Microalgae harvesting via thermoresponsive polymers: Insights into poly(N-isopropylacrylamide) efficacy and mechanisms.

Microalgae biorefinery is limited by energy-intensive harvesting. Inspired by fungal self-pelletization, this study explored thermoresponsive polymers (TRPs) for microalgae harvesting. The lower critical solution temperature (LCST) of TRPs in cultivation media was significantly lower than that in water, facilitating temperature-induced phase separation. Harvesting efficiency depends on TRP molecular weight (MW), concentration, and algal species (size, medium, cell wall composition). PNIPAM-300  kDa achieved > 90% efficiency for Nannochloropsis oculata, a seawater microalga whose glucosamine-rich cell walls could partly explain its pronounced MW sensitivity. Introducing cationic groups (e.g., allylamine) reduced efficiency, suggesting a physical entrapment mechanism rather than charge-based flocculation. After harvesting, all tested microalgae species consistently exhibited wrinkled walls, dimmed chlorophyll fluorescence, and reduced viability, suggesting that TRP processing could facilitate intracellular product release. This TRP-induced method enables rapid harvesting with potential downstream advantages, marking a step toward sustainable microalgae processing.