Extracellular electron uptake for CO₂ fixation by Rhodopseudomonas palustris during electro-cultivation in darkness

As a vital part of the global carbon cycle, photosynthesis helps in fixing CO₂ to produce diverse biomass. However, over-reliance on optical density results in inadequate photosynthesis under limited light sources. The coupling of extracellular respiration and photosynthetic chain via the quinone pool provides a possibility for electrically driven photosynthesis in darkness, which is not well understood. In this study, CO₂ fixation of photosynthetic bacteria Rhodopseudomonas palustris was enhanced in the dark via extracellular electron uptake from the electrode at -0.4 V. The copy number of R. palustris increased by 35 folds during 28 days of operation, accompanied by the increase of ATP content, NADH/NAD⁺, and NADPH/NADP⁺ of cells. Especially, the activity of Rubisco, the key enzyme of the Calvin cycle, increased by 28 % during electro-cultivation. Accordingly, the electrochemical activity of R. palustris was found to increase, which might be attributed to the structural modification of protein-like substances due to the enhanced proton-coupled electron transfer (PCET) process in electro-cultivation, which was further confirmed by in situ Fourier transform infrared spectroscopy and kinetic isotope effect tests. This study indicated that extracellular respiration could be electrostimulated via PCET to maintain photosynthesis in R. palustris in the dark.