Exceptionally stable membrane lipid composition of the marine facultative anaerobe and piezotolerant ‘ <i>Labilibaculum euxinus</i> ’ under variable pressure and nutrients

The lipid bilayer is a dynamic barrier that plays a critical role as a frontier between cellular elements and the environment. The marine environment presents a unique set of conditions, including variations in nutrient availability, temperature, and high hydrostatic pressure (HHP). To survive these conditions, deep-sea microbes have developed adaptive strategies to preserve the integrity of their lipid membranes, particularly in response to HHP. Here, we assessed these adaptations by determining changes in the membrane lipids of a piezotolerant bacterium, 'Labilibaculum euxinus' from the phylum Bacteroidota (family Marinifilaceae) isolated from 2000 meters depth in the Black Sea, under different hydrostatic pressures and phosphate concentrations. Lipid analysis of 'L. euxinus' grown in a medium replete in phosphate revealed the presence of lipids with nonphosphate containing amino acid headgroups, such as ornithine lipids, flavolipins, and capnine lipids, typically associated with adaptation to phosphate limitation. Microscopy analysis revealed cell elongation under HHP, suggesting cell adaptation. Despite this morphological change, the distribution of membrane lipids remained stable in terms of polar headgroups. Nevertheless, HHP affected the unsaturation of the fatty acyl chain and the relative abundance of cardiolipins. Our study showcases the adaptability of certain extremophiles, particularly piezotolerant organisms.