Microbial Communities in Full-Scale Wastewater Treatment Systems Exhibit Deterministic Assembly Processes and Functional Dependency over Time

Microbial communities constitute the core component of biological wastewater treatment processes. We conducted a meta-analysis based on the 16S rRNA gene of temporal samples obtained from diverse full-scale activated sludge and anaerobic digestion systems treating municipal and industrial wastewater (collected in this study and published previously) to investigate their community assembly mechanism and functional traits over time, which are not currently well understood. The influent composition was found to be the main driver of the microbial community's composition, and relatively large proportions of specialist (26.1% and 18.6%) and transient taxa (67.2% and 68.1%) were estimated in both systems. Deterministic processes, especially homogeneous selection events (accounting for >53.8% of assembly events), were consistently identified as the dominant microbial community assembly mechanisms in both systems over time. Significant and strong correlations (Pearson's r = 0.51-0.92) were detected between the dynamics of the temporal community and the functional compositions in both systems, which suggests functional dependency. In contrast, the occurrence of sludge bulking and foaming in the activated sludge system led to an increase in stochastic assembly processes (i.e., limited dispersal and undominated events), a shift toward functional redundancy and less community diversity, a decreased community niche breadth index, and a more compact co-association network. This study illustrates that the mechanism of microbial community assembly and functional traits over time can be used to diagnose system performance and provide information on potential system malfunction.