The transfer of particulate organic carbon (POC) from the surface layer to ocean's interior constructs the most dynamic and efficient way for marine carbon sink. Recent works have increasingly highlighted the importance of small phytoplankton in deep-ocean POC, leaving the composition and extent to which it contributes largely unknown. We exploit compound-specific carbon isotopes of amino acids in time-series sinking particles from mesopelagic South China Sea and provide a challenging view against prior diatom governed POC export. Cyanobacteria (89% non-diazotrophs) were estimated to contribute 57%, while eukaryotic microalgae (32%) and heterotrophic bacteria (11%) were much less. The export of all components co-varied with primary productivity, whereas the cyanobacteria allocation remained dominate and relative stable, unlike the seasonal shift of overlying phytoplankton community structure. Our results suggest an important but poorly quantified role of small phytoplankton in the biological pump of oligotrophic oceans, underling a rising potential due to global warming.

Biological carbon pump; sediment trap; compound-specific carbon isotope; food web source; oligotrophic ocean