Ding Tan, Xin Li, Yunhui Li, Xingjia Fu, Wenyan Peng, Yun Liu
Microalgal CO2 fixation is constrained by inorganic carbon utilization, photosynthetic capacity, and oxidative stress, and although electrical stimulation can modulate algal physiology, the effective current-density range remains unclear. Here, Scenedesmus quadricauda was cultivated under microcurrent densities of 0, 10, 20, 30, 40, 70, and 100 μA cm-2, with detailed analyses focused on 0-30 μA cm-2. Cultivation and physiological assays were performed with three biological replicates, and metabolomics with five per treatment. Microcurrent stimulation exhibited current-density dependence, with 20 μA cm-2 yielding the best performance. After 7 days, biomass reached 1.19 g L-1, 63.0% above the control, and the biomass-based apparent CO2 fixation rate increased to 0.30 g L-1 d-1. Chlorophyll a and b contents increased, and maximum photochemical efficiency of photosystem II was maintained. Carbonic anhydrase, ribulose-1,5-bisphosphate carboxylase/oxygenase, and acetyl coenzyme A levels were highest at 20 μA cm-2, indicating enhanced carbon assimilation and central carbon metabolism. This treatment promoted extracellular polymeric substance secretion while maintaining the lowest reactive oxygen species accumulation. 13CO2 tracing confirmed supplied CO2 as the dominant biomass carbon source, with 13C abundance exceeding 90% across treatments. In contrast, 30 μA cm-2 promoted lipid accumulation, whereas higher densities triggered stress responses and severe inhibition at 100 μA cm-2. Untargeted metabolomics further indicated that 20 μA cm-2 favored carbon assimilation, energy metabolism, and antioxidant protection, whereas 30 μA cm-2 shifted metabolism toward stress adaptation and lipid-related responses. These findings identify a moderate microcurrent-density window for enhancing growth-coupled CO2 fixation in S. quadricauda and guiding electro-assisted algal CO2 bioconversion.