Low indoor light-driven CO2 conversion into visible C4 bioplastic via homogeneous non-metal-based biohybrids under photoexcitation

Abstract

The conversion of CO₂ into C₄ bioplastic through biohybrid systems comprising microorganisms and non-metal-based photocatalysts represents a promising strategy for high-value carbon utilization. However, existing non-metal-based biohybrids necessitate the addition of auxiliary agents or heterotrophic carbon sources due to the limited contact and interfacial mass transfer caused by the aggregation of photocatalysts and bacteria. In this work, a homogeneously autotrophic K/O co-doped g-C₃N₄ (K/O-CN) and Ralstonia eutropha (R. eutropha) biohybrid was developed to efficiently convert CO₂ into poly-β-hydroxybutyrate (PHB) without any auxiliary agents. Under low indoor light of 4000 lux (0.75 mW cm⁻²), the K/O-CN–R. eutropha biohybrid achieved a PHB yield of 49.35 ± 0.85 mg L⁻¹ d⁻¹ surpassing non-metal-based biohybrids with added co-factors and reached a quantum efficiency of 5.88 ± 0.16% exceeding most metal-based biohybrids. The K/O co-doping changed stacked bulk layers to nanorods, which enveloped the bacterial cells with uniform dispersion, establishing a tight interaction, strong coupling, and effective electron transfer between them. Upon photoexcitation, K/O-CN catalyzed H₂O splitting to generate H₂. Intracellularly, H₂ was oxidized to H⁺, which participated in ATP synthesis and generated additional reducing equivalents (NADPH), thereby enhancing the carbon metabolic cycle and promoting the formation of key intermediates, ultimately driving the conversion of CO₂ to PHB. This work offers a streamlined and economically viable pathway for obtaining C₄ products from CO₂ and providing key insights into interfacial mass transfer and carbon metabolism changes within the biohybrid.