Energy-carbon flow reallocation in microalgae under high CO2: Platform for carbon utilization and wastewater valorization in steel industry.

The steel industry faces dual challenges of extensive CO2 emissions and intricate wastewater management. This study investigated a novel microalgal platform using Chlorella pyrenoidosa for simultaneous CO2 utilization and valorization of cold-rolling emulsion wastewater. Optimal biomass production (1.80 ± 0.07 g/L) was achieved at 5 % CO2, whereas 20 % CO2 caused severe inhibition of growth and carbon fixation. Mechanistic investigation revealed that 20 % CO2 triggered systemic energy restriction, evidenced by depleted intracellular ATP and downregulated metabolic pathways (TCA cycle, oxidative phosphorylation). Furthermore, essential energy was diverted to counteract oxidative and acidification stress, upregulating multiple DNA repair pathways. Concurrently, 20 % CO2 triggered metabolic overflow that actively rerouted 16.8 % of carbon towards extracellular organic matter. A regulatory nexus was established: energy restriction drove overall inhibition, while energy reallocation induced carbon flow alteration. These insights provide a theoretical basis for engineering microalgae with enhanced tolerance for industrial carbon utilization and bioresource recovery.