Effects of air bubble size on algal growth rate and lipid accumulation using fine-pore diffuser photobioreactors

Abstract Microalgae are a potential feedstock for biodiesel and aeration is a reliable method to improve the growth of microalgae. Previous studies revealed the influence of bubbles on a variety of microorganisms, yet so far few studies have focused on the effects of bubble size on microalgal lipid production. In this study, four different fine-pore diffusers that could produce different bubble sizes were used for aeration in the photobioreactors. The equivalent diameters ( d e ) of the bubbles in the four systems were 1.5 mm, 2.5 mm, 3.5 mm and 6.0 mm, respectively. The system with d e  = 3.5 mm obtained the highest biomass productivity (104.1 mg·L −1 ·d −1 ), which was 12% higher than that with d e  = 1.5 mm. However, the lipid content in the system with d e  = 1.5 mm was 30% higher than that for d e  = 3.5 mm, and it also achieved the highest lipid productivity of 30.4 mg·L −1 ·d −1 . Results from transmission electron microscope indicated that there is more starch in the cells when d e  = 3.5 mm and more lipid droplets in the cells when d e  = 1.5 mm. The largest bubble size was not beneficial for supplementing CO 2 to the algae, which may have limited the growth. Therefore the air bubble size of d e  = 1.5 mm is the best when using aeration as a strategy for enhancing lipid productivity, since it could provide appropriate external stress for lipid accumulation, but did not greatly inhibit the growth of microalgae.