The prokaryotic lipid divide: how strict is it?

A long-lasting dogma regarding the membranes of prokaryotes is the distinct lipid composition of archaea and bacteria. Bacterial phospholipids consist of acyl chains that are esterified to a glycerol-3-phosphate lipid backbone. Instead, in archaeal phospholipids, isoprenoid chains are coupled to the enantiomeric glycerol-1-phosphate via ether bonds. This separation in (1) lipid tail configuration, (2) lipid tail coupling, and (3) lipid backbone chirality is also referred to as the 'lipid divide'. However, accumulating evidence is indicating that this divide is not as strict as currently assumed. In this review, we provide an overview of the latest scientific advances, highlighting the observed variety in prokaryotic lipid content, as well as promiscuity in the involved lipid synthesizing enzymes. Finally, to put these new findings in the context of the evolution of early prokaryotic live, we further reassess current theories underlying the origin of the prokaryotic lipid divide.