A central problem in microbial ecology is explaining coexistence among competing functional guilds and unexpected departures of community composition from theoretical steady states. In enhanced biological phosphorus removal systems, polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs) compete for the same niche. PAOs use polyphosphate as an energy and redox buffer; GAOs lack polyphosphate synthesis yet can persist and at times dominate. The study asked how heterotrophic lineages that do not synthesize polyphosphate nevertheless manage equivalent intracellular redox imbalances and achieve what the authors call metabolic parity.
The authors used an enhanced biological phosphorus removal macrocosm to study competing PAO and GAO lineages. They integrated quantitative stoichiometric modeling with metaproteomic analyses to resolve alternative metabolic strategies that underpin cellular homeostasis in heterotrophs. The combination of bulk stoichiometry and protein-level evidence was used to identify pathways and biochemical features that could support redox buffering and energy conservation in GAOs relative to PAOs.
The core finding is that GAOs perform heterotrophic re-assimilation of inorganic carbon (CO2/HCO3-) through the ethylmalonyl-CoA pathway, providing a parallel redox-buffering mechanism to that of PAOs. This dark CO2 fixation couples structural carbon conservation to redox control, thereby mitigating intracellular electron overflow. The authors argue that this inorganic carbon fixation mechanism enables GAOs to reach baseline metabolic parity with PAOs, countering the long-standing assumption that GAOs are bioenergetically inferior because they lack polyphosphate storage.
Beyond CO2 re-assimilation through the ethylmalonyl-CoA pathway, GAOs were characterized by several bioenergetic and metabolic traits that increase efficiency:
Stoichiometric analysis and proteomic signatures support that this suite of metabolic wiring collectively fortifies GAO energy balance and redox control, enabling parity with PAOs despite differing storage strategies.
The authors report that minor formate co-feeding disrupted the established PAO/GAO parity. Stoichiometric simulations indicated that formate supplementation produced an asymmetric bioenergetic niche that conferred per-cycle energy gains uniquely to GAOs. In other words, formate addition altered available electron and energy flows in a manner that advantaged GAOs at the level of single metabolic cycles.
In biomass-retaining systems where hydraulic throughput is decoupled from solids retention time, cells that accumulate intracellular inventories (for example, PHA) are retained across cycles. The authors propose that the small per-cycle stoichiometric advantage that GAOs obtained under formate co-feeding is translated into a generational ratchet: solids retention time allows these subtle energetic edges to compound over multiple generations, driving a rapid community transition from PAO/GAO co-dominance to GAO dominance. This mechanism links single-cycle stoichiometry to long-term community assembly outcomes.
The findings challenge traditional steady-state models that assume single-substrate and single-cycle conditions. By showing that inorganic carbon management (heterotrophic CO2 fixation) and generational metabolic compounding can determine community composition, the study highlights the need to account for inorganic resource management, multi-cycle retention, and compound stoichiometric effects in predictive ecological models for biomass-retaining microbial ecosystems.
The source summary does not provide specific experimental parameters, quantitative values for stoichiometric gains, detailed metaproteomic identifications, or exact simulation settings. The authors declared no competing interests. Readers interested in experimental replicates, raw data, or parameter values should consult the full preprint or supplementary materials for those details, which were not reported in the provided summary.