Upper-Layer Bacterioplankton Potentially Impact the Annual Variation and Carbon Cycling of the Bathypelagic Communities in the South China Sea

Abstract

Pelagic bacterioplankton exhibit biogeographical patterns linked with exporting organic carbon and energy fluxes into the deep ocean. However, knowledge of the mechanisms shaping deep-sea bacterial communities remains largely elusive. In this study, we used high throughput sequencing of the 16S rRNA gene to reveal significant annual bacterioplankton community dynamics in the South China Sea during three summer cruises (2016–2018). As we expected, the epipelagic–bathypelagic connective amplicon sequence variants (ASVs, mostly belonging to Actinobacteriota, Firmicutes, and Cyanobacteria) suggested that they not only affect the community structure but also influence the carbon cycling functions of bathypelagic bacterioplankton in different years. However, the microbial source tracking (MST) analysis indicated that the directly linked proportions between the bathypelagic and epipelagic samples were minimal. Thus, the epipelagic bacteria communities may form “seeds” rather than directly sinking into the deep ocean to influence bathypelagic bacteria. This study provides a new perspective on the mechanisms shaping the deep ocean bacterioplankton communities.