Transcriptomic analysis unveils survival strategies of autotrophic Haematococcus pluvialis against high light stress

Haematococcus pluvialis, a commercial microalga, accumulates massive astaxanthin under biotic and abiotic stress conditions, particularly under high light conditions. Upon exposure to high light stress, the green motile cells rapidly differentiate into non-motile red cyst, which undergo dramatic morphology changes during subsequent encystment, including fall off the flagella, and formation of a thick secondary cell wall, finally accumulate abundant astaxanthin. Cell photosynthetic activities are inhibited first at day 1 under high light, while enhanced later when astaxanthin is highly accumulated after adapting to the stress condition. To further understand the adaptive responses of H. pluvialis to high light stress at transcriptional level, cells exposed to high light for 0, 1, 5 and 10 days were subjected to transcriptome analysis. High light caused significant transcriptomic alterations in cells. High light promoted astaxanthin biosynthesis by enhancing the expression of key genes involved in the MEP and astaxanthin biosynthesis pathway, and simultaneously inhibiting the expression of genes in the branch pathways. High light down-regulated the expression of genes associated with photosynthesis at day 1 while up-regulated those at day 5 and day 10. All those transcriptomic changes were consistent with above mentioned morphological and physiological changes. In addition, high light stressed cells maintained a ROS homeostasis via simultaneously up- and down-regulating different sets of paralogs genes of antioxidant enzyme. Furthermore, high light up-regulated constantly the expression of 29 transcriptional factors, potential key players in the defense response to high light stress and in the regulation of astaxanthin biosynthesis. This study reveals the survival strategies of H. pluvialis against high light stress and lays a theoretic foundation for metabolic engineering of astaxanthin in H. pluvialis.