Light intensity control as a strategy to improve lipid productivity in Chlorella sp. HS2 for biodiesel production

Abstract The light greatly affects the physiological properties of microalgae, but the outcome is species-specific. To improve the traits of microalgae suitable for biofuel production, we investigated the influence of the light on biomass and lipid productivity of Chlorella sp. HS2. Photosynthetic performance in terms of relative electron transport rate (rel. ETR) was maximized at 450 μmol photons m−2 s−1, which allowed cells to reach high biomass productivity of 1.10 g L−1 day−1. Although the stress was induced by excessive irradiance under 750 μmol photons m−2 s−1, Chlorella sp. HS2 was able to maintain a high growth rate and achieved higher biomass productivity (1.34 g L−1 day−1) compared to that of other Chlorella species. In an effort to understand the relationship between photosynthesis and biomass productivity, we found out that effective quantum yield [Y(II)] and rel. ETR of Chlorella sp. HS2 is relatively higher than that of Chlorella vulgaris. The total lipid content decreased as the light intensity increased at the exponential phase. Interestingly, however, this trend was reversed during the stationary phase. This implies that the high light may synergistically induce lipids when the cells undergo nutrient starvation. The lipid content and productivity continued to increase up to 0.25 g L−1 day−1 even after the light saturation point. The fraction of esterifiable lipid suitable for biodiesel production rapidly increased from 50% to 99% during the stationary phase. These results suggest that species-specific high light based cultivation can be an effective strategy to improve the lipid productivity in Chlorella sp. HS2.