Constructed wetlands treating synthetic wastewater in response to day-night alterations: Performance and mechanisms

Constructed wetlands (CWs), as nature-based wastewater treatment systems, continuously maintain exchanges of material and energy with the natural environment. However, it is unclear how pollutant removal performance in CWs occurs in response to diurnal variation. Herein, we investigated the effects of day-night alterations on the rhizosphere microbial community in CWs. The results revealed that a dynamic alternate mechanism existed in the inner environment when the CWs were in steady operation, which was validated by rhythmic changes in the core microbial community, microbial metabolism activity, and pollutant removal shown in this study. The results showed that pollutant removal (e.g., total nitrogen) due to microbial processes was 1.31 times higher under daytime conditions than under nighttime conditions. Core microbial taxa of the rhizosphere that evolved with circadian rhythm (e.g., Chloroflexus and Beijerinckia) were mostly associated with carbon, nitrogen, phosphorus, and energy metabolism, with lower average relative abundance identified at night. Although higher activity of microbial metabolism was also observed in the daytime, nighttime conditions demonstrated higher gene levels that were related to carbon and nitrogen metabolic functions. Structural equation model (SEM) analysis further suggested that environmental factors exerted a significant influence in driving microbial processes for pollutant removal and that the microbial community played a greater role in promoting pollutant removal under nighttime conditions than under daytime conditions.