Multiple extracellular polymeric substances pathways expressed by Accumulibacter and the flanking community during aerobic granule formation and after influent modification

<p><span>Aerobic granular sludge is a biological wastewater treatment process in which a microbial community forms a granular biofilm. The role of <em>Candidatus</em> 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 <em>de novo</em> metatranscriptomics analysis. Finally, the effect of decreasing the influent phosphate concentration on the granule stability and microbial activity was investigated. </span></p> <p><span>A few weeks after the reactor start-up, the microbial community was dominated by <em>Accumulibacter</em> 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 <em>Propionivibrio</em>, with some <em>Accumulibacter</em> species still abundant. <em>De novo</em> metatranscriptomics analysis indicated a high diversity of potential extracellular substances produced mainly by <em>Accumulibacter</em>, <em>Azonexus</em>, <em>Candidatus</em> Contendobacter, and <em>Propionivibrio</em>. Moreover, the results suggest that <em>Azonexus</em>, <em>Contendobacter</em> and <em>Propionivibrio</em> recycle the neuraminic acids produced by <em>Accumulibacter</em>. Changes in the microbial community did not cause the granules to disintegrate, indicating that a <em>Propionivibrio</em>-dominated community can maintain stable granules.</span></p>