Deep-sea dissolved organic carbon (DOC) is an important reservoir for long-term carbon storage, yet its relatively stable concentration may conceal continuous microbial transformation. High hydrostatic pressure (HHP), as an environmental constraint, influences microbial community composition and cellular metabolism. These, in turn, determine how microorganisms utilize DOC. However, the chain from pressure to biological responses and ultimately to carbon fate, remains insufficiently understood. To investigate how pressure affects DOC transformation, we established simplified, controlled incubation systems with pressure as the primary experimental variable.
In the natural-DOC system, deep seawater was used as the DOC substrate and microbial community from hadal zone as the inoculum. Incubations were conducted for 25 days at 4°C under 7 pressures ranging from 0.1 to 115 MPa. Although DOC concentrations remained relatively stable, its molecular composition was shifted toward more oxidized and potentially less bioavailable. With increasing pressure, the overall O/C ratio significantly increased. Newly formed and persistent molecules exhibited higher O/C, nominal oxidation state of carbon (NOSC), and modified aromaticity index (AI_mod), together with lower H/C ratios. Meanwhile, microbial abundance first increased and then decreased with pressure, and distinct bacterial taxa dominated across different pressure ranges. Metatranscriptomic responses associated with complex organic matter degradation, central carbon metabolism and antioxidant defense were also enhanced. Together, these results suggest that pressure regulated microbial community and energy strategies, ultimately leaving a potentially recalcitrant molecular imprint on the extracellular DOC pool.
In single-strain model system, a hadal piezotolerant Alcanivorax xenomutans A28 was incubated under 0.1, 40, and 80 MPa with n-C₁₆. HHP decreased the apparent n-C₁₆ consumption rate but increased the proportion of consumed carbon recovered as dissolved inorganic carbon. Biomass formation and intracellular carbon storage were suppressed, whereas fatty acid oxidation, the tricarboxylic acid cycle and stress responses were enhanced. These findings suggest that HHP may redirect a greater proportion of utilized organic carbon toward mineralization, leaving less carbon available for growth and storage.
Through pressure-controlled incubation experiments, this study reveals a potential link that HHP alters DOC fate via microbial responses. HHP may reshape microbial community and cellular energy strategies, thereby altering how microorganisms utilize DOC. Thus, the same organic carbon may follow different transformation pathways under different pressure conditions, highlighting pressure as an important environmental constraint shaping the microbial fate of carbon in the deep ocean.
 

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