Observational time-series analysis reveals cyanobacteria dominate deep ocean carbon export despite eukaryotic blooms, indicating their critical role in stabilizing marine carbon storage.
Key Points
To assess how phytoplankton community composition controls the efficiency and magnitude of deep ocean carbon sequestration via the biological carbon pump.
Analyzed time-series sediment trap samples collected across varying seasons in the mesopelagic zone.
Measured compound-specific stable carbon isotopes of amino acids to quantify taxon-specific contributions of cyanobacteria and eukaryotic microalgae to sinking particulate organic carbon (POC).
Mesopelagic sinking POC remained persistently cyanobacteria-rich at 67% ± 2%, even during high-productivity periods when surface community dominance shifted to eukaryotes.
Bulk POC sequestration efficiency remained statistically invariant because eukaryotic sequestration efficiency halved while cyanobacterial efficiency nearly doubled.
Eukaryote-derived organic carbon underwent heavy microbial remineralization in the twilight zone, whereas small non-ballasted cyanobacteria formed aggregates that compensated for eukaryotic losses.