Vertical variations of bacterial growth, mortality loss to nanoflagellates, and viruses in the subtropical northwestern Pacific Ocean

Nanoflagellate grazing and viral lysis are the two main causes of mortality losses of marine bacterioplankton. Deciphering the mortality losses across the water column helps us understand their ecological and biogeochemical consequences. In this study, we implemented the two-point modified dilution method consisting of treatment of undiluted and 25% nanoflagellates and/or virus density at the surface (3 m), deep chlorophyll maximum (DCM), and the mesopelagic zone (300 m and 500 m deep) in the subtropical northwestern Pacific Ocean in summer. We found that the bacterial per capita growth (2.3 ± 0.6 d⁻¹) and production (12.4 ± 6.9 μgC L⁻¹ d⁻¹) were significantly higher at the DCM layer than at the mesopelagic zone, possibly because of tight bacteria-phytoplankton coupling and trophic interactions between bacteria, nanoflagellates, and viruses. Further, we found that ∼70% of the bacterial mortality loss can be attributed to nanoflagellate grazing in the DCM layer, while most of the mortality loss in the surface and the mesopelagic zone can be attributed to viral lysis. We argue that while bacterial production is more efficiently transferred to higher trophic levels at the DCM layer, it is predominately recycled in the viral loop on the surface and the mesopelagic zone. Our results reveal the vertical variation of bacterial growth, production, mortality loss to nanoflagellate grazing, and viral lysis, from which we could deduct their depth-dependent impacts on carbon flux in the water column. Our study facilitates the understanding of the impacts of nanoflagellates and viruses on bacterioplankton and the bacteria-mediated biogeochemical cycling.