Bacterial Carbon Cycling in the River Plume in the Northern South China Sea During Summer

Heterotrophic bacterioplankton play a significant role in carbon cycle in oceans. It is crucial to identify regulators of bacterial carbon processing across marine environments. In summer 2016, a cruise was conducted to examine the links between bacterial metabolic rates and community composition, and potential mechanisms regulating bacterial growth efficiency in the plume-impacted coastal area. Our results showed that the two aspects of bacterial metabolism responded differently. Bacterial production (3.69 to 57.1gC L -1 day( -1)) at the surface increased by 40% to 21-fold in response to the freshwater input. The enhanced bacterial production was attributed to an increase in bacterial abundance and cell-specific bacterial production, which was linked to shifts in bacterial community composition and changing partition between anabolic and catabolic pathways. Bacterial groups that grew fast and preferred high molecular weight dissolved organic carbon were responsible for the increase in bacterial production. However, bacterial respiration increased (less than fourfold) to lesser extent than bacterial production. Consequently, bacterial growth efficiency increased dramatically (up to sevenfold) in response to the plume input. Bacterial respiration was primarily dependent on environmental conditions rather than bacterial community composition. The increased phytoplankton biomass modulated bacterial respiration in two contrasting ways likely by providing phytoplankton-derived dissolved organic carbon, which not only improved bacterial abundance but also lowered cell-specific bacterial respiration because of mitigating energy limitation. Our findings elucidated bacterial carbon processing in the plume-impacted coastal waters and highlighted the potential role of bacterial community composition in regulating carbon cycling in oceans. Plain Language Summary Heterotrophic bacterioplankton play a significant role in carbon cycle in oceans. The two aspects of bacterial metabolism responded differently to the river plume. Bacterial production increased by 40% to 21-fold, in response to the river plume. The enhanced bacterial production was attributed to an increase in bacterial abundance and cell-specific bacterial production, which was linked to shifts in bacterial community composition and changing partition between anabolic and catabolic pathways. Bacterial group that grew fast and preferred high molecular weight dissolved organic carbon (DOC) was responsible for the increase in bacterial production. However, bacterial respiration increased (less than fourfold) to lesser extent than bacterial production. Consequently, bacterial growth efficiency increased dramatically (up to sevenfold) in response to the plume input. Bacterial respiration was primarily dependent on environmental conditions rather than shifts in bacterial community composition. The increased phytoplankton biomass modulated bacterial respiration in two contrasting ways likely by providing phytoplankton-derived DOC, which not only improved bacterial abundance but also lowered cell-specific bacterial respiration because of mitigating energy limitation. Our findings elucidated bacterial carbon processing in the plume-impacted coastal waters and highlighted the potential role of bacterial community composition in regulating carbon cycling in oceans.