Copepod grazing and prokaryotic decomposition amplify the effect of diatom/dinoflagellate regime change on biological carbon pump efficiency
The Biological Carbon Pump (BCP) plays a critical role in global carbon sequestration by exporting particulate organic carbon (POC) from surface waters to the ocean's depths. Copepod fecal pellets (FPs) are an important but highly variable component of the global BCP. This study decoupled and quantified how copepod grazing and prokaryotic activities affect BCP efficiency under different phytoplankton dietary regimes. Controlled experiments revealed that, with a diet of diatoms, copepods double FP production rates, triple FP sinking rates, and significantly lower FP decomposition rates relative to those withe dinoflagellate diets. These biological activities synergistically enhance the efficiency of diatom FP exports to the deep ocean. Opportunistic particle-attached (PA) prokaryotes were crucial to FP decomposition. Metagenomic analyses revealed that functional enzymes targeting phytoplankton- and copepod intestine-derived macromolecules from the PA prokaryotic communities were key to FP decomposition. Genomic properties of Planctomycetota revealed that the strong motile ability, detoxification systems and macromolecule degradation enzymes enabled the success of these opportunistic prokaryotes. Elevated temperatures amplified decomposition rates by enhancing enzyme activities, thus reducing BCP efficiency as recycled organic material would remain in surface waters. Our findings highlight the synergistic biological activities amplifying the effects of phytoplankton composition changes on BCP efficiency. This underscores the importance of considering grazing regimes in combination with microbial dynamics in assessing oceanic carbon cycling.