Using resonant ultrasound field-incorporated dynamic photobioreactor system to enhance medium replacement process for concentrated microalgae cultivation in continuous mode

Holding cells in the cultivation broth for subsequent medium replacement is often considered as one of the most difficult procedures during perfusion cell culture. Although a number of harvest approaches have been reported in the past decades, their drawbacks such as high cost, increased contamination, and/or labor consuming remain obstacles for practical application. To overcome these challenges, a novel resonant ultrasound field-incorporated dynamic photobioreactor system (RUF-DPBS) was developed in which the cell retention and medium replacement are carried out by acoustic radiation forces and gravity, respectively. Based on the collection efficiency of microalgae, the RUF operation was optimized by 1 MHz and output intensity of 8 W/cm²with circulating velocity of 2 mL/min whereby 93% of Nannochloropsis oculata in 30 mL can be collected within 2-h operation. Moreover, the cells cultured with RUF-DPBS in which the medium was changed every three days exhibited increased volumetric productivity that the yields of biomass, total lipid, and eicosapentaenoic acid of the N. oculata after 12-days cultivation significantly enhanced 2.6, 2.1, and 2.5 folds (P < 0.05 for each), respectively, as compared to the group without medium replacement. In summary, the semiautomatic RUF-DPBS offers a non-fouling, labor-efficient, and cost-effective means for high-density microalgae culture in continuous mode.