Free-living marine bacterioplankton composition and diversity along the Kuroshio region

We investigated how free-living bacterioplankton's diversity and taxonomic composition vary along the Kuroshio from south Taiwan to the east coast of Honshu Japan (∼22–35 °N). We collected samples from both surface and deep chlorophyll-a maximum (DCM) layers and then applied high-throughput sequencing techniques to study communities of bacterioplankton. Along the Kuroshio, we found a general decreasing richness (R² = 0.077, p = 0.02, regression coefficient = −13.092) and Shannon diversity (R² = 0.327, p = 0.027, regression coefficient = −0.029), which was best explained by decreasing temperature and phosphate concentration. Whereas bacterial evenness increased in the surface (R² = 0.3, p = 0.002, regression coefficient = 0.545) but decreased in the DCM along the Kuroshio (R² = 0.2, p = 0.014, regression coefficient = −0.451); this pattern coincided with the patterns that chlorophyll-a concentration significantly increased in the surface but decreased in the DCM layer. The results suggest that physiological constrain and nutrient availability influenced the richness and Shannon diversity, while productivity influenced the evenness of bacterioplankton. In terms of bacterial community composition, the surface and the DCM layers were significantly different. The compositional change of bacterioplankton along the Kuroshio was also different in the two layers because different bacterial taxa exhibited distinct responses to different environments. In the surface layer, compositional change of bacterioplankton was associated with temperature, dissolved inorganic nitrogen, and salinity, while in the DCM layer, phosphate concentration was also important. In terms of the bacterial taxa that showed significant changes along the Kuroshio, the relative abundance of genus NS5_marine_group significantly decreased, while genus Sva0996_marine_group and Pseudohongiella significantly increased in the surface layer. In addition, genus Clade_Ia and HIMB11 showed significantly lower relative abundance around the transect near Taiwan, while genus Alteromonas was significantly more abundant at the transect around 30 °N. In the DCM layer, the relative abundance of genus Candidatus_Actinomarina, Sva0996_marine_group, and NS5_marine_group significantly decreased along the Kuroshio. We argue that bacteria's preference for temperature, salinity, and resource availability are related to their compositional variation along the Kuroshio. Our findings, for the first time, document how bacterial diversity and composition change along the Kuroshio, providing the foundation to studying the bacterioplankton in the northwestern Pacific Ocean.