Contrasting Drivers of Bacterial Metabolism in the Euphotic and Mesopelagic Zones of Tropical Oligotrophic Oceans

Abstract Microbial metabolism plays a critical role in global carbon cycling; however, our understanding of bacterial metabolic processes across the full depth of tropical oligotrophic oceans remains incomplete. The South China Sea (SCS) and Western Pacific (WP), as contrasting oligotrophic environments, provide an ideal setting to investigate this unresolved issue. This study presented a comprehensive analysis of bacterial carbon demand (BCD) from the euphotic to the mesopelagic zone in both regions, revealing distinct drivers of bacterial metabolism at different depths. In the euphotic zone, BCD was closely linked to biotic factors such as bacterial abundance and net primary production, with the SCS exhibiting higher bacterial metabolic activity compared to the WP. Below the euphotic zone, dissolved organic carbon availability became the critical limiting factor, with the WP supporting stronger bacterial metabolism due to more efficient organic matter retention. These findings highlighted the regional variability in carbon sequestration efficiency between the SCS and WP, offering new insights into the marine biological carbon pump. As climate change intensifies, understanding how microbial metabolism modulates carbon export and long‐term storage is increasingly critical for predicting shifts in the carbon sink capacity of marine ecosystems.