Researchers at the Indian Institute of Technology Guwahati have developed a two-stage cultivation process that improves carbon dioxide (CO?) capture, microalgal biomass production, efficient self-harvesting and bioenergy generation simultaneously. The process is intended to allow industrial facilities to use flue gas as a continuous carbon source while reducing the energy required for downstream biomass recovery. The team reported that microalgae convert CO? into biomass that can be processed into renewable fuels and other bioenergy products.

To balance high carbon availability with sustained photosynthetic performance, the team led by Professor Kaustubha Mohanty implemented an initial high-CO? phase followed by a lower-CO? phase with nutrient amendments. In the first stage cultures were grown under 15 per cent CO? to promote rapid growth, but prolonged exposure led to acidification and reduced growth rate. In the second stage CO? concentration was reduced to five per cent and calcium and phosphorus were added to stabilise pH, promote biomass aggregation and induce self-flocculation.

In a two-litre bubble-column photobioreactor the two-stage strategy produced measurable gains over single-stage cultivation, including 25.7 per cent higher biomass production, 35.4 per cent higher CO? fixation and one point eight six times higher lipid productivity. Total intracellular bioenergy efficiency improved by 37.65 per cent and the energy value of the biomass increased, while biodiesel derived from the algal biomass met standards applicable in India, the United States and Europe. The calcium-assisted aggregation enabled auto-sedimentation, which raised biomass recovery efficiency to 98.46 per cent.

The researchers concluded that the nutrient-assisted CO? modulation approach reduces the energy penalty associated with harvesting and offers a practical pathway to pilot-scale integration with CO?-rich industrial exhaust streams and microalgal biorefineries. The study is presented as a potential means to maintain efficient carbon fixation during prolonged cultivation and lower downstream separation energy, two commercially relevant challenges for algal biorefineries. Further work is indicated to scale the system and validate continuous operation on industrial flue gas.