Transcriptomic analysis of astaxanthin hyper-producing Coelastrum sp. mutant obtained by chemical mutagenesis

A newly isolated green microalga,  Coelastrum  sp. has the capability to produce and accumulate astaxanthin under various stress conditions. At present, a mutant G1-C1 of  Coelastrum  sp. obtained through chemical mutagenesis using  ethyl methane sulfonate  displayed an improvement in astaxanthin accumulation,  which was  2-fold higher than that of the wild-type. However,  lack of genomic information limits the understanding of the molecular mechanism that leads to a high level of astaxanthin in the mutant  G1-C1. In this study, transcriptome sequencing was performed to compare the transcriptome of astaxanthin hyper-producing mutant G1-C1 and wild-type of  Coelastrum  sp. with respect to astaxanthin biosynthesis. This is to clarify why the mutant produced higher astaxanthin yield compared to the wild-type strain. Based on the transcriptomic analysis, the differentially expressed genes involved in astaxanthin biosynthesis were significantly upregulated in the mutant G1-C1 of  Coelastrum  sp.  Genes coding phytoene synthase, phytoene desaturase,  ζ -carotene desaturase, and lycopene  β-cyclase involved in β-carotene biosynthesis in the mutant cells were upregulated by  10-, 9.2-, 8.4-, and 8.7-fold, respectively.  Genes coding beta-carotene ketolase and beta-carotene 3-hydroxylase involved in converting β-carotene into astaxanthin were upregulated by  7.8- and 8.0-fold, respectively . In contrast, the  lycopene e-cyclase gene was downregulated by 9.7-fold in mutant G1-C1.  Together, these results contribute to higher astaxanthin accumulation in mutant G1-C1. Overall, the data in this study provided molecular insight for a better understanding of the differences in astaxanthin biosynthesis between the wild-type and mutant G1-C1 strains.