A quantitative assessment of pressure dependent adaptive changes in the membrane lipids of a piezosensitive deep sub-seafloor bacterium

The finding of microbial life in the Earth's deep subsurface raises the question as to how deep microbial ecosystems are able to survive under these extreme environmental conditions. To investigate the effect of increasing ambient pressure, a new quantitative approach was developed to monitor adaptive changes in the cell membrane phospholipid composition of a piezosensitive bacterium, strain LT25, in response to different growth pressure conditions (0.1 and 25 MPa) using liquid chromatography coupled via an electrospray interface to a mass spectrometer (HPLC-ESI-MS) and collisionally activated dissociation (CAD) experiments (MS-MS). Strain LT25 was isolated from deep subsurface sediments in the Nankai Trough [offshore Japan; Ocean Drilling Program (ODP), Leg 190] and was cultivated under atmospheric and high hydrostatic pressure conditions. The HPLC-ESI-MS-MS method allows detailed structural elucidation and quantification of individual phospholipids, taking into account the different response factors of individual phospholipids in the ESI-MS detection system. Strain LT25 contains phosphatidylglycerol and phosphatidylethanolamine esters with saturated and/or mono-unsaturated fatty acids containing mainly 14, 16 and 18 carbon atoms. Quantitative assessment of the phospholipids and their fatty acyl side chain inventory illustrates complex restructuring processes in the microbial cell membrane of strain LT25: cells cultivated under high pressure conditions (25 MPa) show higher proportions of phosphatidylglycerol esters and mono-unsaturated fatty acids. Both changes are associated with a high degree of cell membrane disturbance and, with that, a lowering of the solid to liquid phase transition temperature of the cell membrane. Since this counterbalances the effect of increasing pressure, it is suggested that an adaptive alteration of the membrane phospholipid composition occurs, in response to increasing growth pressure conditions, to maintain the membrane fluidity.