Abstract
To elucidate the mechanism underlying increased fatty acid and astaxanthin accumulation in Haematococcus pluvialis, transcriptome analysis was performed to gain insights into the multiple defensive systems elicited by salicylic acid combined with sodium acetate (SAHS) stresses with a time course. Totally, 112,886 unigenes and 61,323 non-repeat genes were identified, and genes involved in carbon metabolism, primary and secondary metabolism, and immune system responses were identified. The results revealed that SA and NaAC provide both energy and precursors to improve cell growth of H. pluvialis and enhance carbon assimilation, astaxanthin, and fatty acids production in this microalga with an effective mechanism. Interestingly, SA was considered to play an important role in lowering transcriptional activity of the fatty acid and astaxanthin biosynthesis genes through self-protection metabolism in H. pluvialis, leading to its adaption to HS stress and finally avoiding massive cell death. Moreover, positive correlations between 15 key genes involved in astaxanthin and fatty acid biosynthesis pathways were found, revealing cooperative relation between these pathways at the transcription level. These results not only enriched our knowledge of the astaxanthin accumulation mechanism in H. pluvialis but also provided a new view on increasing astaxanthin production in H. pluvialis by a moderate and sustainable way in the future.
Highlights
Astaxanthin (C40H52O4, 3, 3 -dihydroxy-β, β-carotene-4, 4 dione) is a bright red secondary carotenoid from the xanthophylls group, which is considered to be the most potent antioxidant in nature, and an important pigment for industrial markets (Machado et al, 2016; Shah et al, 2016; Molino et al, 2018)
Morphological changes of H. pluvialis cells at the different salicylic acid combined with sodium acetate (SAHS) treatment stages were observed, displaying that the orange-red partial of the cells increased with treatment time extension (Control, SAHS_1, SAHS_6, SAHS_12, SAHS_24, and SAHS_48) (Figure 1), which might indicate that the content of carotenoid or astaxanthin in H. pluvialis cells increased during the 2-day incubation
According to the results reported by Kobayashi (2003), astaxanthin is produced from β-carotene via two pathways, i.e., hydroxylation and oxidation of β-carotene catalyzed by β-carotene 3-hydroxylase (CrtZ, EC: 1.14.15.24) and β-carotene 4-ketolase (CrtW, EC: 1.14.99.63), which were upregulated by 3.65- and 4.24-fold, indicating enhanced astaxanthin production in H. pluvialis cells as a quick response to SAHS stresses
Summary
Astaxanthin (C40H52O4, 3, 3 -dihydroxy-β, β-carotene-4, 4 dione) is a bright red secondary carotenoid from the xanthophylls group, which is considered to be the most potent antioxidant in nature, and an important pigment for industrial markets (Machado et al, 2016; Shah et al, 2016; Molino et al, 2018). Haematococcus pluvialis, a unicellular microalga distributed in various habitats worldwide, is considered the best astaxanthin natural source (Ranga Rao et al, 2010). H. pluvialis cells accumulate large amounts of neutral lipids in the aplanospore phase. The increased neutral lipids in the cells are hypothesized to be a matrix for solubilizing of the esterified astaxanthin, and the suitable profile of lipids indicates a possibility of biodiesel production from H. pluvialis (Holtin et al, 2009; Saha et al, 2013; Shah et al, 2016). The ability to produce additional high-value compounds in addition to astaxanthin will enhance economic value of H. pluvialis (Le-Feuvre et al, 2020)
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