Unusual Butane- and Pentanetriol-Based Tetraether Lipids in Methanomassiliicoccus luminyensis, a Representative of the Seventh Order of Methanogens
ABSTRACTA new clade of archaea has recently been proposed to constitute the seventh methanogenic order, theMethanomassiliicoccales, which is related to theThermoplasmatalesand the uncultivated archaeal clades deep-sea hydrothermal ventEuryarchaeotagroup 2 and marine group IIEuryarchaeotabut only distantly related to other methanogens. In this study, we investigated the membrane lipid composition ofMethanomassiliicoccus luminyensis, the sole cultured representative of this seventh order. The lipid inventory ofM. luminyensiscomprises a unique assemblage of novel lipids as well as lipids otherwise typical for thermophilic, methanogenic, or halophilic archaea. For instance, glycerol sesterpanyl-phytanyl diether core lipids found mainly in halophilic archaea were detected, and so were compounds bearing either heptose or methoxylated glycosidic head groups, neither of which have been reported so far for other archaea. The absence of quinones or methanophenazines is consistent with a biochemistry of methanogenesis different from that of the methanophenazine-containing methylotrophic methanogens. The most distinctive characteristic of the membrane lipid composition ofM. luminyensis, however, is the presence of tetraether lipids in which one glycerol backbone is replaced by either butane- or pentanetriol, i.e., lipids recently discovered in marine sediments. Butanetriol dibiphytanyl glycerol tetraether (BDGT) constitutes the most abundant core lipid type (>50% relative abundance) inM. luminyensis. We have thus identified a source for these unusual orphan lipids. The complementary analysis of diverse marine sediment samples showed that BDGTs are widespread in anoxic layers, suggesting an environmental significance ofMethanomassiliicoccalesand/or related BDGT producers beyond gastrointestinal tracts.IMPORTANCECellular membranes of members of all three domains of life,Archaea,Bacteria, andEukarya, are largely formed by lipids in which glycerol serves as backbone for the hydrophobic alkyl chains. Recently, however, archaeal tetraether lipids with either butanetriol or pentanetriol as a backbone were identified in marine sediments and attributed to uncultured sediment-dwelling archaea. Here we show that the butanetriol-based dibiphytanyl tetraethers constitute the major lipids inMethanomassiliicoccus luminyensis, currently the only isolate of the novel seventh order of methanogens. Given the absence of these lipids in a large set of archaeal isolates, these compounds may be diagnostic for theMethanomassiliicoccalesand/or closely related archaea.