Abstract
Abstract. Butanetriol and pentanetriol dialkyl glycerol tetraethers (BDGTs and PDGTs) are membrane lipids, recently discovered in sedimentary environments and in the methanogenic archaeon Methanomassiliicoccus luminyensis. They possess an unusual structure, which challenges fundamental assumptions in lipid biochemistry. Indeed, they bear a butanetriol or a pentanetriol backbone instead of a glycerol at one end of their core structure. In this study, we unambiguously located the additional methyl group of the BDGT compound on the C3 carbon of the lipid backbone via high-field nuclear magnetic resonance (NMR) experiments. We further systematically explored the abundance, distribution and isotopic composition of BDGTs and PDGTs as both intact polar and core lipid forms in marine sediments collected in contrasting environments of the Mediterranean Sea and Black Sea. High proportions of intact polar BDGTs and PDGTs in the deeper methane-laden sedimentary layers and relatively 13C-depleted BDGTs, especially in the Rhone Delta and in the Black Sea, are in agreement with a probable methanogenic source for these lipids. However, contributions from heterotrophic Archaea to BDGTs (and PDGTs) cannot be excluded, particularly in the eastern Mediterranean Sea, and contrasting BDGT and PDGT headgroup distribution patterns were observed between the different sites studied. This points to additional, non-methanogenic, archaeal sources for these lipids.
Highlights
Unique membrane lipids formed a key argument in the postulation of the existence of Archaea as a third and independent domain of life, as distantly related to Bacteria as to Eukarya, when Woese et al (1990) proposed their revised tree of life
3.1 nuclear magnetic resonance (NMR) analysis of BDGT-0 extracted from M. luminyensis
We found 13 genes annotated as belonging to the radical SAM superfamily in the permanent draft genome of M. luminyensis B10 (IMG/M website, IMG Submission ID 11458; Chen et al, 2019)
Summary
Unique membrane lipids formed a key argument in the postulation of the existence of Archaea as a third and independent domain of life, as distantly related to Bacteria as to Eukarya, when Woese et al (1990) proposed their revised tree of life. Membrane lipids from members of all domains of life share some common characteristics, such as their amphiphilic nature. That is, they all possess apolar alkyl chains and polar headgroups held together by a glycerol moiety (Lombard et al, 2012). Membrane lipids of Archaea fundamentally differ from those of Bacteria and Eukarya in that they contain (bi)phytanyl chains constituted from the condensation of several isoprenoid units and ether linkages to the sn and sn carbons of a glycerol backbone (De Rosa and Gambacorta, 1988; Koga and Morii, 2005). Intensive exploratory analyses of lipid extracts from pure cultures and environmental samples over the last decades
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