Low pH induces simultaneous fungal bulking and nitrogen removal deterioration in an activated sludge reactor: Mechanisms based on microbial transboundary coevolution

Fungal bulking caused by the abnormal acidic pH in influents stimuli is always accompanied by the deterioration of nitrogen removal in conventional activated sludge process. This is probably the result of co-evolution between dominant fungi and nitrogen-cycling bacterial communities. However, little is known about how they interact across borders in low pH stimuli. In this work, we explored the effects of fungal bloom on the community networks and metabolic pathways of nitrogen-cycling bacteria using an activated sludge reactor. We induced Trichoderma (54%) sludge bulking by lowering the pH, leading to high SVI (301 mL/g) and ammonia nitrogen (10.6 mg/L) in effluent. The abundances and expressions of classic nitrogen cycle genes all decreased, involving in ammoxidation (amo and hao), nitrification (nirS and nirK), and denitrification (napA and nrfA). With the occurrence of Trichoderma bloom, the relative abundances of major N cycling bacteria, such as Nitrosomonas, Nitrospira, and Dechoromonas, significantly reduced by 72%, 60%, and 74%, respectively. As a result, the nitrogen cycling metabolic network collapsed. In contrast, dominant autotrophic denitrifying bacteria Rhodanobacter, clustered with Trichoderma on the phylogenetic tree, increased from 2% to 47%. Trichoderma exhibited symbiotic relationships with certain enriched denitrifiers. Herein, these findings demonstrate that the succession between eukaryotic-prokaryotic microbes can impair nitrogen metabolism functioning in activated sludge systems.