Algal Photosensitization of Methanosarcina barkeri for CO₂-to-CH₄ Production

Anaerobic digestion is applied to organic wastes, yet a substantial fraction of carbon is released as CO₂ because methanogens lack of low potential reducing equivalents. Here we show that heat-treated residues of algae Chlorella sorokiniana can be repurposed as chlorophyll-based biophotosensitizers to couple visible light harvesting to CO₂ methanation by Methanosarcina barkeri (M. barkeri), providing a route supplying light-derived reducing power. Under autotrophic conditions with CO₂ as the sole carbon source, residue supplementation enhanced methane formation in a chlorophyll-dependent manner and yielded 420 µmol L^-1 CH₄ at 16 mg L^-1 chlorophyll after 7 d, with most of the increase occurring in the first 3 d. The residues retained functional photosystem II centers that supported charge separation and interfacial electron export, increasing the apparent electron transfer rate from 22.21 to 34.41 s^-1 and extending the fluorescence lifetime to 1.89 ns under illumination. Methane enhancement was mainly governed by photosystem II-derived electron generation and cytochrome-mediated electron entry into methanogens. Membrane-bound hydrogenases may also contribute to archaeal electron uptake at the interface, while inhibition of photosystem II or cytochrome-associated electron transfer reduced CH₄ formation by approximately 70%. Illumination hyperpolarized the proton motive force of M. barkeri by 40 mV, consistent with incoming electrons and protons integrated through proton-coupled electron transfer steps that feed membrane energy-conserving modules and strengthen methanogenesis. This study demonstrates that discarded algal residues can function as renewable light-responsive inputs that introduce photogenerated reducing flux into the methanogenic network and redirect carbon flow from CO₂ loss toward methane formation in anaerobic digestion.