(191z) Using Ultrasound Standing Wave-Incorporated Dynamic Photobioreactor System to Enhance Medium Replacement Efficiency for Concentrated Microalgae Cultivation in Continuous Mode

Introduction: Microalgae have long been recognized as promising biosources for live feed, pharmaceuticals, and alternative fuel since they can provide abundance of essential chemicals such as vitamins, polysaccharides, and fatty acids. To minimize the potential contamination and provide quality cell products, microalgae culture in closed setting (i.e., photobioreactor) is generally considered as a favorable approach since all the microalgae growth-related factors, such as light, pH, temperature, and microorganism can be strictly controlled. However, decrease of nutrient and/or accumulation of cellular metabolites in the culture medium including excretions and dissolved oxygen generated from the microalgal photosynthesis may severely inhibit cell growth. Therefore, renewing the growth medium to maintain the balance and sufficiency of micro- and macronutrients in the living environment is crucial for massive microalgae cultivation. Although quite a few harvesting techniques including flotation, centrifugation, sedimentation, flocculation, filtration, and/or combinations of above methods have been achieved in the past decades, several drawbacks, such as increase of contamination risk, complexity of the process, labor consuming, and cost-ineffectiveness still remain obstacles for their practical use. To circumvent these issues, a synthetic ultrasound standing wave-incorporated dynamic photobioreactor system (USW-DPBRS) for high-efficiency microalgae culture was explored in this study.

Materials and Methods: The USW-DPBRS is consisting of 1) a culture chamber, 2) an acoustic chamber, 3) two medium reservoirs, and 4) three ultrasonic devices. The microalgae in the USW-DPBRS were harvested/separated from the culture medium driven by the USW-induced the primary and secondary acoustic radiation forces. The 1-MHz sinusoidal wave with 50% duty cycle and defined output intensity was served as the ultrasonic setting. To verify the efficacy of USW-DPBRS on cell culture, microalgae Nannochloropsis oculatawere cultivated with and without medium change using USW-DPBRS with optimal settings including USW intensity, exposure time, and flow rate for 12 days, followed by analyses of cell growth and productions of microalgae including biomass and lipid.

Results and Discussion: Both mobile cells with and without USW exhibited similar profiles of cell growth within 7 days (growth rate = 0.802 ± 0.014 day^-1) and that was comparable to the group with neither circulation nor USW treatment (growth rate = 0.807 ± 0.037 day^-1). These results clearly show that the developed USW is harmless to microalgae. Based on the collection efficiency of microalgae, the USW operation was optimized by 1 MHz and output intensity of 8 W/cm² with circulating velocity of 2 mL/min whereby 93% of N. oculata in 30 mL can be collected within 2-h operation. The cells culture under USW-DPBRS in which the medium was changed every three days by optimal settings (growth rate = 0.033 day^-1) exhibited increased microalgae productions that the yields of biomass, total lipid, and eicosapentaenoic acid (EPA) 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 (growth rate = 0.01 day^-1), and were all similar to the values obtained from the group with centrifugation (P= NS for each). These results clearly demonstrated the applicability of the USW-DPBRS for use in the practice.

Conclusions: In this study, we have successfully developed a synthetic photobioreactor system, named USW-DPBRS for microalgae culture in which the medium replacement can be semi-automatically carried out through USW-mediated separation technique and gravity-driven perfusion. In comparison to conventional separation methods, the USF-DPBRS offers a non-fouling, labor-efficient, and cost-effective means for high-density microalgae culture in continuous mode.