Multiple extracellular polymeric substance pathways transcribed by <i>Accumulibacter</i> and the flanking community during aerobic granule formation and after influent modification

Aerobic granular sludge is a biological wastewater treatment process in which a microbial community forms a granular biofilm. The role of <i>Candidatus</i> Accumulibacter in the production of a biofilm matrix composed of extracellular polymeric substances was studied in a sequencing batch reactor enriched with polyphosphate-accumulating organisms. The metabolisms of the microbial populations were investigated using <i>de novo</i> metatranscriptomics analysis. Finally, the effect of decreasing the influent phosphate concentration on the granule stability and microbial activity was investigated. A few weeks after the reactor start-up, the microbial community was dominated by <i>Accumulibacter</i> with up to nine species active in parallel. However, the most active species differed according to sampling time. Decreasing drastically the influent phosphate concentration led to a dominance of the glycogen-accumulating organism <i>Propionivibrio</i>, with some <i>Accumulibacter</i> species still abundant. <i>De novo</i> metatranscriptomics analysis indicated a high diversity of potential extracellular substances produced mainly by <i>Accumulibacter</i>, <i>Azonexus</i>, <i>Candidatus</i> Contendobacter, and <i>Propionivibrio</i>. Moreover, the results suggest that <i>Azonexus</i>, <i>Contendobacter,</i> and <i>Propionivibrio</i> recycle the neuraminic acids produced by <i>Accumulibacter</i>. Changes in the microbial community did not cause the granules to disintegrate, indicating that a <i>Propionivibrio</i>-dominated community can maintain stable granules.IMPORTANCEOne of the main advantages of the aerobic granular sludge wastewater treatment process is the higher settling velocities compared to the conventional activated sludge-based process. In aerobic granular sludge, the biomass is concentrated into a biofilm matrix composed of biopolymers, providing micro-niches to different types of microbial populations. We demonstrate with the help of <i>de novo</i> metatranscriptomics analysis that the formation of granules is a highly dynamic microbial process, even when enriching for a microbial guild, such as phosphate-accumulating organisms. Often underestimated, the flanking community of the main phosphate-accumulating organisms population enriched in the reactor is nonetheless active and transcribing genes related to different extracellular polymeric substance pathways. The multiplicity of the extracellular polymeric substances produced probably helped the matrix to remain stable, thanks to their specific properties. Moreover, the results suggest microbial interactions in extracellular polymeric substance recycling between different microbial populations that can be helpful to prevent a disruption of the granules while stressing out the microbial community.