Microbial Membrane-Spanning Lipids: Determination of Biological Sources and Evolutionary Implications

Summary Bacterial membrane lipids are composed of a glycerol-3-phosphate backbone attached via ester bonds to fatty acid molecules, while those of Archaea have a glycerol-1-phosphate backbone linked to isoprenoids via ether bonds. The cytoplasmic membrane is generally organized as a bilayer, however the majority of archaeal membranes are organized as membrane-spanning lipids (MSLs), forming a monolayer of isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs). Some bacterial species produce lipids similar to the archaeal MSLs but non-isoprenoidal, i.e., branched glycerol dialkyl glycerol tetraethers (brGDGTs) that are likely produced by soil bacteria. During the past decade, these types of MSL have been increasingly used in molecular proxies. Some isoGDGTs are used in the paleotemperature proxy TEX86 to reconstruct past sea surface temperatures, while the BIT index evaluating terrestrial input is based on isoGDGTs and brGDGTs. Recently, novel GDGTs have been detected in various environments and proposed to have biomarker potential, however little is known about their sources, which is instrumental for their application as paleoproxies. Here, we investigated the biosynthetic pathway leading to the formation of bacterial MSLs by using cultivation and genomic analyses. In addition, we also investigated two anoxic and highly sulfidic environmental settings to determine the biological sources of specific isoGDGTs.