Algal Photosensitization of Methanosarcina barkeri for CO 2 ‐to‐CH 4 Production

ABSTRACT Anaerobic digestion is applied to organic wastes, yet a substantial fraction of carbon is released as CO 2 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 CO 2 methanation by Methanosarcina barkeri ( M. barkeri ), providing a route supplying light‐derived reducing power. Under autotrophic conditions with CO 2 as the sole carbon source, residue supplementation enhanced methane formation in a chlorophyll‐dependent manner and yielded 420 µmol L −1 CH 4 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 CH 4 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 CO 2 loss toward methane formation in anaerobic digestion.