Energy-efficient selection of high-yield PHA producers via microaerophilic uncoupled feeding

Uncoupled feeding (UnF), supplying organic carbon and nutrients separately, is a promising strategy to enrich polyhydroxyalkanoates (PHA)-accumulating mixed microbial communities (MMCs) and has shown higher productivity than conventional approaches. UnF enables improved oxygen control, which is particularly relevant for PHA production, where aeration constitutes a major operational cost. However, the effect of dissolved oxygen (DO) on PHA production remains unclear. To address this gap, this study investigates the effect of microaerophilic conditions on biomass enrichment during the growth phase of PHA-accumulating bacteria in an UnF regime. Reduced DO is expected to lower energy demands and mitigate bacterial decay and predation, which are typically intensified under aerobic conditions. Two lab-scale sequencing batch reactors (R1-R2) were operated under identical conditions, differing only in DO during the growth phase of PHA-accumulating bacteria: R1 was maintained under aerobic conditions (3-4 mg O₂ L^-1), while R2 was operated under microaerophilic conditions (0-1 mg O₂ L^-1). After 100 days of enrichment with a synthetic acetate-propionate influent, the microaerophilic reactor achieved a maximum PHA accumulation of 82 ± 11%, while the aerobic reactor reached 55.1 ± 0.3% (volatile suspended solids basis). Microaerophilic conditions promoted greater PHA accumulation, enhancing PHA yield of 17% (0.88 vs 0.75 gCOD_PHA gCOD_VFA^-1) and storage capacity (49% increase), also affecting the polymer composition and both eukaryotic and prokaryotic community, while achieving an estimated 32% reduction in aeration energy demand compared to conventional aerobic operation. These findings show that integrating microaerophilia into UnF enrichments offers a simple, energy-efficient strategy for selecting MMCs with high PHA accumulation potential.