Variation in abundance and community structure of particle-attached and free-living bacteria in the South China Sea

Bacteria play a crucial role in the biological transfer of carbon in the ocean, especially in the transformation of particulate organic carbon (POC) to dissolved organic carbon (DOC), yet little information is available for their abundance and community structures in mesopelagic and bathypelagic waters. Here we present results of our investigation on the spatial variations of abundance and community composition of the particle-attached (PA) and free-living (FL) bacterial assemblages at different depths (500 m, 800 m, and 1500 m) in the South China Sea (SCS). Our results showed that the abundance of FL bacteria was an order of magnitude higher than that of PA bacteria. The relative abundance of FL bacteria increased from 82% at 500 m to 93% at 1500 m. In all cases, the FL bacteria assemblages were more diverse than the PA fraction. The PA bacterial assemblage was dominated by members of the Rhodobacterales of the α-Proteobacteria and the Alteromonadales of the γ-Proteobacteria. In contrast, the FL bacteria assemblages displayed distinctive spatial heterogeneity and were dominant by different phylotypes at different depths. However, certain bacterial taxa of the α- and γ-Proteobacteria co-occurred in both the FL and PA fractions, with proportions being varied with depth. These results highlight the coexistence of particle-attached and free-living bacteria specialists and generalists, and provide valuable information in understanding the role of different bacterial assemblages in the deep-ocean carbon cycle.