Absolute concentrations of BDGTs and PDGTs in sediments from expeditions M84/1 and POS450

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 two-dimensional 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. In addition, relatively 13C-depleted BDGTs from the Rhone delta and from the Black Sea are in agreement with a probable methanogenic source for these lipids. In line with this interpretation, high proportions of intact polar BDGTs and PDGTs were detected in the deeper methane-laden sedimentary layers. However, relatively 13C enriched BDGTs and contrasting headgroup distribution patterns of BDGTs and PDGTs in sediments of the Eastern Mediterranean Sea imply that additional archaeal groups also produce these unique lipids.