Polar lipids of non-alkaliphilic Halococci

Proposed minimal standards for description of new taxa of the class Halobacteria

Halophilic archaea of the class Halobacteria are the most salt-requiring prokaryotes within the domain Archaea. In 1997, minimal standards for the description of new taxa in the order Halobacteriales were proposed. From then on, the taxonomy of the class Halobacteria provides an excellent example of how changing concepts on prokaryote taxonomy and the development of new methods were implemented. The last decades have witnessed a rapid expansion of the number of described taxa within the class Halobacteria coinciding with the era of genome sequencing development. The current members of the International Committee on Systematics of Prokaryotes Subcommittee on the Taxonomy of Halobacteria propose these revisions to the recommended minimal standards and encourage the use of advanced technologies in the taxonomic description of members of the Halobacteria. Most previously required and some recommended minimal standards for the description of new taxa in the class Halobacteria were retained in the present revision, but changes have been proposed in line with the new methodologies. In addition to the 16S rRNA gene, the rpoB' gene is an important molecular marker for the identification of members of the Halobacteria. Phylogenomic analysis based on concatenated conserved, single-copy marker genes is required to infer the taxonomic status of new taxa. The overall genome relatedness indexes have proven to be determinative in the classification of the taxa within the class Halobacteria. Average nucleotide identity, digital DNA-DNA hybridization, and average amino acid identity values should be calculated for rigorous comparison among close relatives.

Homeoviscous Adaptation of Membranes in Archaea

Because membranes play a central role in regulating fluxes inward and outward from the cells, maintaining the appropriate structure of the membrane is crucial to maintain cellular integrity and functions. Microbes often face contrasted and fluctuating environmental conditions, to which they need to adapt or die. Membrane adaptation is achieved by a modification of the membrane lipid composition, a strategy termed homeoviscous adaptation. Homeoviscous adaptation in archaea involves strategies similar to that observed in bacteria and eucarya, such as the regulation of lipid chain length or saturation levels, as well as strategies specific to archaea, such as the regulation of the number of cycles along the isoprenoid chains or the regulation of the ratio between mono and bipolar lipids. Although not described yet described in hyperthermophilic bacteria, it is possible that these two strategies also apply to these latter organisms.

Novel sulfated phosphoglycolipids from Natronomonas moolapensis

Polar lipid pattern determination is often used for the taxonomic classification of halophilic Archaea in addition to a genomic characterization. During the analysis of polar lipid extracts from the recently described haloarchaeon Natrononomonas moolapensis, an unknown glycolipid was detected. Fragmentation patterns observed from preliminary mass spectrometric analysis initially suggested the presence of a sulfo-hexosyl-phosphatidylglycerol. However, by NMR spectroscopy and enzymatic assays the existence of two isomeric molecules with different hexoses (1-(6-sulfo-d-glcp/galf-β1,2-glycero)-phospho-2,3-diphytanylglycerol) could be shown. The structural origin from phosphatidylglycerol distinguishes these glycolipids within Archaea, because all other characterized haloarchaeal glycolipids consist of diphytanylglycerol directly linked to an oligoglycosyl moiety. Now the door is open to investigate the physical and functional consequences of these architectural differences of the head groups.

The Santa Pola saltern as a model for studying the microbiota of hypersaline environments

Multi-pond salterns constitute an excellent model for the study of the microbial diversity and ecology of hypersaline environments, showing a wide range of salt concentrations, from seawater to salt saturation. Accumulated studies on the Santa Pola (Alicante, Spain) multi-pond solar saltern during the last 35 years include culture-dependent and culture-independent molecular methods and metagenomics more recently. These approaches have permitted to determine in depth the microbial diversity of the ponds with intermediate salinities (from 10% salts) up to salt saturation, with haloarchaea and bacteria as the two main dominant groups. In this review, we describe the main results obtained using the different methodologies, the most relevant contributions for understanding the ecology of these extreme environments and the future perspectives for such studies.

Archaeal lipids and their biotechnological applications

The lipids of Archaea, based on glycerol isopranoid ethers, can be used taxonomically to distinguish between phenotypic subgroups of the domain to delineate them clearly from all other organisms. This review is a general survey of the structural features of archaeal lipids and how they relate to survival in the harsh environments in which the Archaea live. The molecular organization of archaeal lipids in monolayers, artificial black membranes and vesicles and the unique properties and possible biotechnological applications of liposomes of the lipids are presented. The results with these liposomes are compared with similar data obtained with synthetic compounds which mimic the structure of archaeal lipids. Studies on computer simulation are also reported.

Adaptation of the membrane in Archaea

Microbes often face contrasted and fluctuating environmental conditions, to which they need to adapt or die. Because membranes play a central role in regulating fluxes inward and outward from the cells, maintaining the appropriate structure of the membrane is crucial to maintain cellular integrity and functions. This is achieved in bacteria and eucarya by a modification of the membrane lipid compositions, a strategy termed homeoviscous adaptation. We review here evidence for homeoviscous adaptation in Archaea, and discuss the limits of this strategy and our knowledge in this very peculiar domain of life.

A dendrogram of archaea based on lipid component parts composition and its relationship to rRNA phylogeny

The results of two objective and quantitative, computer-assisted analyses of the lipid component parts distribution pattern among various archaeal organisms belonging to Euryarchaeota are reported. One was a cluster analysis and the other a selection of unique combinations of lipid component parts found exclusively in a given taxon. The cluster analysis revealed that the distribution of lipid component parts was correlated with phylogeny based on small subunit rRNA sequences, although there was some discrepancy with rRNA phylogeny. A hypothesis that may explain the reason for the correlation and the discrepancy is proposed. In our scenario, we assumed that random and independent mutations on the rRNA and lipid biosynthesis genes may result largely in coincided evolution. The fact that RNA and lipid are semantide and episemantic molecules, respectively, is the fundamental difference between the phylogeny of RNA and lipid. Moreover, different selective pressures on RNA and lipids exert different effects on their evolution. Unique lipid component parts were detected for eight out of nine orders, 14 families, and 22 genera of the Euryarchaeota analyzed. A unique lipid component parts combination pattern characterized the taxon. The results confirm and extend a previously reported conclusion based on a more statistical basis.

Unsaturated diether lipids in the psychrotrophic archaeon Halorubrum lacusprofundi

The major phospholipids of Halorubrum lacusprofundi grown at 25 degrees C were archaeol phosphatidylglycerol, archaeol phosphatidylglycerylsulphate and archaeol phosphatidylglycerylphosphate methyl ester. Glycolipids included a monoglycosyl archaeol and the sulphate ester of a diglycosyl archaeol. Cultures grown at 12 degrees C contained the same suite of phospho- and glycolipids, with the addition of a series of unsaturated analogues with up to six double bonds. The patterns of unsaturation were similar for all the phospholipid series, but a different pattern occurred in the glycolipids. The analytical techniques used in this study allow facile detection of unsaturated archaeal cell membrane lipids that are degraded by commonly used chemical derivatization procedures.

Characterisation of membrane phospholipids and glycolipids from a halophilic archaebacterium by high-performance liquid chromatography/electrospray mass spectrometry

Combined high-performance liquid chromatography and electrospray mass spectrometry (LC/ES-MS) has been used for direct characterisation of the polar membrane lipids in total lipid extracts from Halobacterium salinarium, a species of halophilic archaebacterium. The principle phospholipids found were the diphytanyl archaeol phosphatidylglycerol and diphytanyl archaeol phosphatidylglycerolphosphate methyl ester. The application of LC/ES-MS revealed the additional presence of diphytanyl archaeol phosphatidylglycerol sulphate The extracts also contained an archaeol glycolipid, initially detected in preliminary offline ES-MS studies, which was further characterised by LC/ES-MS and by product ion tandem mass spectrometry (MS/MS) as a sulphate ester of diglycosyl-2,3-di-O-phytanyl-sn-glycerol. Whilst archaeol phospho- and glycolipids containing a (C(20)-C(20))-isopranyl glycerol ether core predominated, LC/ES-MS of the extracts from Halobacterium salinarium indicated the presence of an analogue containing one double bond in its isoprenyl ether core as a minor component of the phosphatidylglycerolphosphate methyl ester fraction, providing a further example of the previously recognised existence of isoprenologues of diphytanyl archaeols which occur as minor components of archaebacterial membrane lipids. The value of these techniques in compositional analysis of archaebacterial lipid extracts is discussed.

The terpenoid theory of the origin of cellular life: the evolution of terpenoids to cholesterol

Terpenoids have an apparently essential function in modern cellular membranes, reinforcing them against shear stresses. Primitive membranes could initially have been formed by simple terpenoids, and vesicles formed from these membranes may have evolved into progressively more complex units, more and more similar to protocells.

Polar lipids of a non-alkaliphilic extremely halophilic archaebacterium strain 172: a novel bis-sulfated glycolipid

Extremely halophilic archaebacteria which require high salt concentrations for growth and survival contain glycerol diether analogues of phospholipids and sulfated glycolipids as major membrane polar lipids. A non-alkaliphilic, non-pigmented rod-shaped extreme halophile, isolated from sea sand in Japan and designated 'strain 172', was found to contain two phospholipids, phosphatidylglycerol (PG) and phosphatidylglyceromethylphosphate (PGP-Me), derived from both C20-C20- and C20-C25-glycerol diethers, and a novel major glycolipid (designated SGL-X). This glycolipid has been identified as a bis-sulfated diglycosyl C20-C20- or C20-C25-glycerol diether, on the basis of its TLC mobility, positive-staining behavior with sugar and sulfate-staining reagents, its mole ratio sulfate/glycolipid = 2.2, and by spectrometric analysis (IR and FAB-MS) of the intact and the desulfated SGL-X. The sugars were identified as mannose and glucose, after acid hydrolysis of SGL-X, by paper chromatography of the free sugars and GC-MS of the derivatized sugars (alditol acetates). Permethylation analysis and 1H- and 13C-NMR analysis established the position and configuration of the sugar linkages and the positions of the sulfate groups. The final structure of SGL-X (now designated S2-DGD-1) is proposed to be: 2,3-diphytanyl- or phytanyl-sesterterpenyl-1-[2,6-(HSO3)2-alpha-Manp-1--> 2- Glcp]-sn-glycerol. This lipid is the first bis-sulfated glycolipid to be reported in extremely halophilic archaebacteria, and is the first in the biosphere that possesses two sulfate groups attached to the same monosaccaride.

A novel prenyltransferase, farnesylgeranyl diphosphate synthase, from the haloalkaliphilic archaeon, Natronobacterium pharaonis

A novel prenyltransferase, farnesylgeranyl diphosphate (FGPP) synthase (EC 2.5.1.X), which synthesizes C25-prenyl diphosphate, was found in the haloalkaliphilic archaeon Natronobacterium pharaonis. It was separated from geranylgeranyl diphosphate (GGPP) synthase (EC 2.5.1.29), which synthesizes C20-prenyl diphosphate, a major prenyltransferase in this organism. The highest activity of FGPP synthase was observed when GGPP was used as the allylic substrate. FGPP synthase may synthesize a precursor for the C25 moiety of C20, C25 diether lipids using a longer allylic diphosphate, such as GGPP synthesized by GGPP synthase, rather than dimethylallyl diphosphate, which is the product of isopentenyl diphosphate isomerase.

Biology of halophilic bacteria, Part II. Membrane lipids of extreme halophiles: biosynthesis, function and evolutionary significance

Archaebacteria (archaea) are comprised of three groups of prokaryotes: extreme halophiles, methanogens and thermoacidophiles (extreme thermophiles). Their membrane phospholipids and glycolipids are derived entirely from a saturated, isopranoid glycerol diether, sn-2,3-diphytanylglycerol ('archaeol') and/or its dimer, dibiphytanyldiglyceroltetraether ('caldarchaeol'). In extreme halophiles, the major phospholipid is the archaeol analogue of phosphatidylglycerolmethylphosphate (PGP-Me); the glycolipids are sulfated and/or unsulfated glycosyl archaeols with diverse carbohydrate structure characteristic of taxons on the generic level. Biosynthesis of these archaeol-derived polar lipids occurs in a multienzyme, membrane-bound system that is absolutely dependent on high salt concentration (4 M). The highly complex biosynthetic pathways involve intermediates containing glycerol ether-linked C20-isoprenyl groups which are reduced to phytanyl groups to give the final saturated polar lipids. In methanogens, polar lipids are derived both from archaeol and caldarchaeol, and thermoacidophiles contain essentially only caldarchaeol-derived polar lipids. The function of these membrane polar lipids in maintaining the stability, fluidity and ionic properties of the cell membrane of extreme halophiles, as well as the evolutionary implications of the archaeol and caldarchaeol-derived structures will be discussed.

The glycolipid of Halobacterium trapanicum

The structural elucidation of the polar lipids in Halobacterium trapanicum is reported with particular emphasis on a new sulfated disaccharide derivative of 2,3-di-O-phytanyl-sn-glycerol. The full structural designation of this glycolipid is 2,3-di-O-phytanyl-1-O- (mannopyranosyl-(2-sulfate)-alpha-D-1-2-glucopyranosyl-alpha-D)-sn-glyce rol. The value of glycolipid structures in the taxonomy of halophilic Archaea is also discussed.

On the revised structure of the major phospholipid of Halobacterium salinarium

Recent fast atom bombardment-mass spectrometry (FABMS) studies (Tsujimoto, K., Yorimitsu, S., Takahashi, T. and Ohashi, M. (1989) J. Chem. Commun. 668-670; Frederickson, H.L., De Leeuw, J.W., Tas, A.C., Van der Greef, J., LaVos, G.F. and Boon, J.J. (1989) Biomed. Environ. Mass. Spectrom. 18, 96-105; Kloppel, K.D. and Fredrickson, H.L. (1991) J. Chromatogr. 562, 369-376) have indicated that the structure of the major phospholipid of Halobacterium salinarium (formerly Halobacterium cutirubrum) is not 2,3-diphytanyl-sn-glycerol-1-phospho-3'-sn-glycerol-1'- phosphate (PGP), but the monomethylated derivative, 2,3-diphytanyl-sn-glycerol-1-phospho-3'-sn-glycerol-1'-methylphosphate (PGP-Me). We have now confirmed the structure of the major phospholipid of extremely halophilic archaebacteria as being this methylated structure (PGP-Me) by 1H- and 13C-NMR, FABMS and TLC of the native phospholipid and its product of mild acid hydrolysis PGP. The methylated structure (PGP-Me), rather than PGP itself, is also the major phospholipid in species of other genera of extreme halophiles examined so far, such as, Haloferax, Haloarcula, Halococcus, Natronobacterium and Natronococcus.