Transcriptomic and metabolic analysis of an astaxanthin-hyperproducing Haematococcus pluvialis mutant obtained by low-temperature plasma (LTP) mutagenesis under high light irradiation

Haematococcus pluvialis can accumulate sufficient levels of astaxanthin under various stress conditions. High light irradiation is essential for astaxanthin accumulation in Haematococcus pluvialis. However, a lack of genomic information limits the understanding of its physiological metabolism for high astaxanthin production. In this work, we investigated the wild-type (WT) strain and mutant strain (named as M3 which was obtained in our previous work) under high light irradiation and compared the difference in astaxanthin biosynthesis. We collected and analyzed the data of de novo transcriptome information of Haematococcus pluvialis at 24 h under high light stress. Our transcriptomic results indicated that M3 strain had higher utilization efficiency of CO2 to provide the precursors of carotenoid and fatty acid biosynthesis by increasing the expression levels of phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), malate dehydrogenase (ME), ribulose bisphosphate carboxylase/oxygenase (Rubisco) activase (RCA) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) while decreasing the expression levels of fructose-1, 6-bisphosphatase (FBP). The analysis of pigments, chlorophyll fluorescence and the quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that M3 strain maintained higher photosynthetic activity by regulating chlororespiration pathway and elevating non-photosynthetic pigment (lutein, beta-carotene, and astaxanthin) content to alleviate photooxidative damage. Moreover, the M3 strain showed higher fatty acid content than the WT strain. Overall, combinative analysis of de novo transcriptomic and physiological data provided information necessary for not only a better understanding of the difference in astaxanthin biosynthesis between WT and M3 strains but also the feasibility of genetic engineering of Haematococcus pluvialis in the future.