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 CO2-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.