Fig. 3. A light-driven iron redox cycle drives and enhances CH₄ formation.

Upon illumination, water is photolytically split into hydroxyl radicals (·OH) and hydrogen forming H₂ and H₂O₂. Organic Fe³⁺-complexes (Fe³⁺-[L¹]) are converted into Fe²⁺ and organic radicals (·L¹) via ligand-to-metal charge transfer (LMCT). The generated Fe²⁺ reacts with H₂O₂ to ·OH or [Fe^IV= O]²⁺ and thereby drives the generation of methyl radicals (·CH₃) from S-/N-methylated compounds. The LMCT-generated ·L¹ decomposes into CO₂ and another organic radical (·L²) that additionally facilitates CH₄ formation upon reacting with S-/N-methylated compounds. Under light, (a) H₂ (gray bars) and (b) H₂O₂ is formed in pure buffer. c Upon illumination, CH₄ formation rates (yellow bars) are increased. Fe²⁺ formation (brown bars) depends on anoxic conditions and is driven by LMCT induced by the addition of citrate. d Light and heat have synergistic effects on CH₄ formation. While heat drives CH₄ formation upon Fe²⁺-supplementation, light increases CH₄ formation upon Fe³⁺- and Fe²⁺-addition. All experiments were conducted in closed glass vials containing a buffered solution (pH 7) supplemented with DMSO, Fe²⁺ or Fe³⁺, N₂ or air atmosphere in the presence or absence of citrate incubated under light or in the dark at 4 °C or 30 °C. The bars are the mean + standard deviation of triplicates, shown as circles. a, b Was created with BioRender.com.