Jan A. J. Geenevasen

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.

Linearly concatenated cyclobutane lipids form a dense bacterial membrane.

Lipid membranes are essential to the functioning of cells, enabling the existence of concentration gradients of ions and metabolites. Microbial membrane lipids can contain three-, five-, six- and even seven-membered aliphatic rings, but four-membered aliphatic cyclobutane rings have never been observed. Here we report the discovery of cyclobutane rings in the dominant membrane lipids of two anaerobic ammonium-oxidizing (anammox) bacteria. These lipids contain up to five linearly fused cyclobutane moieties with cis ring junctions. Such ‘ladderane’ molecules are unprecedented in nature but are known as promising building blocks in optoelectronics. The ladderane lipids occur in the membrane of the anammoxosome, the dedicated intracytoplasmic compartment where anammox catabolism takes place. They give rise to an exceptionally dense membrane, a tight barrier against diffusion. We propose that such a membrane is required to maintain concentration gradients during the exceptionally slow anammox metabolism and to protect the remainder of the cell from the toxic anammox intermediates. Our results further illustrate that microbial membrane lipid structures are far more diverse than previously recognized.

Newly discovered non-isoprenoid glycerol dialkyl glycerol tetraether lipids in sediments

Newly discovered non-isoprenoid dialkyl diglycerol tetraethers containing 13,16-di- or 5,13,16-trimethyloctacosanyl moieties have been identified in peats and coastal marine and lake sediments by HPLC–MS and high-field NMR spectroscopy.

Structural identification of ladderane and other membrane lipids of planctomycetes capable of anaerobic ammonium oxidation (anammox)

The membrane lipid composition of planctomycetes capable of the anaerobic oxidation of ammonium (anammox), i.e. Candidatus‘Brocadia anammoxidans’ and Candidatus‘Kuenenia stuttgartiensis’, was shown to be composed mainly of so‐called ladderane lipids. These lipids are comprised of three to five linearly concatenated cyclobutane moieties with cis ring junctions, which occurred as fatty acids, fatty alcohols, alkyl glycerol monoethers, dialkyl glycerol diethers and mixed glycerol ether/esters. The highly strained ladderane moieties were thermally unstable, which resulted in breakdown during their analysis with GC. This was shown by isolation of a thermal product of these ladderanes and subsequent analysis with two‐dimensional NMR techniques. Comprehensive MS and relative retention time data for all the encountered ladderane membrane lipids is reported, allowing the identification of ladderanes in other bacterial cultures and in the environment. The occurrence of ladderane lipids seems to be limited to the specific phylogenetic clade within the Planctomycetales able to perform anammox. This was consistent with their proposed biochemical function, namely as predominant membrane lipids of the so‐called anammoxosome, the specific organelle where anammox catabolism takes place in the cell.

A reanalysis of phospholipid fatty acids as ecological biomarkers for methanotrophic bacteria

Aerobic methane-oxidizing bacteria (MB) are the primary terrestrial sinks for the greenhouse gas methane. A distinct characteristic of MB is the presence of specific phospholipid ester-linked fatty acids (PLFA) in their membranes that differentiate them from each other and also from all other organisms. These distinct PLFA patterns facilitate microbial ecology studies. For example, the assimilation of C from methane into PLFA can be traced in environmental samples using stable isotope (13C) probing (SIP), which links the activity of MB to their community composition in situ. However, the phylogenetic resolution of this method is low because of a lack of PLFA profiles from cultured MB species. In this study, PLFA profiles of 22 alphaproteobacterial (type II) MB were analysed after growth on methane, methanol or both substrates together. Growth on different substrates did not affect the PLFA profiles of the investigated strains. A number of Methylocystis strains contained novel C18:2 fatty acids (ω7c,12c and ω6c,12c) that can be used as diagnostic biomarkers. The detection of these novel PLFA, combined with the analyses of multiple type II strains, increased the phylogenetic resolution of PLFA analysis substantially. Multivariate analysis of the expanded MB PLFA database identified species groups that closely reflected phylogenies based on 16S rRNA and pmoA gene sequences. The PLFA database therefore provides a robust framework for linking identity to activity in MB communities with a higher resolution than was previously possible.

Structural identification of the C25 highly branched isoprenoid pentaene in the marine diatom Rhizosolenia setigera

2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadeca-2,5E,9E,13-tetraene I possessing a C25 highly branched isoprenoid skeleton has been isolated from the marine diatom Rhizosolenia setigera and identified by 1H and 13C NMR spectroscopy.

An experimental study of the low-temperature sulfurization of carbohydrates

Sulfurization of carbohydrates has been suggested as an important mechanism for the preservation of organic matter. To study this process, different monosaccharides were sulfurized under laboratory conditions at relatively low temperature (50 °C). The products formed after cleavage of polysulfide linkages were analyzed using gas chromatography (GC) and gas chromatography/mass spectrometry after appropriate derivatization. Selected products were isolated by preparative GC and their structures were identified by nuclear magnetic resonance spectroscopy. During these experiments all monosaccharides were completely converted into organic sulfur compounds (OSCs) and monosaccharides with the carbonyl function replaced by sulfur formed a substantial part of the GC-amenable OSCs. The structures of other OSCs formed indicated that cleavage of C C bonds and racemization also took place during these experiments. The yield of recoverable OSCs after cleavage of polysulfide linkages was relatively low (<5% of the starting monosaccharide), indicating that most of the sulfurization products were still non GC-amenable and thus, for example, linked through monosulfide linkages. Flash pyrolysates of the sulfurized carbohydrate material contained in all cases relatively high amounts of short-chain alkylated (C0 C5) thiophenes, comparable to those obtained from S-rich kerogen. The structure of the monosaccharide used in the experiments had no effect on the alkylthiophene distribution. These results provide experimental evidence that sulfurization of monosaccharides at relatively low temperatures can result in the formation of OSCs, most likely starting with sulfurization of the carbonyl functionality. Preservation of carbohydrates through sulfurization may thus be an important pathway of preservation of organic matter in anoxic depositional environments.

A mixed ladderane/n-alkyl glycerol diether membrane lipid in an anaerobic ammonium-oxidizing bacterium

A novel glycerol diether containing ladderane and tetradecyl moieties has been identified in an anaerobic ammonium-oxidizing bacterium by GC/MS and high-field NMR spectroscopy.

Novel intact glycolipids in sediments from an Antarctic lake (Ace Lake)

Abstract Two novel glycolipids, docosanyl 3-O-methyl-α-rhamnopyranoside and docosanyl 3-O-methylxylopyranoside, were identified as the most abundant GC-amenable components of extracts of sediments of Ace Lake, an Antarctic meromictic lake. Docosanyl 3-O-methyl-α-rhamnopyranoside was identified by isolation and high-field two-dimensional NMR techniques. Docosanyl 3-O-methylxylopyranoside was tentatively identified by characterisation of its sugar moiety by methanolysis and demethylation. Both glycolipids also have a minor homologue, containing a C24 instead of a C22 n-alkyl chain. Stable carbon isotope measurements indicate that the sugar moieties of these glycolipids are ca. 8–9‰ enriched relative to the alkyl chains. Concentrations increased markedly with depth reaching 2 mg/g sediment at 25 cm depth. Structurally related glycolipids occur in specialised cells of nitrogen-fixing cyanobacteria, so a cyanobacterial origin is suggested for these sedimentary glycolipids.

All-cis hentriaconta-9,15,22-triene in microbial mats formed by the phototrophic prokaryote Chloroflexus

All-cis hentriaconta-9,15,22-triene (I) has been isolated from Chloroflexus mats, Yellowstone National Park (USA), and identified by GC-(HR)MS analysis of I and its hydrogenated and DMDS-derivatized products and by 1H and 13C NMR spectroscopy.