High-frequency sampling reveals diurnal variability and assembly mechanisms of fungi and bacteria in a reclaimed water impacted ecosystem.

While the long-term effects of reclaimed water are well-studied, its diel ecological impacts remain underexplored. Here, we conducted 24-hour high-frequency monitoring of a reclaimed-water-impacted urban aquatic ecosystem (HY) against two natural reference aquatic systems (SD and XL) to reveal diel ecological dynamics associated with continuous water replenishment. The HY exhibited higher concentrations and pronounced diurnal variations in nutrients (e.g. nitrate, total phosphorus) and micropollutants, creating a dynamic and powerful stress compared to the reference areas. These fluctuations corresponded to rapid and significant shifts in the HY microbiota, evidenced by higher variations in community composition and α-diversity compared to the natural waters. In addition, microbial communities in SD and XL were well fitted with Sloan neutral model by stochastic process, while iCAMP analysis further revealed that microbial communities in reclaimed water system were more impacted by heterogeneous selection and dispersal limitation. Notably, bacteria and fungi showed divergent responses, where fungal community in HY tended to collaborate more in response to the highly variable environment. Our findings provide valuable insights into the influence of reclaimed water and demonstrate that diel-scale analysis is essential for assessing the ecological risks of aquatic environment especially for those with high anthropogenic impacts.