Fig. 4. Transition metals, wavelengths and methylated sulfur- and nitrogen compounds mediate light-driven CH₄ formation.

a Iron, cobalt and cerium enhance light-driven CH₄ formation. No significant CH₄ increase was observed for cobalt, nickel and manganese supplementation. b Light-driven CH₄ formation and Fe²⁺ generation increases in the near-UV spectrum. c Light-driven formation of CH₄ from methylated S-/N-compounds (logarithmic scale). Upon illumination, significant increases in CH₄ levels were measured for dimethyl sulfide, methionine, 2-methylthioethanol, trimethylamine N-oxide and dimethyl sulfoxide (DMSO). All experiments were conducted in closed glass vials containing a buffered solution (pH 7), N₂ and either Fe³⁺ or other transition metals (a), DMSO or other substrates (c) and either ascorbate (a) or citrate (b, c). Samples were incubated under broad-spectrum light (a, c), specific wavelengths (b) or in the dark at 30 °C. The dashed red line depicts the average CH₄ amounts obtained from samples illuminated by a broad-spectrum light source. Statistical analysis was performed using paired two-tailed t tests, *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001. The bars are the mean + standard deviation of triplicates, shown as circles.