Intermittent illumination enables reversible carbon buffering by Rhodopseudomonas palustris for enhanced methane recovery from food waste anaerobic digestion.

Anaerobic digestion (AD) of food waste (FW) is vulnerable to acidification when fermentation outpaces syntrophic oxidation and methanogenesis. To address this temporal imbalance, we introduced Rhodopseudomonas palustris (R. palustris), which can assimilate fermentation-derived volatile fatty acids (VFAs) and store carbon as polyhydroxyalkanoates (PHAs). Intermittent illumination reduced VFA accumulation and end-cycle H2 partial pressure, while yielding the highest mean daily methane production (589.0 mL d-1) and fraction of carbon recovered as methane (37.2 %). A pure-culture assay showed that PHA accumulated during illumination and declined in darkness, indicating reversible intracellular carbon storage and remobilization. The PHA turnover pattern paralleled reactor methane dynamics, which was consistent with a temporal association between carbon storage and downstream methanogenesis. Intermittent illumination yielded the lowest NADH/NAD+ ratio while maintaining high ATP availability. Community analysis revealed a more balanced distribution of functional guilds between the liquid and reactor-wall fractions, while metagenomic analysis indicated functional potential for PHA turnover and methanogenesis. Together, these findings support a carbon-buffering model in which light-dark cycling is associated with transient carbon storage and subsequent remobilization, potentially improving the temporal coupling between acidogenesis and methane formation and enhancing AD stability and methane recovery.