Ozonation as non-thermal option for bacterial load reduction of Chlorella biomass cultivated in airlift photobioreactor

The present study proposes the selective reduction of bacterial load in the microalgal cultures without affecting the microalgae. Bacteria was found to be symbiotically associated with microalgae and exponentially increased in the range of 10³–10¹¹ CFU mL^-1 during Chlorella cultivation in the airlift photobioreactor. The bacterial load needs to be reduced to meet USFDA standards (∼10³ CFU mL^-1) for the use of microalgal biomass in food and feed applications. Ozone was evaluated as a non-thermal option for bacterial load reduction of Chlorella cultivated in the airlift photobioreactor. The results of ozonation w.r.t to time (5, 10, 15, 20, 25, and 30 min) and dissolved ozone concentrations (0.25, 0.49, 0.75, 0.99, and 1.25 mg of O₃ L^-1) showed a significant reduction of bacterial load in the range of 3 – 6.5 log. The bacterial disinfection kinetics of ozonation showed an overall second-order reaction with a rate constant (K) of 0.93 mg⁻¹ min⁻¹ L. Further, confocal laser microscopy imaging of Chlorella and its associated bacterial cells confirmed the selective and significant reduction of associated bacteria. The viability of the Chlorella cells were confirmed with trypan blue viability assay (∼99%) under a fluorescent microscope. The biomass quality with respect to metabolite integrity was also validated. In conclusion, ozonation can be a potential solution for selective reduction of bacterial load associated with Chlorella without affecting Chlorella cells viability.