Water temperature and organic carbon control spatio-temporal dynamics of particle-attached and free-living bacterial communities in a hypereutrophic urban river network

Bacteria are an important component of the aquatic ecosystem playing a key role in the transformation and mineralisation of allochthonous organic matter, nutrients and pollutants. However, there is no clear understanding of spatio-temporal patterns of particle-attached versus free-living bacterial abundances, diversities and communities, which limits our in-depth understanding of hypereutrophic urban river ecosystems and the mechanisms regulating nutrient cycling and biogeochemical transformations. In order to investigate spatio-temporal patterns and associated factors for aquatic particle-attached versus free-living bacterial communities, we conducted a 2-year study of four contrasting habitats in a hypereutrophic urban river network of eastern China. Bacterial abundance and community structure in relation to various environmental variables were assessed by epifluorescence microscopy and high-throughput sequencing techniques. Results revealed that particle-attached bacterial abundance depended mainly on suspended particle concentrations, whereas free-living bacterial abundance was correlated with nutrient availability and water temperature. Organic carbon was related to a decrease of free-living and particle-attached bacterial richness, and particulate matter input increased free-living bacterial diversity. Seasonal changes of water temperature were linked to the temporal dynamics of bacterial community composition, and the differences in total organic carbon concentrations contributed to spatial differences in bacterial community composition. There were significant differences in bacterial community composition between particle-attached and free-living assemblages in low-turbidity/low-nutrient rivers found in land areas comprising mainly natural vegetation. These differences were especially prominent for the relative abundances of Cyanobacteria and Actinobacteria, which were higher in the particle-attached and free-living fractions, respectively. However, no significant differences were found in turbid/nutrient-rich rivers. Total organic carbon was the most significant factor shaping the composition similarity between particle-attached and free-living bacterial communities. Our findings indicate that spatio-temporal variations in water temperature, suspended particle and resource availability, especially for organic carbon, appreciably contribute to changes in the abundance, diversity and composition of aquatic particle-attached versus free-living bacterial communities in the hypereutrophic urban river network. Considering that biodiversity is important for supporting ecosystem functioning, this study informs spatio-temporal patterns of particle-attached and free-living bacterial abundance, diversity and composition of communities, and their underlying control mechanisms, thereby providing fundamental knowledge of microbial ecological functions for remediation and sustainable management of hypereutrophic urban river ecosystems.