Enhancing carotenoid accumulation in Dunaliella bardawil by combined treatments with fulvic acid and optimized culture conditions

Abstract

Natural carotenoids from microalgae have received more attention as an alternative source. In this study, fulvic acid (FA), a plant growth regulator, was used to enhance carotenoid accumulation in microalgae Dunaliella bardawil rich in lutein. However, the addition of FA promoted pigment synthesis but also exhibited an inhibitory effect on biomass. Therefore, the optimization of culture conditions was performed to further enhance carotenoid accumulation, including high light stress (10,000 lx) and the two-stage cultivation comprising 1-aminocyclopropane-1-carboxylic acid (ACC) and FA. Under both culture conditions, the growth inhibition caused by FA was alleviated, leading to a further increase in the contents of chlorophylls and carotenoids. HPLC analysis revealed that the production of lutein, α-carotene and β-carotene increased by 0.44-, 0.37- and 0.54-fold under the treatment of 400 mg/L FA with high light intensity and 0.91-, 1.15–0.29-fold under the two-stage cultivation comprising 11 mM ACC and 500 mg/L FA. Furthermore, algal cells under FA treatment and the two-stage cultivation stained with Bodipy505/515 emitted stronger fluorescence under a laser confocal microscope, suggesting that lipid accumulation was increased. Additionally, the transcription levels of carotenogenic genes were also found to be up-regulated by qRT-PCR. These results indicated an enhancement in both the storage capacity and synthesis of carotenoids in D. bardawil. This study revealed the potential application of plant growth regulators in promoting carotenoid accumulation in D. bardawil which could be further improved by optimizing the culture conditions, providing a reference for efficient carotenoid production in microalgae.