Microdroplet photobioreactor for the photoautotrophic culture of microalgal cells

Microalgae, unicellular photoautotrophic microorganisms, have attracted great attention for the production of biofuel and high-value products, but the commercial use of microalgae has been limited by low photosynthetic productivity. To overcome this limitation, it is required to develop an efficient platform for the rapid evaluation of photoautotrophic growth performance and productivity of microalgal strains. Here we describe a droplet-based photobioreactor for high-throughput analysis of the photoautotrophic growth of microalgal cells. By integrating micropillar arrays and adjusting the height of the microchamber, we could accurately monitor the growth kinetics of microalgae in an immobilized microdroplet and improve the transfer rate of CO₂ into the microdroplet photobioreactor with an increased contact area between the microdroplet and PDMS surface. The improvement of CO₂ transfer into the microdroplet was also confirmed by improved microalgal cell growth and a decrease in pH measured using colorimetric and fluorescence-based assays. The photoautotrophic growth kinetics of Chlorella vulgaris were measured under different CO₂ concentrations (ambient, 1%, 2.5%, 5% and 7.5%) and light intensity (35, 55, 100, 150, and 200 μmol photons per m² per s) conditions, which are key factors for photoautotrophic growth. Chlorella vulgaris in a microdroplet showed better cell growth performance compared to a flask culture due to the reduced shading effects and improved mass transfer. Finally, we could evaluate the photoautotrophic growth performance of four microalgal strains (Chlorella vulgaris, Chlorella protothecoides, Chlorella sorokiniana and Neochloris oleoabundans) for 120 hours. These results demonstrate that our microdroplet system can be used as an efficient photobioreactor for the rapid evaluation of the photoautotrophic growth of microalgal strains under various conditions.