Why do anaerobes like the light stimulation: Enhanced anaerobic digestion at different wavelengths under ammonia stress

Strengthening microbes with light during anaerobic digestion (AD) has emerged as a promising approach for effective waste-to-energy conversion, yet the underlying mechanisms remain elusive. This study delved into the complexities of microbial behavior, metabolic pathways, and digestion efficiency to explore the light-stimulating effects of various wavelengths (blue, green, red and mixed wavelengths) on AD under ammonia stress. Different light wavelengths induced distinct responses in anaerobic consortia and cell metabolism. Blue and green light impacted the Energy and Methane metabolism, while red light regulated the Cell cycle and motility-related genes. Although Methanosarcina dominance was observed across all the lighted groups, the dominant pathway shifted from hydrogenotrophic to acetoclastic methanogenesis specifically under the mix-color lighting. This characteristic was attributed to the collaborative effects of short and long wavebands, enhancing the diversity of microflora and triggering the cellular processes more effectively. Moreover, the enrichment of syntrophic bacteria and Methanosarcina (91.3% of archaeal community) facilitated the complete degradation of organic acid and outperformed methanation under mixed wavelengths. Furthermore, metagenetic predictions elucidated that critical metabolic processes regulating organic conversion (Carbohydrate metabolism), microbial response (Signal transduction, Membrane transports system) and cross-population cooperation (Quorum sensing) were significantly activated under the mixed wavelengths. Notably, the mixed-wavelength light stimulation upregulated a c-type cytochrome-mediated interspecies communication, fostering an energy-conserving bionetwork via electronic signals. From the academic to the practical viewpoint, this study unveiled the mechanisms and potential of a visible light-stimulated system for waste-to-energy conversion, highlighting the feasibility of sustainable sunlight-mediated waste management and energy recovery on a larger scale.