Differences in metabolic potential between particle-associated and free-living bacteria along Pearl River Estuary

Particulate organic matter (POM) in aquatic ecosystem is critical for biogeochemical cycling and host distinct communities of microbes, compared to its surrounding water. In this study, the structures and functional potentials of microbial communities associated with particles or free-living in water samples from the Pearl River Estuary were investigated using 16S rRNA gene sequencing and GeoChip 5.0 analysis. Significant differences in the community structure and genetic functional potentials between particle-associated bacteria and free-living bacteria were observed across all eight sampling sites. In particle-associated bacteria communities, Rhodobacteraceae and Flavobacteriaceae were more abundant, while SAR11 clade and SAR86 clade were the most abundant in free-living bacteria communities. The richness and abundance of functional genes involved in nutrient cycling and stress response, including carbon degradation, nitrogen fixation, DMSP degradation, and polyphosphate degradation, were much higher in particle-associated bacteria compared with free-living bacteria. Thus, the particle-associated bacteria seem to play a much more important role in the biogeochemical cycles than free-living bacteria. In conclusion, the results from this study highlight the central role played by particle-associated bacteria in structuring microbial assemblages, and their importance for mediating biogeochemical cycling in the estuarine ecosystem.