A moderate microcurrent-density window enhances apparent CO₂ fixation in Scenedesmus quadricauda

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.