Aggregation and remineralization of Trichodesmium unveil potential for ocean carbon sequestration

Recent studies have shown that diazotrophs can form aggregates sinking at velocities up to 400 m d^-1, challenging the long-standing assumption that these organisms are confined to the surface ocean, and suggesting an understimated role in carbon (C) sequestration in warm oligotrophic waters. Yet, the extent to which their biomass escapes remineralization in the mesopelagic zone remains poorly constrained. Here, we experimentally simulated the aggregation and sinking of the filamentous diazotroph Trichodesmium erythraeum using roller tanks, following organic matter distribution over a 10-day period-equivalent to a 1000 m descent at a sinking velocity of ~100 m d^-1. Our results show that 33% of organic C and 36% of N remained in the particulate fraction at the end of the experiment, indicating that microbial remineralization was incomplete and relatively proportional. Remineralization was most intense during the first 3 to 5 days of descent (0-500 m), after which potential C flux declined. We also estimated that a substantial portion of bacterial biomass was incorporated into the aggregates and may contribute to the C export, revealing a dual role for bacteria as both recyclers and exporters of organic matter. Given the widespread distribution and high productivity of Trichodesmium erythraeum in the expanding (sub)tropical ocean, our results highlight the need to include its fate in global biogeochemical models.