Dark CO2 fixation via the ethylmalonyl-CoA pathway establishes metabolic parity: A stoichiometric basis for compounding ecosystem shifts.

Glycogen-accumulating Ca. Competibacter GAO achieves stoichiometric yield parity with polyphosphate-accumulating Ca. Dechloromonas PAO on acetate by using ethylmalonyl-CoA–mediated CO2/HCO3− fixation for redox buffering.

• Why it matters: It remains unclear how heterotroph lineages lacking polyphosphate synthesis maintain comparable redox balance and coexistence, especially in engineered phosphorus-removal communities.
• What they did: In an enhanced biological phosphorus removal system, the authors combined quantitative stoichiometry with metaproteomics to compare acetate-grown PAO and GAO metabolism, then tested how minor formate co-feeding and solids retention time alter outcomes.
• The result: CO2/HCO3− fixation via the ethylmalonyl-CoA pathway mitigates electron overflow and removes the assumed GAO bioenergetic inferiority; formate creates an asymmetric energy niche favoring GAOs, and solids retention time preserves inventory that compounds this advantage across generations.

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