Identification and quantification of polyfunctionalized hopanoids by high temperature gas chromatography-mass spectrometry

Hopanoid lipids promote soybean-Bradyrhizobium symbiosis

The symbioses between leguminous plants and nitrogen-fixing bacteria known as rhizobia are well known for promoting plant growth and sustainably increasing soil nitrogen. Recent evidence indicates that hopanoids, a family of steroid-like lipids, promote Bradyrhizobium symbioses with tropical legumes. To characterize hopanoids in Bradyrhizobium symbiosis with soybean, we validated a recently published cumate-inducible hopanoid mutant of Bradyrhizobium diazoefficiens USDA110, Pcu-shc::∆shc. GC-MS analysis showed that this strain does not produce hopanoids without cumate induction, and under this condition, is impaired in growth in rich medium and under osmotic, temperature, and pH stress. In planta, Pcu-shc::∆shc is an inefficient soybean symbiont with significantly lower rates of nitrogen fixation and low survival within the host tissue. RNA-seq revealed that hopanoid loss reduces the expression of flagellar motility and chemotaxis-related genes, further confirmed by swim plate assays, and enhances the expression of genes related to nitrogen metabolism and protein secretion. These results suggest that hopanoids provide a significant fitness advantage to B. diazoefficiens in legume hosts and provide a foundation for future mechanistic studies of hopanoid function in protein secretion and motility.

A major problem for global sustainability is feeding our exponentially growing human population while available arable land decreases. Harnessing the power of plant-beneficial microbes is a potential solution, including increasing our reliance on the symbioses of leguminous plants and nitrogen-fixing rhizobia. This study examines the role of hopanoid lipids in the symbiosis between Bradyrhizobium diazoefficiens USDA110, an important commercial inoculant strain, and its economically significant host soybean. Our research extends our knowledge of the functions of bacterial lipids in symbiosis to an agricultural context, which may one day help improve the practical applications of plant-beneficial microbes in agriculture.

Hopanoid lipids promote soybean- Bradyrhizobium symbiosis

The symbioses between leguminous plants and nitrogen-fixing bacteria known as rhizobia are well known for promoting plant growth and sustainably increasing soil nitrogen. Recent evidence indicates that hopanoids, a family of steroid-like lipids, promote Bradyrhizobium symbioses with tropical legumes. To characterize hopanoids in Bradyrhizobium symbiosis with soybean, the most economically significant Bradyrhizobium host, we validated a recently published cumate-inducible hopanoid mutant of Bradyrhizobium diazoefficiens USDA110, Pcu- shc ::Δ shc . GC-MS analysis showed that this strain does not produce hopanoids without cumate induction, and under this condition, is impaired in growth in rich medium and under osmotic, temperature, and pH stress. In planta , Pcu- shc ::Δ shc is an inefficient soybean symbiont with significantly lower rates of nitrogen fixation and low survival within host tissue. RNA-seq revealed that hopanoid loss reduces expression of flagellar motility and chemotaxis-related genes, further confirmed by swim plate assays, and enhances expression of genes related to nitrogen metabolism and protein secretion. These results suggest that hopanoids provide a significant fitness advantage to B. diazoefficiens in legume hosts and provide a foundation for future mechanistic studies of hopanoid function in protein secretion and motility.

IMPORTANCE

A major problem for global sustainability is feeding our exponentially growing human population while available arable land is decreasing, especially in areas with the greatest population growth. Harnessing the power of plant-beneficial microbes has gained attention as a potential solution, including the increasing our reliance on the symbioses of leguminous plants and nitrogen-fixing rhizobia. This study examines the role of hopanoid lipids in the symbiosis between Bradyrhizobium diazoefficiens USDA110, an important commercial inoculant strain, and its economically important host soybean. Our research extends our knowledge of the functions of bacterial lipids in symbiosis to an agricultural context, which may one day help improve the practical applications of plant-beneficial microbes in agriculture.

Apportioning sedimentary organic matter sources and its degradation state: Inferences based on aliphatic hydrocarbons, amino acids and δ15N

The sources and state of sedimentary organic matter (SOM) in fresh water aquatic systems are important to understand the carbon cycling in terrestrial environments. The composition of organic matter in the lake sediments demonstrates the physical and chemical condition of the lake ecosystems. However, the systematic and structured investigations focussed on to understand the source and fate of organic matters within eutrophic lakes is still far from clear. The present study is focusing on the implications of amino acids (AA), aliphatic hydrocarbons and bulk geochemical (C/N, δ¹⁵N) proxies to understand the distribution, sources and state of sedimentary organic matter in Ahansar Lake from Kashmir valley, India. The relatively low C/N ratios along with high AA contents indicate enhanced aquatic productivity in the lake system. Likewise, the dominance of the mid-chain monomethyl alkanes (MMAs), highly branched isoprenoids (HBIs), botryococcenes, steroids and triterpenoids suggest OM sourced from periphyton remains. Furthermore, the presence of C₂₇, C₂₈ and C₂₉ diagenetically altered steroids also reflects a major algal contribution. The spatial variability of P_aq demonstrates their applicability as a proxy for the contribution of aquatic vegetation. The ratio of individual amino acids (oxic/anoxic ratio) and low Pr/Ph (pristane/phytane) values indicate anoxic nature of the current depositional environment. This also leads to significant organic matter preservation as revealed by amino acid indices (e.g., degradation index - DI and reactivity index - RI). These data collectively demonstrate the systematic investigation and comprehensive understanding of source of sedimentary organic matters and respective depositional condition via multiple indicators. Overall, understanding the OM molecular composition and its spatial heterogeneity in a lake system is important to better constrain the fate of organic carbon, and assess the pollution risks as well as adopt relevant management strategies.

Analysis of Bacteriohopanoids from Thermophilic Bacteria by Liquid Chromatography-Mass Spectrometry

Background: Hopanoids modify plasma membrane properties in bacteria and are often compared to sterols that modulate membrane fluidity in eukaryotes. In some microorganisms, they can also allow adaptations to extreme environments. Methods: Hopanoids were identified by liquid chromatography-mass spectrometry in fourteen strains of thermophilic bacteria belonging to five genera, i.e., Alicyclobacillus, Brevibacillus, Geobacillus, Meiothermus, and Thermus. The bacteria were cultivated at temperatures from 42 to 70 °C. Results: Regardless of the source of origin, the strains have the same tendency to adapt the hopanoid content depending on the cultivation temperature. In the case of aminopentol, its content increases; aminotetrol does not show a significant change; and in the case of aminotriol the content decreases by almost a third. The content of bacteriohopanetetrol and bacteriohopanetetrol glycoside decreases with increasing temperature, while in the case of adenosylhopane the opposite trend was found. Conclusions: Changes in hopanoid content can be explained by increased biosynthesis, where adenosylhopane is the first intermediate in the biosynthesis of the hopanoid side chain.

Anaerobic 3-methylhopanoid production by an acidophilic photosynthetic purple bacterium

Bacterial lipids are well-preserved in ancient rocks and certain ones have been used as indicators of specific bacterial metabolisms or environmental conditions existing at the time of rock deposition. Here we show that an anaerobic bacterium produces 3-methylhopanoids, pentacyclic lipids previously detected only in aerobic bacteria and widely used as biomarkers for methane-oxidizing bacteria. Both Rhodopila globiformis, a phototrophic purple nonsulfur bacterium isolated from an acidic warm spring in Yellowstone, and a newly isolated Rhodopila species from a geochemically similar spring in Lassen Volcanic National Park (USA), synthesized 3-methylhopanoids and a suite of related hopanoids and contained the genes encoding the necessary biosynthetic enzymes. Our results show that 3-methylhopanoids can be produced under anoxic conditions and challenges the use of 3-methylhopanoids as biomarkers of oxic conditions in ancient rocks and as prima facie evidence that methanotrophic bacteria were active when the rocks were deposited.

A squalene-hopene cyclase in Schizosaccharomyces japonicus represents a eukaryotic adaptation to sterol-limited anaerobic environments

Biosynthesis of sterols, which are key constituents of canonical eukaryotic membranes, requires molecular oxygen. Anaerobic protists and deep-branching anaerobic fungi are the only eukaryotes in which a mechanism for sterol-independent growth has been elucidated. In these organisms, tetrahymanol, formed through oxygen-independent cyclization of squalene by a squalene-tetrahymanol cyclase, acts as a sterol surrogate. This study confirms an early report [C. J. E. A. Bulder, Antonie Van Leeuwenhoek, 37, 353-358 (1971)] that Schizosaccharomyces japonicus is exceptional among yeasts in growing anaerobically on synthetic media lacking sterols and unsaturated fatty acids. Mass spectrometry of lipid fractions of anaerobically grown Sch. japonicus showed the presence of hopanoids, a class of cyclic triterpenoids not previously detected in yeasts, including hop-22(29)-ene, hop-17(21)-ene, hop-21(22)-ene, and hopan-22-ol. A putative gene in Sch. japonicus showed high similarity to bacterial squalene-hopene cyclase (SHC) genes and in particular to those of Acetobacter species. No orthologs of the putative Sch. japonicus SHC were found in other yeast species. Expression of the Sch. japonicus SHC gene (Sjshc1) in Saccharomyces cerevisiae enabled hopanoid synthesis and stimulated anaerobic growth in sterol-free media, thus indicating that one or more of the hopanoids produced by SjShc1 could at least partially replace sterols. Use of hopanoids as sterol surrogates represents a previously unknown adaptation of eukaryotic cells to anaerobic growth. The fast anaerobic growth of Sch. japonicus in sterol-free media is an interesting trait for developing robust fungal cell factories for application in anaerobic industrial processes.

Variation of salinity and nitrogen concentration affects the pentacyclic triterpenoid inventory of the haloalkaliphilic aerobic methanotrophic bacterium Methylotuvimicrobium alcaliphilum

The occurrence and activity of aerobic methanotrophs are influenced by environmental conditions, including pH, temperature, salinity, methane and oxygen concentrations, and nutrient availability. Aerobic methanotrophs synthesize a variety of lipids important for cell functions. However, culture-based experiments studying the influence of environmental parameters on lipid production by aerobic methanotrophs are scarce. Such information is crucial to interpret lipid patterns of methanotrophic bacteria in the environment. In this study, the alkaliphilic strain Methylotuvimicrobium alcaliphilum was cultivated under different salinities and different nitrate concentrations to assess the effect of changing conditions on the inventory of pentacyclic triterpenoids. The results indicate that hopanoid abundance is enhanced at lower salinity and higher nitrate concentration. The production of most pentacyclic triterpenoids was favored at low salinity, especially for aminotriol. Interestingly, 3-methyl-aminotetrol and tetrahymanol were favored at higher salinity. Bacteriohopanepolyols (BHPs), particularly aminotriol and 3-methyl-aminotriol, increased considerably at higher nitrate concentrations. Four novel N-containing BHPs—aminodiol, 3-methyl-aminodiol, and isomers of aminotriol and 3-methyl-aminotriol—were identified. This study highlights the significance of environmental factors for bacterial lipid production and documents the need for cultivation studies under variable conditions to utilize the full potential of the biomarker concept.

Hopanoid lipids: from membranes to plant-bacteria interactions

Lipid research represents a frontier for microbiology, as showcased by hopanoid lipids. Hopanoids, which resemble sterols and are found in the membranes of diverse bacteria, have left an extensive molecular fossil record. They were first discovered by petroleum geologists. Today, hopanoid-producing bacteria remain abundant in various ecosystems, such as the rhizosphere. Recently, great progress has been made in our understanding of hopanoid biosynthesis, facilitated in part by technical advances in lipid identification and quantification. A variety of genetically tractable, hopanoid-producing bacteria have been cultured, and tools to manipulate hopanoid biosynthesis and detect hopanoids are improving. However, we still have much to learn regarding how hopanoid production is regulated, how hopanoids act biophysically and biochemically, and how their production affects bacterial interactions with other organisms, such as plants. The study of hopanoids thus offers rich opportunities for discovery.

Bacteriohopanepolyols along redox gradients in the Humboldt Current System off northern Chile

Marine oxygen minimum zones (OMZs) are characterized by the presence of subsurface suboxic or anoxic waters where diverse microbial processes are responsible for the removal of fixed nitrogen. OMZs have expanded over past decades and are expected to continue expanding in response to the changing climate. The implications for marine biogeochemistry, particularly nitrogen cycling, are uncertain. Cell membrane lipids (biomarkers), such as bacterial bacteriohopanepolyols (BHPs) and their degradation products (hopanoids), have distinctive structural attributes that convey information about their biological sources. Since the discovery of fossil hopanoids in ancient sediments, the study of BHPs has been of great biogeochemical interest due to their potential to serve as proxies for bacteria in the geological record. A stereoisomer of bacteriohopanetetrol (BHT), BHT II, has been previously identified in OMZ waters and has as been unequivocally identified in culture enrichments of anammox bacteria, a key group contributing to nitrogen loss in marine OMZs. We tested BHT II as a proxy for suboxia/anoxia and anammox bacteria in suspended organic matter across OMZ waters of the Humboldt Current System off northern Chile, as well as in surface and deeply buried sediments (125-150 ky). The BHT II ratio (BHT II/total BHT) increases as oxygen content decreases through the water column, consistent with previous results from Perú, the Cariaco Basin and the Arabian Sea, and in line with microbiological evidence indicating intense anammox activity in the Chilean OMZ. Notably, BHT II is transported from the water column to surface sediments, and preserved in deeply buried sediments, where the BHT II ratio correlates with changes in δ¹⁵ N sediment values during glacial-interglacial transitions. This study suggests that BHT II offers a proxy for past changes in the relative importance of anammox, and fluctuations in nitrogen cycling in response to ocean redox changes through the geological record.

Analysis of non-derivatised bacteriohopanepolyols by ultrahigh-performance liquid chromatography/tandem mass spectrometry

RATIONALE

Traditional investigation of bacteriohopanepolyols (BHPs) has relied on derivatisation by acetylation prior to gas chromatography/mass spectrometry (GC/MS) or liquid chromatography/MS (LC/MS) analysis. Here, modern chromatographic techniques (ultrahigh-performance liquid chromatography (UPLC)) and new column chemistries were tested to develop a method for BHP analysis without the need for derivatisation.

METHODS

Bacterial culture and sedimentary lipid extracts were analysed using a Waters Acquity Xevo TQ-S triple quadrupole mass spectrometer in positive ion atmospheric pressure chemical ionisation (APCI) mode. Waters BEH C18 and ACE Excel C18 were the central columns evaluated using a binary solvent gradient with 0.1% formic acid in the polar solvent phase in order to optimise performance and selectivity.

RESULTS

Non-amine BHPs and adenosylhopane showed similar performance on each C18 column; however, BHP-containing terminal amines were only identified eluting from the ultra-inert ACE Excel C18 column. APCI-MS/MS product ion scans revealed significant differences in fragmentation pathways from previous methods for acetylated compounds. The product ions used for targeted multiple reaction monitoring (MRM) are summarised.

CONCLUSIONS

UPLC/MS/MS analysis using an ACE Excel C18 column produced superior separation for amine-containing BHPs and reduced run times from 60 to 9 min compared with previous methods. Unexpected variations in fragmentation pathways between structural subgroups must be taken into account when optimising MRM transitions for future quantitative studies. Copyright © 2016 John Wiley & Sons, Ltd.

Salinicoccus amylolyticus sp. nov., isolated from a saltern

A Gram-stain-positive coccus, strain JC304T, was isolated from a saltern of Nari along the Bhavnagar Coast, Gujarat, India. The 16S rRNA gene sequence analysis and sequence comparison data indicated that JC304T represented a member of the genus Salinicoccus and was most closely related to Salinicoccus roseus 9T (99.6 %), Salinicoccus luteus YIM 70202T (97.0 %), Salinicoccus hispanicus J-82T (97.0 %) and the remaining species of the genus Salinicoccus (<97 %). Genome relatedness based on DNA-DNA hybridization of JC304T with the type strains of the most closely related species was less than 46 % and the ΔTmwas >5 °C indicating that the strain represents a novel species of the genus Salinicoccus. Independent and concatenated phylogenetic analysis of recA/fusA gene translated product showed a clear distinction of JC304T from its phylogenetic neighbors. Diphosphotidylglycerol, phosphatidylglycerol, an unidentified glycolipid and three unidentified lipids (L1, L2 and L3) were the polar lipids of JC304T. Iso-C15 : 0 and anteiso-C15 : 0 were the major (>10 %) fatty acids in strain JC304T. The cell-wall amino acids were l-lysine and d-glycine. Hopanoids were not detected. The major isoprenoid quinone was menaquinone (MK-6). The DNA G+C content of JC304T was 48 mol%. On the basis of physiological, genotypic, phylogenetic and chemotaxonomic analyses, strain JC304T is considered to represent a novel species of the genus Salinicoccus, for which the name Salinicoccusamylolyticus sp. nov. is proposed. The type strain is JC304T (=KCTC 33661T=LMG 28757T).

Sterol Synthesis in Diverse Bacteria

Sterols are essential components of eukaryotic cells whose biosynthesis and function has been studied extensively. Sterols are also recognized as the diagenetic precursors of steranes preserved in sedimentary rocks where they can function as geological proxies for eukaryotic organisms and/or aerobic metabolisms and environments. However, production of these lipids is not restricted to the eukaryotic domain as a few bacterial species also synthesize sterols. Phylogenomic studies have identified genes encoding homologs of sterol biosynthesis proteins in the genomes of several additional species, indicating that sterol production may be more widespread in the bacterial domain than previously thought. Although the occurrence of sterol synthesis genes in a genome indicates the potential for sterol production, it provides neither conclusive evidence of sterol synthesis nor information about the composition and abundance of basic and modified sterols that are actually being produced. Here, we coupled bioinformatics with lipid analyses to investigate the scope of bacterial sterol production. We identified oxidosqualene cyclase (Osc), which catalyzes the initial cyclization of oxidosqualene to the basic sterol structure, in 34 bacterial genomes from five phyla (Bacteroidetes, Cyanobacteria, Planctomycetes, Proteobacteria, and Verrucomicrobia) and in 176 metagenomes. Our data indicate that bacterial sterol synthesis likely occurs in diverse organisms and environments and also provides evidence that there are as yet uncultured groups of bacterial sterol producers. Phylogenetic analysis of bacterial and eukaryotic Osc sequences confirmed a complex evolutionary history of sterol synthesis in this domain. Finally, we characterized the lipids produced by Osc-containing bacteria and found that we could generally predict the ability to synthesize sterols. However, predicting the final modified sterol based on our current knowledge of sterol synthesis was difficult. Some bacteria produced demethylated and saturated sterol products even though they lacked homologs of the eukaryotic proteins required for these modifications emphasizing that several aspects of bacterial sterol synthesis are still completely unknown.

A distinct pathway for tetrahymanol synthesis in bacteria

Tetrahymanol is a polycyclic triterpenoid lipid first discovered in the ciliate Tetrahymena pyriformis whose potential diagenetic product, gammacerane, is often used as a biomarker for water column stratification in ancient ecosystems. Bacteria are also a potential source of tetrahymanol, but neither the distribution of this lipid in extant bacteria nor the significance of bacterial tetrahymanol synthesis for interpreting gammacerane biosignatures is known. Here we couple comparative genomics with genetic and lipid analyses to link a protein of unknown function to tetrahymanol synthesis in bacteria. This tetrahymanol synthase (Ths) is found in a variety of bacterial genomes, including aerobic methanotrophs, nitrite-oxidizers, and sulfate-reducers, and in a subset of aquatic and terrestrial metagenomes. Thus, the potential to produce tetrahymanol is more widespread in the bacterial domain than previously thought. However, Ths is not encoded in any eukaryotic genomes, nor is it homologous to eukaryotic squalene-tetrahymanol cyclase, which catalyzes the cyclization of squalene directly to tetrahymanol. Rather, heterologous expression studies suggest that bacteria couple the cyclization of squalene to a hopene molecule by squalene-hopene cyclase with a subsequent Ths-dependent ring expansion to form tetrahymanol. Thus, bacteria and eukaryotes have evolved distinct biochemical mechanisms for producing tetrahymanol.

Specific hopanoid classes differentially affect free-living and symbiotic states of Bradyrhizobium diazoefficiens

A better understanding of how bacteria resist stresses encountered during the progression of plant-microbe symbioses will advance our ability to stimulate plant growth. Here, we show that the symbiotic system comprising the nitrogen-fixing bacterium Bradyrhizobium diazoefficiens and the legume Aeschynomene afraspera requires hopanoid production for optimal fitness. While methylated (2Me) hopanoids contribute to growth under plant-cell-like microaerobic and acidic conditions in the free-living state, they are dispensable during symbiosis. In contrast, synthesis of extended (C35) hopanoids is required for growth microaerobically and under various stress conditions (high temperature, low pH, high osmolarity, bile salts, oxidative stress, and antimicrobial peptides) in the free-living state and also during symbiosis. These defects might be due to a less rigid membrane resulting from the absence of free or lipidA-bound C35 hopanoids or the accumulation of the C30 hopanoid diploptene. Our results also show that C35 hopanoids are necessary for symbiosis only with the host Aeschynomene afraspera but not with soybean. This difference is likely related to the presence of cysteine-rich antimicrobial peptides in Aeschynomene nodules that induce drastic modification in bacterial morphology and physiology. The study of hopanoid mutants in plant symbionts thus provides an opportunity to gain insight into host-microbe interactions during later stages of symbiotic progression, as well as the microenvironmental conditions for which hopanoids provide a fitness advantage.

IMPORTANCE

Because bradyrhizobia provide fixed nitrogen to plants, this work has potential agronomical implications. An understanding of how hopanoids facilitate bacterial survival in soils and plant hosts may aid the engineering of more robust agronomic strains, especially relevant in regions that are becoming warmer and saline due to climate change. Moreover, this work has geobiological relevance: hopanes, molecular fossils of hopanoids, are enriched in ancient sedimentary rocks at discrete intervals in Earth history. This is the first study to uncover roles for 2Me- and C35 hopanoids in the context of an ecological niche that captures many of the stressful environmental conditions thought to be important during (2Me)-hopane deposition. Though much remains to be done to determine whether the conditions present within the plant host are shared with niches of relevance to the rock record, our findings represent an important step toward identifying conserved mechanisms whereby hopanoids contribute to fitness.

Specific hopanoid classes differentially affect free-living and symbiotic states of Bradyrhizobium diazoefficiens

A better understanding of how bacteria resist stresses encountered during the progression of plant-microbe symbioses will advance our ability to stimulate plant growth. Here, we show that the symbiotic system comprising the nitrogen-fixing bacterium Bradyrhizobium diazoefficiens and the legume Aeschynomene afraspera requires hopanoid production for optimal fitness. While methylated (2Me) hopanoids contribute to growth under plant-cell-like microaerobic and acidic conditions in the free-living state, they are dispensable during symbiosis. In contrast, synthesis of extended (C35) hopanoids is required for growth microaerobically and under various stress conditions (high temperature, low pH, high osmolarity, bile salts, oxidative stress, and antimicrobial peptides) in the free-living state and also during symbiosis. These defects might be due to a less rigid membrane resulting from the absence of free or lipidA-bound C35 hopanoids or the accumulation of the C30 hopanoid diploptene. Our results also show that C35 hopanoids are necessary for symbiosis only with the host Aeschynomene afraspera but not with soybean. This difference is likely related to the presence of cysteine-rich antimicrobial peptides in Aeschynomene nodules that induce drastic modification in bacterial morphology and physiology. The study of hopanoid mutants in plant symbionts thus provides an opportunity to gain insight into host-microbe interactions during later stages of symbiotic progression, as well as the microenvironmental conditions for which hopanoids provide a fitness advantage.

IMPORTANCE

Because bradyrhizobia provide fixed nitrogen to plants, this work has potential agronomical implications. An understanding of how hopanoids facilitate bacterial survival in soils and plant hosts may aid the engineering of more robust agronomic strains, especially relevant in regions that are becoming warmer and saline due to climate change. Moreover, this work has geobiological relevance: hopanes, molecular fossils of hopanoids, are enriched in ancient sedimentary rocks at discrete intervals in Earth history. This is the first study to uncover roles for 2Me- and C35 hopanoids in the context of an ecological niche that captures many of the stressful environmental conditions thought to be important during (2Me)-hopane deposition. Though much remains to be done to determine whether the conditions present within the plant host are shared with niches of relevance to the rock record, our findings represent an important step toward identifying conserved mechanisms whereby hopanoids contribute to fitness.

Lipid remodeling in Rhodopseudomonas palustris TIE-1 upon loss of hopanoids and hopanoid methylation

The sedimentary record of molecular fossils (biomarkers) can potentially provide important insights into the composition of ancient organisms; however, it only captures a small portion of their original lipid content. To interpret what remains, it is important to consider the potential for functional overlap between different lipids in living cells, and how the presence of one type might impact the abundance of another. Hopanoids are a diverse class of steroid analogs made by bacteria and found in soils, sediments, and sedimentary rocks. Here, we examine the trade-off between hopanoid production and that of other membrane lipids. We compare lipidomes of the metabolically versatile α-proteobacterium Rhodopseudomonas palustris TIE-1 and two hopanoid mutants, detecting native hopanoids simultaneously with other types of polar lipids by electrospray ionization mass spectrometry. In all strains, the phospholipids contain high levels of unsaturated fatty acids (often >80%). The degree to which unsaturated fatty acids are modified to cyclopropyl fatty acids varies by phospholipid class. Deletion of the capacity for hopanoid production is accompanied by substantive changes to the lipidome, including a several-fold rise of cardiolipins. Deletion of the ability to make methylated hopanoids has a more subtle effect; however, under photoautotrophic growth conditions, tetrahymanols are upregulated twofold. Together, these results illustrate that the 'lipid fingerprint' produced by a micro-organism can vary depending on the growth condition or loss of single genes, reminding us that the absence of a biomarker does not necessarily imply the absence of a particular source organism.

Recent advances in hopanoids analysis: Quantification protocols overview, main research targets and selected problems of complex data exploration

Pentacyclic triterpenoids, particularly hopanoids, are organism-specific compounds and are generally considered as useful biomarkers that allow fingerprinting and classification of biological, environmental and geological samples. Simultaneous quantification of various hopanoids together with battery of related non-polar and low-molecular mass compounds may provide principal information for geochemical and environmental research focusing on both modern and ancient investigations. Target compounds can be derived from microbial biomass, water columns, sediments, coals, crude fossils or rocks. This create number of analytical problems due to different composition of the analytical matrix and interfering compounds and therefore, proper optimization of quantification protocols for such biomarkers is still the challenge. In this work we summarizing typical analytical protocols that were recently applied for quantification of hopanoids like compounds from different samples. Main steps including components of interest extraction, pre-purification, fractionation, derivatization and quantification involving gas (1D and 2D) as well as liquid separation techniques (liquid-liquid extraction, solid-phase extraction, planar and low resolution column chromatography, high-performance liquid chromatography) are described and discussed from practical point of view, mainly based on the experimental papers that were published within last two years, where significant increase in hopanoids research was noticed. The second aim of this review is to describe the latest research trends concerning determination of hopanoids and related low-molecular mass lipids analyzed in various samples including sediments, rocks, coals, crude oils and plant fossils as well as stromatolites and microbial biomass cultivated under different conditions. It has been found that majority of the most recent papers are based on uni- or bivariate approach for complex data analysis. Data interpretation involves number of physicochemical parameters and hopanoids quantities or given biomarkers mass ratios derived from high-throughput separation and detection systems, typically GC-MS and HPLC-MS. Based on quantitative data reported in recently published experimental works it has been demonstrated that multivariate data analysis using e.g. principal components computations may significantly extend our knowledge concerning proper biomarkers selection and samples classification by means of hopanoids and related non-polar compounds.

Quantitative hopanoid analysis enables robust pattern detection and comparison between laboratories

Hopanoids are steroid-like lipids from the isoprenoid family that are produced primarily by bacteria. Hopanes, molecular fossils of hopanoids, offer the potential to provide insight into environmental transitions on the early Earth, if their sources and biological functions can be constrained. Semiquantitative methods for mass spectrometric analysis of hopanoids from cultures and environmental samples have been developed in the last two decades. However, the structural diversity of hopanoids, and possible variability in their ionization efficiencies on different instruments, have thus far precluded robust quantification and hindered comparison of results between laboratories. These ionization inconsistencies give rise to the need to calibrate individual instruments with purified hopanoids to reliably quantify hopanoids. Here, we present new approaches to obtain both purified and synthetic quantification standards. We optimized 2-methylhopanoid production in Rhodopseudomonas palustris TIE-1 and purified 2Me-diplopterol, 2Me-bacteriohopanetetrol (2Me-BHT), and their unmethylated species (diplopterol and BHT). We found that 2-methylation decreases the signal intensity of diplopterol between 2 and 34% depending on the instrument used to detect it, but decreases the BHT signal less than 5%. In addition, 2Me-diplopterol produces 10× higher ion counts than equivalent quantities of 2Me-BHT. Similar deviations were also observed using a flame ionization detector for signal quantification in GC. In LC-MS, however, 2Me-BHT produces 11× higher ion counts than 2Me-diplopterol but only 1.2× higher ion counts than the sterol standard pregnane acetate. To further improve quantification, we synthesized tetradeuterated (D4) diplopterol, a precursor for a variety of hopanoids. LC-MS analysis on a mixture of (D4)-diplopterol and phospholipids showed that under the influence of co-eluted phospholipids, the D4-diplopterol internal standard quantifies diplopterol more accurately than external diplopterol standards. These new quantitative approaches permit meaningful comparisons between studies, allowing more accurate hopanoid pattern detection in both laboratory and environmental samples.

Hopanoid inventory of Rhodoplanes spp

Hopanoids are pentacyclic triterpenoid lipids and are important for bacterial membrane stability and functioning. These pentacyclic triterpenoids of hopane series are biomarkers for eubacteria and can be used as chemotaxonomic markers. Anoxygenic phototrophic bacteria are good producers of hopanoids, and their inventory to date is restricted to a few members. Rhodoplanes spp. are phototrophic prokaryotes which grow and thrive in subsurface and sediment environments. A study on the diversity of hopanoids of several species of Rhodoplanes revealed a rich diversity of hopanoids with carbon length of C30/C31 and C35. Hop-22(29)-ene (II), diplopterol (V), tetrahymanol (VII), 2-methyldiplopterol (VI), 2-methyltetrahymanol (VIII), bacteriohopanetetrol (IX), bacteriohopaneaminotriol (X) and bacteriohopanepolyols [BHP-492 (XIII), BHP-550 (XIV), BHP-508 (XII)] are the major hopanoids of the genus Rhodoplanes. Tetrahymanol (VII) content is high (38-60 %) among all the members, except for Rhodoplanes elegans. Hopanoid fingerprints allowed differentiation of species of the genus Rhodoplanes. Statistical analyses also indicate hopanoids as good chemotaxonomic markers to distinguish species of the genus Rhodoplanes.

High-throughput characterization of sediment organic matter by pyrolysis-gas chromatography/mass spectrometry and multivariate curve resolution: A promising analytical tool in (paleo)limnology

Molecular-level chemical information about organic matter (OM) in sediments helps to establish the sources of OM and the prevalent degradation/diagenetic processes, both essential for understanding the cycling of carbon (C) and of the elements associated with OM (toxic trace metals and nutrients) in lake ecosystems. Ideally, analytical methods for characterizing OM should allow high sample throughput, consume small amounts of sample and yield relevant chemical information, which are essential for multidisciplinary, high-temporal resolution and/or large spatial scale investigations. We have developed a high-throughput analytical method based on pyrolysis-gas chromatography/mass spectrometry and automated data processing to characterize sedimentary OM in sediments. Our method consumes 200 μg of freeze-dried and ground sediment sample. Pyrolysis was performed at 450°C, which was found to avoid degradation of specific biomarkers (e.g., lignin compounds, fresh carbohydrates/cellulose) compared to 650°C, which is in the range of temperatures commonly applied for environmental samples. The optimization was conducted using the top ten sediment samples of an annually resolved sediment record (containing 16-18% and 1.3-1.9% of total carbon and nitrogen, respectively). Several hundred pyrolytic compound peaks were detected of which over 200 were identified, which represent different classes of organic compounds (i.e., n-alkanes, n-alkenes, 2-ketones, carboxylic acids, carbohydrates, proteins, other N compounds, (methoxy)phenols, (poly)aromatics, chlorophyll and steroids/hopanoids). Technical reproducibility measured as relative standard deviation of the identified peaks in triplicate analyses was 5.5±4.3%, with 90% of the RSD values within 10% and 98% within 15%. Finally, a multivariate calibration model was calculated between the pyrolytic degradation compounds and the sediment depth (i.e., sediment age), which is a function of degradation processes and changes in OM source type. This allowed validation of the Py-GC/MS dataset against fundamental processes involved in OM cycling in aquatic ecosystems.

Covalently linked hopanoid-lipid A improves outer-membrane resistance of a Bradyrhizobium symbiont of legumes

Lipopolysaccharides (LPSs) are major components of the outer membrane of Gram-negative bacteria and are essential for their growth and survival. They act as a structural barrier and play an important role in the interaction with eukaryotic hosts. Here we demonstrate that a photosynthetic Bradyrhizobium strain, symbiont of Aeschynomene legumes, synthesizes a unique LPS bearing a hopanoid covalently attached to lipid A. Biophysical analyses of reconstituted liposomes indicate that this hopanoid-lipid A structure reinforces the stability and rigidity of the outer membrane. In addition, the bacterium produces other hopanoid molecules not linked to LPS. A hopanoid-deficient strain, lacking a squalene hopene cyclase, displays increased sensitivity to stressful conditions and reduced ability to survive intracellularly in the host plant. This unusual combination of hopanoid and LPS molecules may represent an adaptation to optimize bacterial survival in both free-living and symbiotic states.

Diverse capacity for 2-methylhopanoid production correlates with a specific ecological niche

Molecular fossils of 2-methylhopanoids are prominent biomarkers in modern and ancient sediments that have been used as proxies for cyanobacteria and their main metabolism, oxygenic photosynthesis. However, substantial culture and genomic-based evidence now indicates that organisms other than cyanobacteria can make 2-methylhopanoids. Because few data directly address which organisms produce 2-methylhopanoids in the environment, we used metagenomic and clone library methods to determine the environmental diversity of hpnP, the gene encoding the C-2 hopanoid methylase. Here we show that hpnP copies from alphaproteobacteria and as yet uncultured organisms are found in diverse modern environments, including some modern habitats representative of those preserved in the rock record. In contrast, cyanobacterial hpnP genes are rarer and tend to be localized to specific habitats. To move beyond understanding the taxonomic distribution of environmental 2-methylhopanoid producers, we asked whether hpnP presence might track with particular variables. We found hpnP to be significantly correlated with organisms, metabolisms and environments known to support plant-microbe interactions (P-value<10(-6)); in addition, we observed diverse hpnP types in closely packed microbial communities from other environments, including stromatolites, hot springs and hypersaline microbial mats. The common features of these niches indicate that 2-methylhopanoids are enriched in sessile microbial communities inhabiting environments low in oxygen and fixed nitrogen with high osmolarity. Our results support the earlier conclusion that 2-methylhopanoids are not reliable biomarkers for cyanobacteria or any other taxonomic group, and raise the new hypothesis that, instead, they are indicators of a specific environmental niche.

Oxidation of Fe(II) leads to increased C-2 methylation of pentacyclic triterpenoids in the anoxygenic phototrophic bacterium Rhodopseudomonas palustris strain TIE-1

Hopanoids are among the most widespread biomarkers of bacteria that are used as indicators for past and present bacterial activity. Our understanding of the production, function, and distribution of hopanoids in bacteria has improved greatly, partly due to genetic, culture-independent studies. Culture-based studies are important to determine hopanoid function and the environmental conditions under which these compounds are produced. This study compares the lipid inventory of Rhodopseudomonas palustris strain TIE-1 under anoxic photoautotrophic conditions using either H2 or Fe(II) as electron donor. The high amount to which adenosylhopane is produced irrespective of the used electron donor suggests a specific function of this compound rather than its exclusive role as an intermediate in bacteriohopanepolyol biosynthesis. C-2 methylated hopanoids and tetrahymanol account for as much as 59% of the respective C-2 methylated/non-methylated homologs during growth with Fe(II) as electron donor, as compared with 24% C-2 methylation for growth with H2 . This observation reveals that C-2 methylated hopanoids have a specific function and are preferentially synthesized in response to elevated Fe(II) concentrations. The presence of C-2 methylated pentacyclic triterpenoids has commonly been used as a biosignature for the interpretation of paleoenvironments. These new findings suggest that increased C-2 methylation may indicate anoxic ferrous conditions, in addition to other environmental stressors that have been previously reported.