MDEA-assisted CO2 absorption coupled with IAA-induced metabolic regulation enhances lipid production and biodiesel quality in the oleaginous microalga Mychonastes rotundus.

Microalgae had high potential for CO2 fixation and lipid synthesis, and indole-3-acetic acid (IAA) promoted the growth and antioxidant defense capability of microalgae. Nevertheless, sole IAA supplementation failed to suppress CO2 volatilization in the culture system or alleviate the bottleneck of insufficient carbon supply. Herein, N-methyldiethanolamine (MDEA) was co-supplemented with IAA at its optimal predetermined concentration to explore their influences on carbon fixation and lipid accumulation of Mychonastes rotundus. The results demonstrated that the combined addition of 20 mg·L-1 MDEA and 20 mg·L-1 IAA significantly improved microalgal growth, photosynthetic activity, and carbon assimilation. Mechanistically, MDEA enhanced CO2 hydration and mass transfer, while IAA promoted intracellular carbon utilization, collectively prolonging CO2 saturation time by 50 % and increasing biomass productivity by 28.7 %. Notably, the contents of proteins, polysaccharides, and lipids were each elevated by approximately 60 % and the ratio of C18:3 and C18:1 was also optimized. The fatty-acid profile yielded predicted biodiesel indices including a cetane number of 52.01 and higher heating value (HHV) of 41.4MJ·kg-1, while differential scanning calorimetry (DSC) suggested improved low-temperature behavior. Transcriptomic analysis revealed that MDEA and IAA redirected metabolic flux toward lipid synthesis by upregulating central carbon metabolism and fatty acid biosynthesis genes and downregulating protein synthesis-related pathways, and thus energy and carbon skeletons were conserved for lipid accumulation. This study substantially enhanced CO2 sequestration efficiency and lipid productivity, providing a viable technical strategy for flue gas carbon capture and sustainable biomass energy generation.