(224f) Using Fluoroalkylated Polyethylene Glycol-Stabilized Perfluorocarbon Nanodroplet As Oxygen Carriers to Reduce Oxygen Inhibition Effect for Microalgal Growth

Introduction: Dissolved oxygen (DO) built up in the photobioreactor and/or its excessive accumulation in the medium have long been considered as one of the major limitations for closed and massive cultivation of microalgae. Although the DO may be removed via air sparging-based photobioreactor technology, disadvantages such as high cost, increased contamination, and severe cell damage caused by hydrodynamic stress remain obstacles for microalgal culture. Perfluorocarbon (PFC), a fluorine-substituted derivative of hydrocarbons which can dissolve large respiratory and other non-polar gases as compared to water, has been widely used as an oxygen/carbon dioxide transporter in various fields. To overcome the effect of oxygen inhibition on the microalgal growth, a strategy of using perfluorocarbon nanodroplets (PNDs) as oxygen scavengers was explored.

Materials and Methods: The PNDs were synthesized by perfluorooctyl bromide (PFOB) and fluoroalkylated methoxy polyethylene glycol (R_F-MPEG, Mw of PEG molecule = 2000 or 5000) with various formulations through single PFC-in-water emulsification in association with solvent evaporation approach and were characterized using Fourier Transform Infrared spectroscopy. The stability of each type of PND under dynamic environment was evaluated by analyzing their changes in size and concentration while circulating in the photobioreactor system for 7 days. The deoxygenability of each type of PND under 7-days oxygenated photobioreactor operation was determined based on the mass accumulation rate coefficient (K_La^ac) of DO in the medium by use of dynamic method. To verify the effectiveness of PNDs-aided photobioreactor on Nannochloropsis Oculata (N. oculata)growth under a hypercritical oxygen environment, the cellular growth rate in each group was examined through measurement of the cell concentrations by hemocytometry every 24 h for up to 7 days.

Results and Discussion: Our results showed that all of the PNDs expanded during circulation in the photobioreactor and the one prepared by R_F-MPEG-2000 (MW of PEG = 2000) and perfluorooctyl bromide (PFOB) with a volume ratio of 9:1 (PNDs-2000/PFOB (9:1)) exhibited the lowest size increase of 19% (from 200 ± 2.43 to 238 ± 1.99 nm) after 7 days. Moreover, the concentration of DO in the medium approximately decreased 25%, 15%, 9%, and 5% at day 7 where the value of K_La^ac was about 4.7, 5.92, 6.49, and 7.4 Ã?10^-3 h^-1 for the group with 1% (v/v) PNDs-2000/PFOB (9:1), PNDs-2000/PFOB (7:3), PNDs-5000/PFOB (9:1), and PNDs-5000/PFOB (7:3), respectively, as compared to the group without PND. These results clearly exhibited that the PNDs-2000/PFOB (9:1) can provide the highest stability and capacity of deoxygenation for the defined photobioreactor operation. Under oxygenated cultivation, the growth of N. oculata (μ) without PNDs was tremendously arrested (μ = 0.366 day^-1) within seven days, while the cells with aid of 1% (v/v) PNDs-2000/PFOB (9:1) successfully multiplied in the hyperoxygenated environment that the growth rate of 0.467 day^-1 was comparable to the normal breeding with neither PNDs nor oxygen injection (μ = 0.498 day^-1), and the biomass production was five-fold higher than the O₂-treated group without PND after cultivation for seven days.

Conclusions: In summary, the developed PNDs-2000/PFOB (9:1) enabled to efficiently remove the DO in the photobioreactor that provided a feasible means for massive culture of microalgae in closed setting.