Unraveling an unknown diversity of archaeal and bacterial tetraether membrane lipid producers in a euxinic marine system

Bacterial membrane lipids have been traditionally defined as fatty acids (FAs) bilayers linked through ester bonds, while those of Archaea as ether-linked isoprenoids forming bilayers or monolayers of membrane spanning lipids (MSLs) known as isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs). This paradigm has been challenged with the discovery of branched GDGTs (brGDGTs), membrane spanning ether-bound branched alkyl FAs, that are of bacterial origin but whose specific producers in the environment are often unknown. The limited number of available microbial cultures restricts the knowledge of the biological sources of membrane lipids, which in turn limits their potential applicability as biomarkers. To address this limitation, we detected membrane lipids in the Black Sea using high resolution accurate mass/mass spectrometry and inferred their potential producers by targeting lipid biosynthetic pathways encoded on the metagenome, in metagenome-assembled genomes and unbinned scaffolds. We also detected brGDGTs and overly branched GDGTs in the suboxic and euxinic waters, which are potentially attributed, to members of the Planctomycetota, Cloacimonadota, Desulfobacterota, Chloroflexota, Actinobacteria and Myxococcota--all anaerobic microorganisms. These results open a new chapter in the use of specific brGDGTs as biomarkers of anoxic conditions in marine settings and of the role of these membrane lipids in microbial adaptation.