Quantifying internal CO₂ recycling for high-rate carbon capture in mixotrophic Scenedesmus acuminatus

Mixotrophy, which simultaneously utilizes organic and inorganic carbon, offers a promising alternative to overcome the low biomass productivity and carbon utilization rates of conventional microalgal systems. However, its potential for enhanced CO₂ capture and the contribution of internal respired-CO₂ recycling remain poorly quantified. This study used a stoichiometric carbon-budget framework to estimate CO₂-equivalent carbon capture, carbon utilization, model-estimated internal CO₂ recycling, and evaluated biomass production in Scenedesmus acuminatus under autotrophy, heterotrophy, and mixotrophy, followed by optimization of organic carbon source, glucose loading, and feeding strategy. Under autotrophy, the CO₂-eq capture rate reached only 0.48 g L^-1d^-1, whereas CO₂ emission under heterotrophy reached 0.27g L^-1d^-1. Mixotrophy achieved a net CO₂ eq. capture rate of 1.19 g L^-1 d^-1, 148% higher than autotrophy. Among different organic carbon sources, glucose was the most effective carbon source for enhancing carbon utilization and biomass productivity. Batch glucose optimization showed that 5 g L^-1 glucose showed highest net CO_2-eq capture, whereas 6.5 g L^-1 favored biomass accumulation but reduced capture efficiency, demonstrating that biomass and carbon-capture optimal were not identical. Semi-continuous glucose feeding further improved performance: a total glucose input of 6 g L^-1 increased net CO₂ capture to 1.35 g L^-1 d^-1, enhanced total carbon utilization to 2.15 g L^-1 d^-1, and reduced unrecycled respiratory CO₂ emission by 80% relative to batch cultivation. These findings demonstrate that glucose-controlled mixotrophy enhances microalgal carbon capture by coordinating simultaneous organic/inorganic carbon utilization and internal CO₂ recycling, providing a basis for carbon-negative wastewater treatment and algal bio-refineries.