Microbial electromethanogenesis (EM), as a sustainable bioderived carbon-neutrality catalyzing platform, can be accelerated and regulated by weak power input for carbon fixation into value-added bioenergy. Solar electricity as a day-night intermittent renewable resource has been verified to effectively directly drive microbes to capture CO2. However, understanding the influence mechanisms of higher CO2 loading on EM is of intrinsic significance yet lacking. Herein, natural solar-powered bioelectrocatalytic CO2 reduction to CH4 under increasing bicarbonate concentrations was investigated. CH4 recovery for the long-term measurement showed that CH4 production rate positively responded to improved bicarbonate concentrations from 2.5 to 10 g HCO3-·L-1, exhibiting a robust and potent competence in CH4 yield compared to reported EM. Whereas exceed bicarbonate mainly contributed to raised pH in the solution resulting in the proton limitation despite the intermittent driven-mode could mitigate pH shock. Electrochemistry results demonstrated that higher bicarbonate concentrations promoted the redox activity of electrode biofilm and lowered the system resistances, especially the charge transfer resistance. Adequately improving CO2 loading can dynamically optimize the structure of anodic electroactive microorganisms and facilitate electron transfer. Furthermore, more functional cathodic mcrA genes were upregulated with elevated bicarbonates and the species of basophilic Methanobacterium alcaliphilum occupied predominated at the cathode. These findings open up a perspective avenue to carbon reduction using natural solar intermittent-powered EM.