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Astaxanthin produced by Haematococcus pluvialis is an economically valuable ketocarotenoid pigment that benefits the health of human being. Astaxanthin can be accumulated in H. pluvialis cells up to 3% under high light stress. In this study, by adding 1 mM potassium iodide (KI) into the culture media of H. pluvialis cells under high light condition (HK), 26%–35% more astaxanthin than that of high light alone (HL) was obtained. As compared with HL that all cells turned into astaxanthin-rich non-motile ones at day 5 and day 10, about 30%–35% cells in HK remained flagella, which were motile without thickened secondary cell wall. Notably, these astaxanthin-rich motile cells contained more astaxanthin than astaxanthin-containing non-motile cells. The maximal photochemical efficiency (Fv/Fm) was higher in HK while the actual photochemical efficiency (Fv'/Fm') was lower in HK as compared with those in HL. Transcriptomes of cells in HK and HL groups were significantly different, especially at day 5 and day 10, which were consistent with above-mentioned morphological differences between HK and HL cells. KI promoted astaxanthin accumulation by up-regulating the transcription levels of astaxanthin biosynthetic genes and by simultaneously down-regulating the transcription levels of genes involved in other pathways that compete for precursors of astaxanthin biosynthesis. Transcriptomic data also revealed significant changes between HK and HL cells in genes involved in cell wall biosynthesis (57), flagella biosynthesis and cell movement (620), ROS homeostasis (42), and transcriptional factors (42). The possible mechanisms underlying KI's promoting role in H. pluvialis cells under high light were discussed based on morphological and transcriptomic data. In summary, this study provides a new method to increase astaxanthin accumulation in H. pluvialis , and its output expands current understanding of survival strategies and astaxanthin accumulation of H. pluvialis under high light stress. • Potassium iodide (KI) increases astaxanthin yield in H. pluvialis under high light. • KI induces the formation of astaxanthin-rich motile cells. • KI up- and down-regulate expression of astaxanthin biosynthetic and catabolic genes simultaneously. • KI inhibits secondary cell wall formation via suppressing corresponding gene expression. [ABSTRACT FROM AUTHOR]