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

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 (de) 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 de = 3.5 mm obtained the highest biomass productivity (104.1 mg·L−1·d−1), which was 12% higher than that with de = 1.5 mm. However, the lipid content in the system with de = 1.5 mm was 30% higher than that for de = 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 de = 3.5 mm and more lipid droplets in the cells when de = 1.5 mm. The largest bubble size was not beneficial for supplementing CO2 to the algae, which may have limited the growth. Therefore the air bubble size of de = 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.