Substrate stress response of microbial consortia in a MBR for wastewater treatment as detected through a metaproteomic approach

A lab-scale Membrane BioReactor (MBR) for the treatment of synthetic municipal wastewater was set-up. The functional response of the activated sludge of the plant to a change in the supply of the influent wastewater was investigated. After 60 days at the steady state, the volumetric loading rate (VLR) was suddenly reduced to 65% by reducing the influent flowrate and establishing stress conditions for the biomass. The VLR decrease resulted in a reduction of volatile suspended solids (VSS) in the mixed liquor and in a slight worsening of the chemical oxygen demand (COD) degradation with respect to the previous period. Respirometry results revealed that the autotrophic bacteria more promptly adapted to the organic load perturbation than the heterotrophic ones. More details were provided by the application of metaproteomics. Particularly, several species expressed the same molecular chaperone in different experimental stages, evidencing diverse organic supply stress thresholds by different bacteria. More, coherently with the respirometric approach, some proteins belonging to species based on nitrogen metabolism were found in all samples, confirming the good resistance of autotrophic genera such as Nitrosomonas or Nitrospira to the stress due to VLR decrease. Finally, the expression of 1-butanol dehydrogenase by Thauera butanivorans at the end of the experimentation indicates that some species developed different metabolic pathways such as the degradation of butane, which had not been detected in previous stages.