Astaxanthin biosynthesis in transgenic Dunaliella salina (Chlorophyceae) enhanced tolerance to high irradiation stress

Abstract

Astaxanthin is a highly valuable derivative of β-carotene, as well as an important raw material for pharmacy and healthy industries. Dunaliella salina is a halotolerant green alga that contains manifold nutriments and possesses a remarkable ability to adapt environment. The alga is also able to effectively accumulate β-carotene up to 10% of its dry weight. However, D. salina is lack of the enzymes for astaxanthin synthesis so that the accumulated β-carotene cannot be converted to astaxanthin in the alga. The designed experiment was that the exogenous key enzyme genes for astaxanthin synthesis, β-carotene ketolase and β-carotene hydroxylase cloned from Haematococcus pluvialis (Chlorophyceae), were co-transformed into D. salina by bombardment deliver system. The observation of laser scanning confocal microscope and analysis of PCR amplification confirmed the efficient expression of the transformed genes. Compared with the wild type strain, the transformants showed stronger high irradiation adaptability with higher cell survival rate, light saturation point, maximum relative electron transport rate and lower ROS level. The results of biochemical analysis indicated that the activities of antioxidant enzymes showed no significant difference between the two groups. At the transcriptional level, four carotenogenic genes (PSY, PDS, ZDS, LCYB) expressed 2~4 fold higher in the transformants than in the wild type. The tested transformants treated under high irradiation intensity were able to synthesize astaxanthin and the amount of astaxanthin was 5.56 μg/g dry weight measured by HPLC. This study may provide fundamental data for metabolic engineering to synthesize astaxanthin in D. salina.