Ancient carbon flux into ecosystem biomass in shallow-water hydrothermal systems

Hydrothermal vents introduce substantial amounts of ¹⁴ C-free carbon into the ocean, primarily as CO ₂ . While these emissions influence the global oceanic carbon pool, the mechanisms by which this carbon is integrated into marine ecosystems remain largely unexplored. Here, we present a combination of bulk ¹⁴ C and ¹³ C measurements of particulate organic carbon (POC) from water column filters with compound-specific ² H, ¹³ C, and ¹⁴ C analyses of fatty acids from surface sediments to investigate carbon cycling across benthic and pelagic realms in a low pH, shallow-water hydrothermal system off Taiwan. Using isotope correlations, this study demonstrates that vent-derived CO ₂ is incorporated into the local microbial and faunal ecosystem through chemoautotrophy and supports photoautotrophy in the overlying water column, particularly at the margins of the vent plumes. Our findings reveal carbon uptake levels in vent fluids are substantially higher than previously documented. However, within a 20-meter radius of the vents only up to 3.4% of the daily CO ₂ emissions are sequestered, confirming hydrothermal systems as net contributors to the global oceanic inorganic carbon pool. Our novel isotope-based approach provides in-situ evidence of photoautotrophs incorporating vent carbon, highlighting a yet unrecognized pathway of ancient carbon integration into marine ecosystems and carbon sequestration in hydrothermal regions.