Enhancement of carotenoid biosynthesis in the green microalga Dunaliella salina with light-emitting diodes and adaptive laboratory evolution

Abstract

There is a particularly high interest to derive carotenoids such as β-carotene and lutein from higher plants and algae for the global market. It is well known that β-carotene can be overproduced in the green microalga Dunaliella salina in response to stressful light conditions. However, little is known about the effects of light quality on carotenoid metabolism, e.g., narrow spectrum red light. In this study, we present UPLC-UV-MS data from D. salina consistent with the pathway proposed for carotenoid metabolism in the green microalga Chlamydomonas reinhardtii. We have studied the effect of red light-emitting diode (LED) lighting on growth rate and biomass yield and identified the optimal photon flux for D. salina growth. We found that the major carotenoids changed in parallel to the chlorophyll b content and that red light photon stress alone at high level was not capable of upregulating carotenoid accumulation presumably due to serious photodamage. We have found that combining red LED (75 %) with blue LED (25 %) allowed growth at a higher total photon flux. Additional blue light instead of red light led to increased β-carotene and lutein accumulation, and the application of long-term iterative stress (adaptive laboratory evolution) yielded strains of D. salina with increased accumulation of carotenoids under combined blue and red light.

Fig. 1

Profile of major pigments in D. salina detected by UPLC-UV-MS (LC/MS). The cells were cultivated for 5 days under red LED lighting at 85 μE/m²/s. Extracted ion chromatograms for the carotenoids species in MS detection figure were scaled up by six times. Operation conditions of UPLC-UV-MS were described in the “Materials and methods” section

Fig. 2

Proposed pathway of carotenoid metabolism in D. salina based on C. reinhardtii in KEGG database

Fig. 3

D. salina growth and carotenoid accumulation under red LED lighting. Cells were cultivated for 5 days under different light intensity conditions, while cells of 128 (N-) were cultivated for additional 16 days for nitrogen deprivation. Detailed data were presented in Table S2 in the ESM. a Average growth rate and biomass yield on light energy under varied light intensities. Average growth rate or biomass productivity indicated biomass produced per day during batch culture, and average biomass yield was calculated according to the average growth rate (see Fig. S2 in the ESM). Dotted lines are drawn to guide the eye. b Lycopene, all-trans β-carotene, lutein, and zeaxanthin content. c Chlorophyll b and major carotenoids content. All results are averaged from three independent experiments. Error bars indicate SD

Fig. 4

Effect of adaptive laboratory evolution (ALE) on growth rate and carotenoid accumulation in D. salina. Average growth rate indicated biomass produced per day during one cycle. All the cycles including cycle 0 were performed under a total light intensity of 170 μE/m²/s consisting of 42 μE/m²/s blue LED light and 128 μE/m²/s red LED light. Detailed data were presented in Table S2 in the ESM. The results are averaged from three independent experiments. Error bars indicate SD

Fig. 5

Schematic pathways to developing D. salina with increased yields of carotenoids. BP biomass productivity, CC carotenoids content