Photon-flux-associated performance of engineered methanotrophic photogranules treating synthetic wastewater and real biogas.

Engineered methanotrophic photogranules provide a potential platform for coupling biogas conversion with nutrient-rich wastewater treatment, but their responses to different illumination conditions remain unclear. Here, three photo-sequencing batch reactors were operated for 60 days at photon fluxes of 120, 170, and 240 μmol/(m2·s) using real biogas and synthetic nutrient-rich wastewater. The acclimated photogranules were subsequently subjected to short-term tests at 0, 120, 170, and 240 μmol/(m2·s). Across R1-R3, average total organic carbon and total nitrogen removal increased to 96.1% and 92.6%, respectively. Net CO2 removal increased from 37.6% to 64.7%, whereas CH4 removal remained within 58.6-62.3% and showed substantial temporal variation. R3 had the highest biomass concentration, chlorophyll-a, extracellular polymeric substances, and the lowest sludge volume index. However, its biomass-normalized removal capacities were generally lower than those of R1 or R2 during the short-term tests. Across the three acclimated photogranules, the photon flux associated with the highest biomass-normalized removal capacity was not the same. Multi-omics analyses revealed reactor-specific transcript-abundance patterns, with several photosynthesis-related genes showing higher metatranscriptomic abundances in R2 or R3, while methane- and nitrogen-conversion genes showed non-uniform responses. The findings provide an integrated assessment of illumination responses in engineered methanotrophic photogranules.