Bacteria synthesize various types of isoprenoid quinones. The widely distributed menaquinone (MK) has been typically associated to anaerobic metabolisms, whereas ubiquinone (UQ) is generally considered an aerobic quinone. However, our recent discovery of a UQ biosynthesis pathway independent of O2 supports that UQ may also be involved in anaerobic metabolisms [1], as recently demonstrated in the case of Pseudomonas aeruginosa [2]. I will present our latest data on the regulation and biochemistry of the O2- independent UQ biosynthesis pathway. I will also emphasize how the distribution of UQ and MK pathways in bacterial species sheds new light on the respective roles of these quinones in various metabolisms. [1] L. Pelosi, C.D. Vo, S.S. Abby, L. Loiseau, B. Rascalou, M. Hajj Chehade, B. Faivre, M. Gousse, C. Chenal, N. Touati, L. Binet, D. Cornu, C.D. Fyfe, M. Fontecave, F. Barras, M. Lombard, F. Pierrel, Ubiquinone Biosynthesis over the Entire O2 Range: Characterization of a Conserved O2-Independent Pathway, MBio., 10 (2019) e01319-19. [2] C.-D.-T. Vo, J. Michaud, S. Elsen, B. Faivre, E. Bouveret, F. Barras, M. Fontecave, F. Pierrel, M. Lombard, L. Pelosi, The O2-independent pathway of ubiquinone biosynthesis is essential for denitrification in Pseudomonas aeruginosa, J Biol Chem., 295 (2020) 9021–9032.