Revealing metabolic flexibility of Candidatus Accumulibacter phosphatis through redox cofactor analysis and metabolic network modeling

Candidatus Accumulibacter phosphatis, a polyphosphate accumulating organism (PAO) growing under alternating anaerobic/aerobic conditions in enhanced biological phosphorus removal (EBPR) systems, displays an intricate metabolic strategy. The organism relies on combining features from either anaerobic- or aerobic-only metabolisms and connecting phases using different reserve polymers. Analysis of the metabolic functions is not trivial, specially without isolates of this microorganism available and without employing advanced metabolic modelling techniques. So far, there is no consensus regarding the type of glycolysis nor the operating mode of the tricarboxylic acid cycle anaerobically. Here, we simulated different conditions using flux balance analyses and a reduced central carbon metabolic network based on previously published meta-omics data and validated with enzyme activity assays. Interestingly, we observed that the polyhydroxyalkanoates (PHA) production was NADH preferring instead of the commonly reported NADPH dependency. This feature allows for higher metabolic flexibility while maintaining a near-optimal carbon conservation and without extra energetic costs, which explains PAOs9 competitiveness under anaerobic/aerobic shifting environments.