Long-term responses of microbial plankton to ocean warming reveal shifts in air–sea CO₂ exchange in the late 21st century

Abstract Ongoing ocean warming threatens to disrupt microbial plankton communities and impair the efficiency of the biological carbon pump (BCP), a central mechanism driving air–sea CO₂ exchange and carbon sequestration. The carbon cycle is primarily driven by the metabolic balance of the ocean, which is regulated by the coupling between autotrophic and heterotrophic microbiota. However, the extent to which rising sea surface temperature (SST) will alter these communities remains uncertain. Here, we combine projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6), machine learning techniques, and a global microbial plankton dataset to assess long-term biomass responses for autotrophic and heterotrophic microbial plankton by the late 21st century. Our projections, performed under four Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5), indicate a significant decline in total microbial biomass and the decoupling between autotrophic and heterotrophic metabolism, leading to a near-global prevalence of heterotrophy. These shifts have important implications for the biological carbon pump’s efficiency and, consequently, for the ocean’s role as a CO₂ sink, potentially intensifying atmospheric CO₂ feedbacks. Our predictions highlight the pivotal role of microbial plankton trophic dynamics in climate projections and underscore the importance of incorporating multiple plankton trophic levels into future CMIP models.