Life history strategies determine response to SRT driven crash in anammox bioreactors.

Anaerobic ammonium oxidation (anammox) is a biological process often applied in wastewater treatment plants for nitrogen removal from highly concentrated side-stream effluents from anaerobic digesters. However, they are vulnerable to process instability prompted by operational shocks and microbial community imbalances, resulting in lengthy recovery times. These issues are further compounded by a lack of understanding of how sustained press disturbances influence the microbial ecology of the system. Here we investigate the response and recovery of an anammox membrane bioreactor to a solids retention time (SRT)-induced reactor crash using 16S rRNA gene and shotgun metagenomic sequencing. We observed a strong selection of bacterial groups based on reproduction strategies, with the Orders Rhodospirillales and Sphingobacteriales increasing from 1.0 % and 11.9 % prior to the crash to 31.9 % and 18.1 % during the crash respectively. The Orders Brocadiales and Anaerolineales decreased from 17.3 % and 28.3 % to 7.3 % and 1.4 % over the same time period, respectively. Metagenomic and metatranscriptomic analyses revealed differential crash responses in metabolically distinct groups of bacteria, with increased expression of genes for extracellular carbohydrate active enzymes, peptidases and membrane transporters. Following the crash, the reactor recovered to its prior state of nitrogen removal performance and pathway analysis demonstrated increased expression of genes related to exopolysaccharide biosynthesis and quorum sensing during the reactor recovery period. This study highlights the effects of reactor perturbations on microbial community dynamics in anammox bioreactors and provides insight into potential recovery mechanisms from severe disturbance.