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Researchers at the Indian Institute of Technology Guwahati have developed a two-stage cultivation method that improves carbon dioxide capture, microalgal biomass production, self-harvesting efficiency and bioenergy generation.

The research team, led by Professor Kaustubha Mohanty of the Department of Chemical Engineering and research scholar Deepesh Singh Chauhan, designed the process to address challenges linked to prolonged microalgae exposure to high carbon dioxide levels. While microalgae can capture CO2 and produce biomass that can be converted into renewable fuels such as biodiesel, extended exposure to elevated CO2 concentrations can deplete nutrients and impair photosynthesis.

In the first stage of the process, microalgal cultures were grown under a 15% CO2 concentration, which supported growth by increasing carbon availability. However, the researchers found that continuous exposure to 15% CO2 caused acidification and slowed algal growth. In the second stage, CO2 concentration was reduced to 5%, while calcium and phosphorus were added to stabilise the growth environment and improve biomass aggregation.

The adjustment restored pH balance and maintained photosynthetic activity. Calcium supplementation also promoted self-flocculation of microalgal cells, enabling more efficient biomass recovery.

The study, published in the journal Renewable Energy, was conducted using a two-litre bubble-column photobioreactor, allowing detailed monitoring of microalgal development. The results showed a 25.7% increase in biomass production, a 35.4% rise in CO2 fixation, a 1.86-fold increase in lipid productivity, a 37.65% improvement in total intracellular bioenergy efficiency and a higher energy value in the resulting biomass.

Professor Mohanty said the research could support more effective industrial carbon capture by using industrial flue gas as a continuous CO2 source. He also noted that the method addresses a commercially important challenge in microalgal biorefineries by reducing downstream separation energy, a major cost factor.

“This research paves the way for more effective industrial CO2 capture, where industrial flue gas can be used as a continuous source of CO2,” Mohanty said. “The research also addresses one of the most commercially relevant aspects of CO2 capture, i.e., reducing downstream separation energy, a major cost component of microalgal biorefineries. Its most immediate pathway to impact would be as a pilot-scale technology for CO2-rich industrial exhaust streams and integrated microalgal biorefineries.”

The team also reported strong auto-sedimentation performance. Calcium addition helped microalgal cells form compact flocks, improving biomass recovery efficiency to 98.46%. Biodiesel produced from the microalgae met biodiesel standards used in India, the United States and Europe.

Chauhan said the findings show the potential of combining CO2 capture, microalgal biomass production, renewable bioenergy generation and low-energy biomass recovery within a single cultivation framework.

The study concludes that the two-stage nutrient-assisted CO2 modulation strategy offers a promising pathway for microalgal biorefineries by improving carbon fixation during prolonged cultivation while reducing energy requirements for biomass harvesting.