The three domains of life on Earth comprise the two groups of prokaryotes Archaea and Bacteria whom distinction notably relies on the chemical composition of cellular membranes. Bacteria generally synthesize phospholipids constituted of non-isoprenoid alkyl chains esterified to glycerol, whereas Archaea synthesize isoprenoid ethers of glycerol. These differences have potential implications in term of ecology and evolution of Bacteria and Archaea and are frequently used as distinguishing marks. Non-isoprenoid diethers of glycerol (non-isoprenoid DGD) constitute an exception to these chemical distinctions as they exhibit an intriguing combination of structural features of Bacteria and Archaea. The physiological role and the mode of formation of these lipids are poorly understood. Remarkably, these compounds have never been observed in mesophilic marine bacteria although they are omnipresent in non-extreme interfaces between the Biosphere and the Geosphere where their origin remains speculative.This multidisciplinary project aims at investigating in detail the first isolate of marine mesophilic sulphate-reducing bacterium able to produce non-isoprenoid DGD, and at comparing it with other (un)related strains. It relies on the combination of lipidomics (analysis of lipid biomarkers) with microbial ecology and microscopy coupled with spectroscopy (bacterial ecophysiology based on ultrastructure). Results will allow specifying the use of DGD as biogeochemical proxy and clarifying the link between living bacterial structures and the observation of microbial signatures within the sedimentary record. It should also shed light on crucial aspects of microbial ecology and on the evolution of lipid biosynthesis in prokaryotes.The project is based on preliminary results obtained by the host. Its relevance is supported by complementary expertise in microbiology and microbial ecology (national collaboration) and is strengthened by outstanding international collaboration.