Bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) mediated membrane fouling in membrane bioreactor

This work reveals the key role of bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) in the membrane biofouling process in membrane bioreactor (MBR). Two lab-scale MBRs were operated in parallel with different sludge retention times (30 d for R₁ and 10 d for R₂). Significantly high intracellular c-di-GMP levels were detected in membrane foulants than in the activated sludge in both reactors. Compared to R₁, R₂ showed much higher membrane fouling rates, corresponding to elevated intracellular c-di-GMP levels in both the activated sludge and membrane foulants in this reactor. The extracellular polymeric substances (EPS) contents were strongly correlated to the levels of intracellular c-di-GMP, suggesting that elevated c-di-GMP levels in bacterial cells stimulated increased production of EPS, leading to accelerated membrane fouling rates. 16S rRNA gene metabarcoding showed the enrichment of Acinetobacter, Pseudofulvimonas, Glutamicibacter, and Streptococcus related operational taxonomic units (OTUs) in the membrane foulants. Their relative abundances were highly correlated to the c-di-GMP level, showing the mainstay role of these microorganisms in c-di-GMP mediated biofilm formation and membrane fouling. Sloan neutral community modeling suggested that the enrichment and deposition of these key bacteria were not determined by their inherent tendency to grow on the membrane surface but their lifestyle change driven by c-di-GMP. Genome-wide analysis of these key bacteria together with a community-wide PICRUSt2 survey suggested stronger c-di-GMP metabolic abilities of bacteria enriched in membrane foulants than those in the activated sludge, highlighting the key role of c-di-GMP second-messenger pathway in regulating EPS production thus governing the membrane biofouling process.