Microalgae-bacteria synergy in photosynthetic bio-electrochemical systems supports nitrogen transformation, microbial dynamics and greenhouse gas mitigation

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Photosynthetic microbial fuel cells present an integrated strategy for wastewater treatment, greenhouse gas mitigation and bioenergy recovery. Here we elucidate the bioelectrochemical mechanisms by which external resistance and cathodic configuration regulate nitrogen transformation, carbon dynamics and microalgae-bacteria interactions. A microalgae-based cathode achieved net-negative carbon dioxide flux and approximately 37% lower nitrous oxide emissions than open-circuit systems, corresponding to reduced global warming potential relative to mixed-microbial and abiotic cathodes. This reflected enhanced carbon dioxide fixation and suppression of methane and nitrous oxide emissions. Nitrous oxide reduction in the anodic region was attributed to denitrifying bacteria expressing nitrous oxide reductase, supported by electrons from organic matter oxidation. Photosynthesis stimulated nitrification at the cathode, while microalgae assimilated nutrients, improving removal. Lower external resistance enhanced extracellular electron transfer, correlated with elevated pilA and OmcS gene abundance and enrichment of electrogenic genera including Shinella, Geobacter and Pseudomonas. These findings reveal integrated electrochemical–microbial mechanisms facilitating sustainable treatment. Bioelectrochemical interactions between microalgae and bacteria can improve extracellular electron transfer, nitrogen transformation and greenhouse gas mitigation during wastewater treatment in a photosynthetic microbial fuel cell.