatography-mass spectrometry (MS) and by high-performance liquid chromatography-MS of intact polar lipids. Upon both acid and base hydrolyses of total cell material, the uncommon membrane-spanning lipid 13,16-dimethyl octacosanedioic acid (iso-diabolic acid) was released in substantial amounts (22 to 43% of the total fatty acids) from all of the acidobacteria studied. This lipid has previously been encountered only in thermophilic Thermoanaerobacter species but bears a structural resemblance to the alkyl chains of bacterial glycerol dialkyl glycerol tetraethers (GDGTs) that occur ubiquitously in peat and soil and are suspected to be produced by acidobacteria. As reported previously, most species also contained iso-C(15) and C(16:1ω7C) as major fatty acids but the presence of iso-diabolic acid was unnoticed in previous studies, most probably because the complex lipid that contained this moiety was not extractable from the cells; it could only be released by hydrolysis. Direct analysis of intact polar lipids in the Bligh-Dyer extract of three acidobacterial strains, indeed, did not reveal any membrane-spanning lipids containing iso-diabolic acid. In 3 of the 17 strains, ether-bound iso-diabolic acid was detected after hydrolysis of the cells, including one branched GDGT containing iso-diabolic acid-derived alkyl chains. Since the GDGT distribution in soils is much more complex, branched GDGTs in soil likely also originate from other (acido)bacteria capable of biosynthesizing these components.     

Intact membrane lipids of "Candidatus Nitrosopumilus maritimus," a cultivated representative of the cosmopolitan mesophilic group I Crenarchaeota

In this study we analyzed the membrane lipid composition of "Candidatus Nitrosopumilus maritimus," the only cultivated representative of the cosmopolitan group I crenarchaeota and the only mesophilic isolate of the phylum Crenarchaeota. The core lipids of "Ca. Nitrosopumilus maritimus" consisted of glycerol dialkyl glycerol tetraethers (GDGTs) with zero to four cyclopentyl moieties. Crenarchaeol, a unique GDGT containing a cyclohexyl moiety in addition to four cyclopentyl moieties, was the most abundant GDGT. This confirms unambiguously that crenarchaeol is synthesized by species belonging to the group I.1a crenarchaeota. Intact polar lipid analysis revealed that the GDGTs have hexose, dihexose, and/or phosphohexose head groups. Similar polar lipids were previously found in deeply buried sediments from the Peru margin, suggesting that they were in part synthesized by group I crenarchaeota.     

Multiple environmental parameters impact lipid cyclization in Sulfolobus acidocaldarius

Adaptation of lipid membrane composition is an important component of archaeal homeostatic response. Historically, the number of cyclopentyl and cyclohexyl rings in the glycerol dibiphytanyl glycerol tetraether (GDGT) Archaeal lipids has been linked to variation in environmental temperature. However, recent work with GDGT-making archaea highlight the roles of other factors, such as pH or energy availability, in influencing the degree of GDGT cyclization. To better understand the role of multiple variables in a consistent experimental framework and organism, we cultivated the model Crenarchaeon Sulfolobus acidocaldarius DSM639 at different combinations of temperature, pH, oxygen flux, or agitation speed. We quantified responses in growth rate, biomass yield, and core lipid compositions, specifically the degree of core GDGT cyclization. The degree of GDGT cyclization correlated with growth rate under most conditions. The results suggest the degree of cyclization in archaeal lipids records a universal response to energy availability at the cellular level, both in thermoacidophiles, and in other recent findings in the mesoneutrophilic Thaumarchaea. Although we isolated the effects of key individual parameters, there remains a need for multi-factor experiments (e.g., pH + temperature + redox) in order to more robustly establish a framework to better understand homeostatic membrane responses.     

Temperature and pH controls on glycerol dibiphytanyl glycerol tetraether lipid composition in the hyperthermophilic crenarchaeon <Emphasis Type="Italic">Acidilobus sulfurireducens</Emphasis>

Cyclization in glycerol dibiphytanyl glycerol tetraethers (GDGTs) results in internal cyclopentane moieties which are believed to confer thermal stability to crenarchaeal membranes. While the average number of rings per GDGT lipid (ring index) is positively correlated with temperature in many temperate environments, poor correlations are often observed in geothermal environments, suggesting that additional parameters may influence GDGT core lipid composition in these systems. However, the physical and chemical parameters likely to influence GDGT cyclization which are often difficult to decouple in geothermal systems, making it challenging to assess their influence on lipid composition. In the present study, the influence of temperature (range 65–81°C), pH (range 3.0–5.0), and ionic strength (range 10.1–55.7 mM) on GDGT core lipid composition was examined in the hyperthermoacidophile Acidilobus sulfurireducens, a crenarchaeon originally isolated from a geothermal spring in Yellowstone National Park, Wyoming. When cultivated under defined laboratory conditions, the composition of individual and total GDGTs varied significantly with temperature and to a lesser extent with the pH of the growth medium. Ionic strength over the range of values tested did not influence GDGT composition. The GDGT core lipid ring index was positively correlated with temperature and negatively correlated with pH, suggesting that A. sulfurireducens responds to increasing temperature and acidity by increasing the number of cyclopentyl rings in GDGT core membrane lipids.     

Constraining the formation of authigenic carbonates in a seepage‐affected cold‐water coral mound by lipid biomarkers

Cold‐water coral (CWC) mounds are build‐ups comprised of coral‐dominated intervals alternating with a mixed carbonate‐siliciclastic matrix. At some locations, CWC mounds are influenced by methane seepage, but the impact of methane on CWC mounds is poorly understood. To constrain the potential impact of methane on CWC mound growth, lipid biomarker investigations were combined with mineralogical and petrographic analyses to investigate the anaerobic oxidation of methane (AOM) and authigenic carbonate formation in sediment from a seep‐affected CWC mound in the Gulf of Cadiz. The occurrence of AOM was confirmed by characteristic lipids found within a semi‐lithified zone (SLZ) consisting of authigenic aragonite, high‐magnesium calcite and calcium‐excess dolomite. The formation of high‐Mg calcite is attributed to AOM, acting as a lithifying agent. Aragonite is only a minor phase. Ca‐excess dolomite in the SLZ and upper parts may be formed by organoclastic sulphate reduction, favouring precipitation by increased alkalinity. The AOM biomarkers in the SLZ include isoprenoid‐based archaeal membrane lipids, such as abundant glycerol dibiphytanyl glycerol tetraethers (GDGTs) dominated by GDGT‐2. The δ¹³C values of GDGT‐2, measured as ether‐cleaved monocyclic biphytanes, are as low as ?100‰ versus V‐PDB. Further, bacterial dialkyl glycerol diethers with two anteiso‐C₁₅ alkyl chains and δ¹³C values of ?81‰ are interpreted as biomarkers of sulphate‐reducing bacteria. The lipid biomarker signatures and mineralogical patterns suggest that anaerobic methane‐oxidizing archaea of the ANME‐1 group thrived in the subsurface at times of slow and diffusive methane seepage. Petrographic analyses revealed that the SLZ was exhumed at some point (e.g. signs of bioerosion of the semi‐lithified sediment), providing a hard substrate for CWC larval settlement. In addition, this work reveals that AOM‐induced semi‐lithification likely played a role in mound stabilization. Lipid biomarker analysis proves to be a powerful tool to disentangle early diagenetic processes induced by microbial metabolisms.     

Energy flux controls tetraether lipid cyclization in Sulfolobus acidocaldarius

Microorganisms regulate the composition of their membranes in response to environmental cues. Many Archaea maintain the fluidity and permeability of their membranes by adjusting the number of cyclic moieties within the cores of their glycerol dibiphytanyl glycerol tetraether (GDGT) lipids. Cyclized GDGTs increase membrane packing and stability, which has been shown to help cells survive shifts in temperature and pH. However, the extent of this cyclization also varies with growth phase and electron acceptor or donor limitation. These observations indicate a relationship between energy metabolism and membrane composition. Here we show that the average degree of GDGT cyclization increases with doubling time in continuous cultures of the thermoacidophile Sulfolobus acidocaldarius (DSM 639). This is consistent with the behavior of a mesoneutrophile, Nitrosopumilus maritimus SCM1. Together, these results demonstrate that archaeal GDGT distributions can shift in response to electron donor flux and energy availability, independent of pH or temperature. Paleoenvironmental reconstructions based on GDGTs thus capture the energy available to microbes, which encompasses fluctuations in temperature and pH, as well as electron donor and acceptor availability. The ability of Archaea to adjust membrane composition and packing may be an important strategy that enables survival during episodes of energy stress.     

A combined lipidomic and 16S rRNA gene amplicon sequencing approach reveals archaeal sources of intact polar lipids in the stratified Black Sea water column

Archaea are important players in marine biogeochemical cycles, and their membrane lipids are useful biomarkers in environmental and geobiological studies. However, many archaeal groups remain uncultured and their lipid composition unknown. Here, we aim to expand the knowledge on archaeal lipid biomarkers and determine the potential sources of those lipids in the water column of the euxinic Black Sea. The archaeal community was evaluated by 16S rRNA gene amplicon sequencing and by quantitative PCR. The archaeal intact polar lipids (IPLs) were investigated by ultra‐high‐pressure liquid chromatography coupled to high‐resolution mass spectrometry. Our study revealed both a complex archaeal community and large changes with water depth in the IPL assemblages. In the oxic/upper suboxic waters (<105 m), the archaeal community was dominated by marine group (MG) I Thaumarchaeota, coinciding with a higher relative abundance of hexose phosphohexose crenarchaeol, a known marker for Thaumarchaeota. In the suboxic waters (80–110 m), MGI Nitrosopumilus sp. dominated and produced predominantly monohexose glycerol dibiphytanyl glycerol tetraethers (GDGTs) and hydroxy‐GDGTs. Two clades of MGII Euryarchaeota were present in the oxic and upper suboxic zones in much lower abundances, preventing the detection of their specific IPLs. In the deep sulfidic waters (>110 m), archaea belonging to the DPANN Woesearchaeota, Bathyarchaeota, and ANME‐1b clades dominated. Correlation analyses suggest that the IPLs GDGT‐0, GDGT‐1, and GDGT‐2 with two phosphatidylglycerol (PG) head groups and archaeol with a PG, phosphatidylethanolamine, and phosphatidylserine head groups were produced by ANME‐1b archaea. Bathyarchaeota represented 55% of the archaea in the deeper part of the euxinic zone and likely produces archaeol with phospho‐dihexose and hexose‐glucuronic acid head groups.     

Chemotaxonomic characterisation of the thaumarchaeal lipidome

Thaumarchaeota are globally distributed and abundant microorganisms occurring in diverse habitats and thus represent a major source of archaeal lipids. The scope of lipids as taxonomic markers in microbial ecological studies is limited by the scarcity of comparative data on the membrane lipid composition of cultivated representatives, including the phylum Thaumarchaeota. Here, we comprehensively describe the core and intact polar lipid (IPL) inventory of ten ammonia‐oxidising thaumarchaeal cultures representing all four characterized phylogenetic clades. IPLs of these thaumarchaeal strains are generally similar and consist of membrane‐spanning, glycerol dibiphytanyl glycerol tetraethers with monoglycosyl, diglycosyl, phosphohexose and hexose‐phosphohexose headgroups. However, the relative abundances of these IPLs and their core lipid compositions differ systematically between the phylogenetic subgroups, indicating high potential for chemotaxonomic distinction of thaumarchaeal clades. Comparative lipidomic analyses of 19 euryarchaeal and crenarchaeal strains suggested that the lipid methoxy archaeol is synthesized exclusively by Thaumarchaeota and may thus represent a diagnostic lipid biomarker for this phylum. The unprecedented diversity of the thaumarchaeal lipidome with 118 different lipids suggests that membrane lipid composition and adaptation mechanisms in Thaumarchaeota are more complex than previously thought and include unique lipids with as yet unresolved properties.     

Thermophilic archaeon orchestrates temporal expression of GDGT ring synthases in response to temperature and acidity stress

Glycerol dibiphytanyl glycerol tetraethers (GDGTs) are unique archaeal membrane-spanning lipids with 0–8 cyclopentane rings on the biphytanyl chains. The cyclization pattern of GDGTs is affected by many environmental factors, such as temperature and pH, but the underlying molecular mechanism remains elusive. Here, we find that the expression regulation of GDGT ring synthase genes grsA and grsB in thermophilic archaeon Sulfolobus acidocaldarius is temperature- and pH-dependent. Moreover, the presence of functional GrsA protein, or more likely its products cyclic GDGTs rather than the accumulation of GrsA protein itself, is required to induce grsB expression, resulting in temporal regulation of grsA and grsB expression. Our findings establish a molecular model of GDGT cyclization regulated by environment factors in a thermophilic ecosystem, which could be also relevant to that in mesophilic marine archaea. Our study will help better understand the biological basis for GDGT-based paleoclimate proxies. Archaea inhabit a wide range of terrestrial and marine environments. In response to environment fluctuations, archaea modulate their unique membrane GDGTs lipid composition with different strategies, in particular GDGTs cyclization significantly alters membrane permeability. However, the regulation details of archaeal GDGTs cyclization in response to different environmental factor changes remain unknown. We demonstrated, for the first time, thermophilic archaea orchestrate the temporal expression of GDGT ring synthases, leading to delicate control of GDGTs cyclization to respond environmental temperature and acidity stress. Our study provides insight into the regulation of archaea membrane plasticity, and the survival strategy of archaea in fluctuating environments.     

Intact polar and core glycerol dibiphytanyl glycerol tetraether lipids of group I.1a and I.1b thaumarchaeota in soil

Ecological studies of thaumarchaeota often apply glycerol dibiphytanyl glycerol tetraether (GDGT)-based intact membrane lipids. However, these components have only been characterized for thaumarchaeota from aquatic environments. Thaumarchaeota have been shown to play an important role in the nitrogen cycle in soil as ammonium oxidizers, and GDGTs are common lipids encountered in soil. We report the core and intact polar lipid (IPL) GDGTs produced by three newly available thaumarchaeota isolated from grassland soil in Austria ("Nitrososphaera viennensis," group I.1b) and enriched from agricultural soils in South Korea ("Candidatus Nitrosoarchaeum koreensis" MY1, group I.1a; and "Candidatus Nitrososphaera" strain JG1, group I.1b). The soil thaumarchaeota all synthesize crenarchaeol as their major core GDGT, in agreement with the fact that crenarchaeol has also been detected in thaumarchaeota from aquatic environments. The crenarchaeol regioisomer apparently is produced in significant quantities only by soil thaumarchaeota of the I.1b subgroup. In addition, GDGTs with 0 to 4 cyclopentane moieties and GDGTs containing an additional hydroxyl group were detected. The IPL head groups of their membrane lipids comprised mainly monohexose, dihexose, trihexose, phosphohexose, and hexose-phosphohexose moieties. The hexose-phosphohexose head group bound to crenarchaeol occurred in all soil thaumarchaeota, and this IPL is at present the only lipid that is detected in all thaumarchaeota analyzed so far. This specificity and its lability indicate that it is the most suitable biomarker lipid to trace living thaumarchaeota. This study, in combination with previous studies, also suggests that hydroxylated GDGTs occur in the I.1a, but not in the I.1b, subgroup of the thaumarchaeota.     

Thermotropic properties of bipolar lipids of Sulfolobus solfataricus and of their mixtures with dipalmitoylphosphatidylcholine

The thermotropic properties of the bipolar lipids, glycerol dialkylglycerol tetraether (GDGT) and glycerol dialkylnonitol tetraether (GDNT), were determined at different degrees of hydration and in mixtures with dipalmitoylphosphatidylcholine (DPPC). The number of water molecules rendered unfreezable by the GDNT molecule is 10+/-1.5 and that by the GDGT molecule 2.8+/-0.7 or about 1.1-1.5 H2O molecules per OH group. Binding of water molecules causes randomization of the two polar heads from the oriented form prevailing in the dry state. The hydration seems to be a cooperative process extending over a whole lipid domain. DPPC added in small amounts to GDNT interacts preferentially with the nonitol halves of the molecules separating them from the glycerol half molecules. In the cooperative interaction domain each DPPC molecule is surrounded by up to six GDNT molecules. Cooperative domains formed during the interaction of DPPC with GDGT are less pronounced. In both cases they affect the thermotropic properties of the system.     

Monolayers of ether lipids from archaebacteria

The surface behavior of six different ether lipids from archaebacteria, based on condensation of glycerol or more complex polyols with two isoprenoid alcohols at 20 or 40 carbon atoms, was investigated in monolayers at the air-water interface.The compounds with no complex polar group (GD, GDGT, GDNT) form monolayers showing a reversible collapse at surface pressure as low as 22 dynes/cm. This collapse pressure decrease with temperature in such a way that the film tension remains constant. In condensed films, these molecules do not assume a completely upright position.Lipids with complex polar ends (HL, GLB, PLII) form films more stable to compression. Forcearea characteristics and surface moment values of HL monolayers are similar to those of analogous ester lipids with fatty acid chains. Monolayers of the two bipolar lipids, GLB and PLII, at room temperature present a more condensed state, probably due to the lateral cohesion between long alkyl chains, but a lower collapse pressure.For all bipolar lipids, the area expansion induced by temperature increase is larger than that of monopolar ones.Abbreviations GD Glycerol diether (2,3-di-O-phytanyl-sn-glycerol - GDGT Glycerol-dialkyl-glycerol tetraether - GDNT Glycerol-dialkyl-nonitol tetraether - GLB Glycolipid B - PLII Phospholipid II - HL Total lipid extract from Halobacterium halobium     

Crenarchaeol: the characteristic core glycerol dibiphytanyl glycerol tetraether membrane lipid of cosmopolitan pelagic crenarchaeota

The basic structure and stereochemistry of the characteristic glycerol dibiphytanyl glycerol tetraether (GDGT) membrane lipid of cosmopolitan pelagic crenarchaeota has been identified by high field two-dimensional (2D)-NMR techniques. It contains one cyclohexane and four cyclopentane rings formed by internal cyclisation of the biphytanyl chains. Its structure is similar to that of GDGTs biosynthesized by (hyper)thermophilic crenarchaeota apart from the cyclohexane ring. These findings are consistent with the close phylogenetic relationship of (hyper)thermophilic and pelagic crenarchaeota based 16S rRNA. The latter group inherited the biosynthetic capabilities for a membrane composed of cyclopentane ring-containing GDGTs from the (hyper)thermophilic crenarchaeota. However, to cope with the much lower temperature of the ocean, a small but key step in their evolution was the adjustment of the membrane fluidity by making a kink in one of the bicyclic biphytanyl chains by the formation of a cyclohexane ring. This prevents the dense packing characteristic for the cyclopentane ring-containing GDGTs membrane lipids used by hyperthermophilic crenarchaeota to adjust their membrane fluidity to high temperatures.     

Biomarkers at the microscopic range: ToF-SIMS molecular imaging of Archaea-derived lipids in a microbial mat

Time‐of‐Flight Secondary Ion Mass Spectrometry (ToF‐SIMS) with a bismuth cluster primary ion source was used for analysing microbial lipid biomarkers in 10‐µm‐thick microscopic cryosections of methanotrophic microbial mats from the Black Sea. Without further sample preparation, archaeal isopranyl glycerol di‐ and tetraether core lipids, together with their intact diglycoside (gentiobiosyl‐) derivatives, were simultaneously identified by exact mass determination. Utilizing the imaging capability of ToF‐SIMS, the spatial distributions of these biomarkers were mapped at a lateral resolution of < 5 µm in 500 × 500 µm² areas on the mat sections. Using cluster projectiles in the burst alignment mode, it was possible to reach a lateral resolution of 1 µm on an area of 233 × 233 µm, thus approaching the typical size of microbial cells. The mappings showed different ‘provenances’ within the sections that are distinguished by individual lipid fingerprints, namely (A) the diethers archaeol and hydroxyarchaeol co‐occurring with glycerol dialkyl glycerol tetraethers (GDGT), (B) hydroxyarchaeol and dihydroxyarchaeol, and (C) GDGT and gentiobiosyl‐GDGT. Because ToF‐SIMS is a virtually nondestructive technique affecting only the outermost layers of the sample surface (typically 10–100 nm), it was possible to further examine the studied areas using conventional microscopy, and associate the individual lipid patterns with specific morphological traits. This showed that provenance (B) was frequently associated with irregular, methane‐derived CaCO₃ crystallites, whereas provenance (C) revealed a population of fluorescent, filamentous microorganisms showing the morphology of known methanotrophic ANME‐1 archaea. The direct coupling of imaging mass spectrometry with microscopic techniques reveals interesting perspectives for the in‐situ study of lipids in geobiology, microbial ecology, and organic geochemistry. After further developing protocols for handling different kinds of environmental samples, ToF‐SIMS could be used as a tool to attack many challenging problems in these fields, such as the attribution of biological source(s) to particular biomarkers in question, or the high‐resolution tracking of biogeochemical processes in modern and ancient natural environments.     

Monopolar-bipolar lipid interactions in model membrane systems

1H-NMR, dynamic light scattering and negative staining electron microscopy have been used to study the formation and physico-chemical properties of aqueous dispersions of mixtures of monopolar lipids extracted from Sulfolobus solfataricus. This microorganism is a thermophilic archaeobacterium growing optimally at about 85 degrees C and pH 3. The two hydrolytic fractions of the membrane complex lipids that have been studied are: the symmetric lipid glycerol dialkyl glycerol tetraether (GDGT) and the asymmetric lipid glycerol dialkyl nonitol tetraether (GDNT). Electron micrographs of pure and mixed GDNT and GDGT dispersions show the formation of complex structures. Only above a critical monopolar/bipolar lipid ratio, typical of the bipolar lipid, could closed structures be formed and good agreement was obtained in sizing with NMR, electron microscopy and dynamic light scattering. NMR spectra have been carried out at several temperatures from 25 degrees to 85 degrees C, to obtain information on the temperature-dependent structural, dynamic and permeability properties of the co-dispersed vesicles. The results are discussed in terms of the steric constraints and the chemico-physical interactions occurring among the different parts of the molecules and compared with previous studies performed with different physical techniques.     

Archaeal and bacterial glycerol dialkyl glycerol tetraether lipids in hot springs of yellowstone national park

Glycerol dialkyl glycerol tetraethers (GDGTs) are core membrane lipids originally thought to be produced mainly by (hyper)thermophilic archaea. Environmental screening of low-temperature environments showed, however, the abundant presence of structurally diverse GDGTs from both bacterial and archaeal sources. In this study, we examined the occurrences and distribution of GDGTs in hot spring environments in Yellowstone National Park with high temperatures (47 to 83 degrees C) and mostly neutral to alkaline pHs. GDGTs with 0 to 4 cyclopentane moieties were dominant in all samples and are likely derived from both (hyper)thermophilic Crenarchaeota and Euryarchaeota. GDGTs with 4 to 8 cyclopentane moieties, likely derived from the crenarchaeotal order Sulfolobales and the euryarchaeotal order Thermoplasmatales, are usually present in much lower abundance, consistent with the relatively high pH values of the hot springs. The relative abundances of cyclopentane-containing GDGTs did not correlate with in situ temperature and pH, suggesting that other environmental and possibly genetic factors play a role as well. Crenarchaeol, a biomarker thought to be specific for nonthermophilic group I Crenarchaeota, was also found in most hot springs, though in relatively low concentrations, i.e., <5% of total GDGTs. Its abundance did not correlate with temperature, as has been reported previously. Instead, the cooccurrence of relatively abundant nonisoprenoid GDGTs thought to be derived from soil bacteria suggests a predominantly allochthonous source for crenarchaeol in these hot spring environments. Finally, the distribution of bacterial branched GDGTs suggests that they may be derived from the geothermally heated soils surrounding the hot springs.     

Plasma membrane lipid composition of the halophilic cyanobacterium Aphanothece halophytica

The plasma membrane from Aphanothece halophytica was isolated using both glycerol and sucrose gradient centrifugation. The isolated membrane was characterized for lipid content by TLC and isolated lipids were quantified by chemical analysis. The plasma membrane of A. halophytica was composed of MGDG, DGDG and PG. The sulfur containing lipid SQDG was not detected. The mole percent of each lipid in the plasma membrane varied with the external salinity of the media. MGDG was the most abundant lipid in the plasma membrane of cells grown at one molar external NaCl. At three molar external NaCl, PG was the most abundant lipid. The ratio of uncharged to charged lipids comprising the plasma membrane decreased as the external salinity increased. It is possible that the alteration in lipid composition is of major importance in the adaptation of A. halophytica to changing external salinity.Abbreviations TLC Thin-layer chromatography - MGDG momogalactosyldiacylglycerol - DGDG digaloctosyldiacylglycerol - PG phosphatidylglycerol - SQDG sulphoquinovosyldiacylglycerol     

Archaeal and Bacterial Glycerol Dialkyl Glycerol Tetraether Lipids in Hot Springs of Yellowstone National Park

Glycerol dialkyl glycerol tetraethers (GDGTs) are core membrane lipids originally thought to be produced mainly by (hyper)thermophilic archaea. Environmental screening of low-temperature environments showed, however, the abundant presence of structurally diverse GDGTs from both bacterial and archaeal sources. In this study, we examined the occurrences and distribution of GDGTs in hot spring environments in Yellowstone National Park with high temperatures (47 to 83°C) and mostly neutral to alkaline pHs. GDGTs with 0 to 4 cyclopentane moieties were dominant in all samples and are likely derived from both (hyper)thermophilic Crenarchaeota and Euryarchaeota. GDGTs with 4 to 8 cyclopentane moieties, likely derived from the crenarchaeotal order Sulfolobales and the euryarchaeotal order Thermoplasmatales, are usually present in much lower abundance, consistent with the relatively high pH values of the hot springs. The relative abundances of cyclopentane-containing GDGTs did not correlate with in situ temperature and pH, suggesting that other environmental and possibly genetic factors play a role as well. Crenarchaeol, a biomarker thought to be specific for nonthermophilic group I Crenarchaeota, was also found in most hot springs, though in relatively low concentrations, i.e., <5% of total GDGTs. Its abundance did not correlate with temperature, as has been reported previously. Instead, the cooccurrence of relatively abundant nonisoprenoid GDGTs thought to be derived from soil bacteria suggests a predominantly allochthonous source for crenarchaeol in these hot spring environments. Finally, the distribution of bacterial branched GDGTs suggests that they may be derived from the geothermally heated soils surrounding the hot springs.     

Distribution of intact and core membrane lipids of archaeal glycerol dialkyl glycerol tetraethers among size-fractionated particulate organic matter in hood canal, puget sound

There is great interest in the membrane lipids of archaea (glycerol dialkyl glycerol tetraethers [GDGTs]) as tracers of archaeal biomass because of their utility as paleoproxies and because of the biogeochemical importance of archaea. While core GDGTs (formed by hydrolysis of polar head groups of intact GDGTs after cell death) are appropriate for paleostudies, they have also been used to trace archaeal populations. Also, despite the small size (0.2 by 0.7 μm) of cultivated marine archaea, 0.7-μm glass-fiber filters (GFFs) are typically used to collect GDGTs from natural waters. We quantified both core and intact GDGTs in free-living (0.2- to 0.7-μm), suspended (0.7- to 60-μm), and aggregate (>60-μm) particle size fractions in Puget Sound (Washington State). On average, the free-living fraction contained 36% of total GDGTs, 90% of which were intact. The intermediate-size fraction contained 62% of GDGTs, and 29% of these were intact. The aggregate fraction contained 2% of the total GDGT pool, and 29% of these were intact. Our results demonstrate that intact GDGTs are largely in the free-living fraction. Because only intact GDGTs are present in living cells, protocols that target this size fraction and analyze the intact GDGT pool are necessary to track living populations in marine waters. Core GDGT enrichment in larger-size fractions indicates that archaeal biomass may quickly become attached or entrained in particles once the archaea are dead or dying. While the concentrations of the two pools were generally not correlated, the similar sizes of the core and intact GDGT pools suggest that core GDGTs are removed from the water column on timescales similar to those of cell replication, on timescales of days to weeks.