Transcriptomic Analysis Reveals Common Adaptation Mechanisms Under Different Stresses for Moderately Piezophilic Bacteria

Piezophiles, by the commonly accepted definition, grow faster under high hydrostatic pressure (HHP) than under ambient pressure and are believed to exist only in pressurized environments where life has adapted to HHP during evolution. However, recent findings suggest that piezophiles have developed a common adaptation strategy to cope with multiple types of stresses including HHP. These results raise a question on the ecological niches of piezophiles: are piezophiles restricted to habitats with HHP? In this study, we observed that the bacterial strains Sporosarcina psychrophila DSM 6497 and Lysinibacillus sphaericus LMG 22257, which were isolated from surface environments and then transferred under ambient pressure for half a century, possess moderately piezophilic characteristics with optimal growth pressures of 7 and 20 MPa, respectively. Their tolerance to HHP was further enhanced by MgCl2 supplementation under the highest tested pressure of 50 MPa. Transcriptomic analysis was performed to compare gene expression with and without MgCl2 supplementation under 50 MPa for S. psychrophila DSM 6497. Among 4390 genes or transcripts obtained, 915 differentially expressed genes (DEGs) were identified. These DEGs are primarily associated with the antioxidant defense system, intracellular compatible solute accumulation, and membrane lipid biosynthesis, which have been reported to be essential for cells to cope with HHP. These findings indicate no in situ pressure barrier for piezophile isolation, and cells may adopt a common adaptation strategy to cope with different stresses.