Frequent light/dark alternation improves the redox homeostasis of photosynthetic bacteria and enhances sustainable polyhydroxybutyrate production from food waste

Photosynthetic bacteria (PSB) offer broad potential for waste reduction, environmental remediation, and sustainable manufacturing, including biodegradable plastic production from food waste, yet their wider application still requires further exploration. This study investigated if light/dark (L/D) alternation can enhance polyhydroxybutyrate (PHB) synthesis from food waste broth by PSB. A L/D 6:6 ratio enhanced PHB accumulation, reaching 43 % of cell dry weight (628 mg/L) or 1.57-fold higher than continuous illumination, and alleviated redox imbalance by replenishing NADPH, resulting in a favorable NADPH/NADP⁺ratio. Microbial diversity and richness also increased under the L/D 6:6 photoperiod, with Rhodopseudomonas and Enterobacteriaceae dominating. Functional gene analysis further revealed a 31 % upregulation of phaC, the gene encoding PHB synthase. Metabolic profiling indicated that more frequent L/D alternation optimized carbon allocation by channeling acetyl-CoA flux toward PHB, while upregulation of tricarboxylic acid (TCA) cycle genes and elevated NADPH pools sustained reducing equivalents, facilitating PHB synthesis. These findings demonstrate that frequent L/D alternation can promote food waste valorization and improve light-driven biomanufacturing.