An Emerging Technology for Reduction of Chlorella Associated Bacterial Loads for Quality Biomass Production without Affecting Chlorella

Microalgae are symbiotically associated with bacteria during their growth. Microalgal cultivation in photobioreactors is desirable for high value metabolites due to controlled conditions. During algal cultivation in photobioreactors, bacteria are found to be exponentially increased, which cannot be avoided during the algal growth due to symbiosis. Although the bacteria promote Chlorella growth by providing supplements (vitamins, hormones), they increase exponentially and hence the bacterial load needs to be reduced to meet desirable US-FDA standards (~10³ CFU/mL). This study results showed that higher Chlorella associated bacterial load was in the range of 10³-10¹¹ CFU/mL during Chlorella cultivation in an airlift photobioreactor. Bacterial load reduction concerning time and ozone concentrations were determined. Ozonation had a significant effect on bacterial load reduction, which was 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 indicated a selective reduction of associated bacteria without affecting the Chlorella cells, which was further confirmed with trypan viability assay. It can be concluded that optimised conditions (ozone concentration, time) will help in the production of high-quality Chlorella biomass with reduced bacterial load through ozonation.