Astaxanthin Accumulation In Haematococcus Pluvialis Research Articles

The combination of uridine and nitrogen-deprivation promotes the efficient formation of astaxanthin-rich motile cells in Haematococcus pluvialis

Astaxanthin accumulation in Haematococcus pluvialis typically occurs alongside the formation of secondary cell wall (SCW), hindering astaxanthin extraction and bio-accessibility. A potential solution lies in cultivating astaxanthin-rich motile cells lacking SCW. This study explored the influence and underlying mechanism of nitrogen-deprivation (ND) on SCW formation and established a connection between pyrimidine metabolism and SCW development. Then, various pyrimidine and ND combinations were examined to cultivate astaxanthin-rich motile cells. The results indicated that, compared to the nitrogen-replete group, the combination of uridine and ND increased the proportion of motile cells by 25–33 times, achieving 95 %, and enhanced astaxanthin yield by 26.52 %. Moreover, the efficiency of astaxanthin extraction from intact, wet motile cells was 91 % – 95 %, which was 5.6–9.0 times that from non-motile cells. This study not only presents a promising method for producing astaxanthin-rich motile cells in H. pluvialis but also provides insights into the relationship between pyrimidine metabolism and SCW development.

Enhancement of Astaxanthin Content in Mixed Culture of <I>Dunaliella<I/> sp. and <I>Azospirillum<I/> sp. under Light Intensity Treatment

Highlight Research The growth rate was higher when Chlorella was induced by Azospirillum brasilense. The carbohydrate content increased when combined with Azospirillum brasilense. Determination of astaxanthin accumulation in Haematococcus pluvialis. Define condition favoring astaxanthin accumulation in Haematococcus pluvialis. Abstract Dunaliella sp. is a potential natural source of carotenoid pigments such as astaxanthin, β-carotene, and lutein. Dunaliella sp. can also accumulate other valuable products such as glycerol and protein. Another species is Azospirillum sp., which is known as microalgal growth-promoting bacteria. These bacteria are often cultured with microalgae because they contain indole-3-acetic acid, which can significantly increase the growth of microalgae. This study aimed to examine the pigment content in mixed culture of Dunaliella sp. and Azospirillum sp. after being treated with different light intensity treatment. In this study, Dunaliella sp. were cultivated by mixing with Azospirillum sp. under light stress. Two treatments were performed at light stress intensity of 3000 and 6000 lx. Light intensity is widely used as an important parameter in cultivation, which can affect the growth and production of microalgal biomass. In addition, spectrophotometric UV-Vis based measurement was conducted to investigate every single pigment content in all treatments under light stress for eight days. The number of cells, carotenoid pigments, and astaxanthin had increased significantly. Pigments of chlorophyll a and chlorophyll b also significantly increased at lower light treatments. Based on the results, the bacterium Azospirillum sp. and high light intensity significantly increased the growth and cell division of microalgae. Therefore, the combination of Azospirillum sp. and light stress intensity in microalgae cultivation could increase the growth and pigment of Dunaliella sp.

The Histone Acetyltransferase HpGCN5 Involved in the Regulation of Abiotic Stress Responses and Astaxanthin Accumulation in Haematococcus pluvialis.

The histone acetyltransferases (HATs), together with histone deacetylases, regulate the gene transcription related to various biological processes, including stress responses in eukaryotes. This study found a member of HATs (HpGCN5) from a transcriptome of the economically important microalgae Haematococcus pluvialis. Its expression pattern responding to multiple abiotic stresses and its correlation with transcription factors and genes involved in triacylglycerols and astaxanthin biosynthesis under stress conditions were evaluated, aiming to discover its potential biological function. The isolated HpGCN5 was 1,712 bp in length encoding 415 amino acids. The signature domains of Acetyltransf_1 and BROMO were presented, as the GCN5 gene from Arabidopsis and Saccharomyces cerevisiae, confirming that HpGCN5 belongs to the GCN5 subfamily of the GNAT superfamily. The phylogenetic analysis revealed that HpGCN5 is grouped with GNAT genes from algae and is closer to that from higher plants, compared with yeast, animal, fungus, and bacteria. It was predicted that HpGCN5 is composed of 10 exons and contains multiple stress-related cis-elements in the promoter region, revealing its potential role in stress regulation. Real-time quantitative PCR revealed that HpGCN5 responds to high light and high salt stresses in similar behavior, evidenced by their down-regulation exposing to stresses. Differently, HpGCN5 expression was significantly induced by SA and Nitrogen-depletion stresses at the early stage but was dropped back after then. The correlation network analysis suggested that HpGCN5 has a strong correlation with major genes and a transcription factor involved in astaxanthin biosynthesis. Besides, the correlation was only found between HpGCN5 and a few genes involved in triacylglycerols biosynthesis. Therefore, this study proposed that HpGCN5 might play a role in the regulation of astaxanthin biosynthesis. This study firstly examined the role of HATs in stress regulation and results will enrich our understanding of the role of HATs in microalgae.

Improvement of Astaxanthin Accumulation in Haematococcus Pluvialis Using Polyethylenimine Cross-Linked with Chitosan

Improvement of Astaxanthin Accumulation in Haematococcus Pluvialis Using Polyethylenimine Cross-Linked with Chitosan

Enhancement of astaxanthin accumulation in Haematococcus pluvialis by exogenous oxaloacetate combined with nitrogen deficiency

Effective inducers and stress conditions play an essential role in the regulation of astaxanthin biosynthesis. This study reports a strategy developed by combining exogenous addition of oxaloacetate (OA) with nitrogen deficiency to facilitate astaxanthin production in Haematococcus pluvialis. Significantly, addition of 10 mM-OA enhanced the cellular astaxanthin content about 7.18-fold under nitrogen deficiency on day 7, with the content of astaxanthin esters increased concomitantly. To further elucidate the role and mechanism of OA on astaxanthin synthesis, the physiological and metabolic analyses of H. pluvialis treated with exogenous OA were performed. The results showed that exogenous OA promoted respiration over photosynthesis. Concurrently, the metabolite levels in the Embden-Meyerhof-Parnas pathway, pentose phosphate pathway and tricarboxylic acid cycle obviously increased. The enhancement of respiratory metabolic pathways led to elevated levels of substrates, thus directly promoted astaxanthin synthesis. The present findings provide a new and effective approach for optimizing astaxanthin production.

Haematococcus pluvialis Accumulated Lipid and Astaxanthin in a Moderate and Sustainable Way by the Self-Protection Mechanism of Salicylic Acid Under Sodium Acetate Stress.

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.

Transcriptome analysis reveals the promoting effect of trisodium citrate on astaxanthin accumulation in Haematococcus pluvialis under high light condition

Some organic carbons were able to further promote astaxanthin accumulation in Haematococcus pluvialis under stress conditions. In this study, we demonstrate that adding 0.6 mM trisodium citrate (TSC) into the culture media increased astaxanthin content in the single cell by 2.4 folds and increased astaxanthin yield by 11.9% in H. pluvialis at day 7 under high light stress. In addition, the ROS level, which was highly associated with astaxanthin accumulation, was also increased by TSC. Subsequent comparative transcriptomic analysis revealed the mechanism underlying the promoting effect of trisodium citrate on astaxanthin accumulation. TSC treatment up-regulated significantly the expression of many enzyme encoding genes involved in TCA cycle, glucose metabolism, astaxanthin biosynthesis and fatty acids metabolism. In addition, trisodium citrate treatment significantly changed the expression of genes involved in ROS scavenging and cell wall biosynthesis. Our results indicated that TSC can not only be used as a carbon source but also further increase the ROS level in H. pluvialis under high light stress. Collectively, this study provides a new method that can increase astaxanthin yield, but also insights into the mechanism of trisodium citrate-induced astaxanthin biosynthesis in H. pluvialis under high light.

Transcriptome analysis of signal transduction pathway involved in light inducing astaxanthin accumulation in Haematococcus pluvialis

The unicellular green alga Haematococcus pluvialis is the best source of natural astaxanthin (AST) in the world due to its high content under stress conditions. Although high light (HL) can effectively induce AST biosynthesis, the specific mechanisms of light signal perception and transduction are unclear. In the current study, we used transcriptomic data of normal (N), high white light (W), and high blue light (B) to study the mechanisms of light inducing AST accumulation from the point of photoreceptors. The original data of 4.0 G, 3.8 G, and 3.6 G for N, W, and B were obtained, respectively, by the Illumina Hi-seq 2000 sequencing technology. Totally, 51 954 unigenes (at least 200 bp in length) were generated, of which, 20 537 unigenes were annotated into at least one database (NR, NT, KO, SwissProt, Pfam, GO, or KOG). There were 1 255 DEGs in the W vs N, 1 494 DEGs in the B vs N, and 1 008 DEGs in the both W vs N and B vs N. KEGG enrichment analysis revealed that photosynthesis, oxidative phosphorylation, carotenoid biosynthesis, fatty acids biosynthesis, DNA replication, nitrogen metabolism, and carbon metabolism were the significantly enriched pathways. Moreover, a large number of genes encoding photoreceptors and predicted interacting proteins were predicted in Haematococcus transcriptome data. These genes showed significant differences at transcriptional expression levels. In addition, 15 related DEGs were selected and tested by qRT-PCR and the results were significantly correlated with the transcriptome data. The above results indicate that the signal transduction pathway of "light signal - photoreceptors - interaction proteins - (interaction proteins - transcription factor/transcriptional regulator) - gene expression - AST accumulation" might play important roles in the regulation process, and provide reference for further understanding the transcriptional regulation mechanisms of AST accumulation under HL stress.

Transcriptome-based analysis of the effects of salicylic acid and high light on lipid and astaxanthin accumulation in Haematococcus pluvialis

BackgroundThe unicellular alga Haematococcus pluvialis has achieved considerable interests for its capacity to accumulate large amounts of triacylglycerol and astaxanthin under various environmental stresses. To our knowledge, studies focusing on transcriptome research of H. pluvialis under exogenous hormones together with physical stresses are rare. In the present study, the change patterns at transcriptome level were analyzed to distinguish the multiple defensive systems of astaxanthin and fatty acid metabolism against exogenous salicylic acid and high light (SAHL) stresses.ResultsBased on RNA-seq data, a total of 112,463 unigenes and 61,191 genes were annotated in six databases, including NR, KEGG, Swiss-Prot, PFAM, COG and GO. Analysis of differentially expressed genes (DEGs) in KEGG identified many transcripts that associated with the biosynthesis of primary and secondary metabolites, photosynthesis, and immune system responses. Furthermore, 705 unigenes predicted as putative transcription factors (TFs) were identified, and the most abundant TFs families were likely to be associated with the biosynthesis of astaxanthin and fatty acid in H. pluvialis upon exposure to SAHL stresses. Additionally, majority of the fifteen key genes involved in astaxanthin and fatty acid biosynthesis pathways presented the same expression pattern, resulting in increased accumulation of astaxanthin and fatty acids in single celled H. pluvialis, in which astaxanthin content increased from 0.56 ± 0.05 mg·L−1 at stage Control to 0.89 ± 0.12 mg·L−1 at stage SAHL_48. And positive correlations were observed among these studied genes by Pearson Correlation (PC) analysis, indicating the coordination between astaxanthin and fatty acid biosynthesis. In addition, protein–protein interaction (PPI) network analysis also demonstrated that this coordination might be at transcriptional level.ConclusionThe results in this study provided valuable information to illustrate the molecular mechanisms of coordinate relations between astaxanthin and fatty acid biosynthesis. And salicylic acid might play a role in self-protection processes of cells, helping adaption of H. pluvialis to high light stress.

Transcriptome analysis reveals pathways responsible for the promoting effect of sucrose on astaxanthin accumulation in Haematococcus pluvialis under high light condition

Haematococcus pluvialis accumulates large amount of astaxanthin, a valuable ketocarotenoid with super antioxidant activity, under stress conditions. Providing suitable carbon source has been proved to be an effective method to promote astaxanthin accumulation in H. pluvialis, however, mechanisms remain obscure. Here, we demonstrate that adding sucrose (1–10 mM) into the culture media increase astaxanthin yield by 27.9–34.2% in H. pluvialis under high light stress. Subsequently, comparative transcriptomic analysis revealed the mechanism underling the promoting effect of sucrose on astaxanthin biosynthesis. Sucrose treatment significantly up-regulated the expression of many enzyme encoding genes involved in sucrose and starch metabolism, glycolysis, astaxanthin biosynthesis, and fatty acids metabolism. In addition, sucrose treatment significantly changed the expression of 29 transcriptional factors, potential key regulators in sucrose metabolism, lipid biosynthesis and astaxanthin biosynthesis. Furthermore, exogenous application of metabolites involved in sucrose catabolism and glycolysis pathway (glucose-6-phosphate disodium and fructose-1,6-diphosphate trisodium) significantly promoted astaxanthin yield by 33.1% and 18.1%, respectively. Our results indicated that sucrose treatment exerts remarkable effects on the transcriptomes of H. pluvialis, favoring the shift from carbon metabolism to astaxanthin accumulation under high light condition.

Comparative transcriptome analysis unveils mechanisms underlying the promoting effect of potassium iodide on astaxanthin accumulation in Haematococcus pluvialis under high light stress

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.

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, β-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.

The interrelation between photorespiration and astaxanthin accumulation in Haematococcus pluvialis using metabolomic analysis

To further investigate the interrelation between photorespiration and astaxanthin accumulation, metabolomic analysis of Haematococcus pluvialis during the astaxanthin accumulation period was performed, using gas chromatography–mass spectrometry (GC–MS), in the absence or presence of carboxymethoxylamine (CM), an inhibitor of the photorespiratory pathway. Algal samples were collected on days 0, 6 and 12, which were named as the green phase with green, non-motile cells, the brown phase with astaxanthin-synthesizing, non-motile cells, and the red phase with astaxanthin-enriched, red, non-motile cells, respectively. The inhibition of photorespiration by CM did not affect the biomass, had a slight effect on chlorophyll at the end of the incubation, but significantly suppressed astaxanthin accumulation. A total of 78 metabolites were identified by GC–MS, including 27 amino acids, 16 organic acids, 12 fatty acids, 7 polyols, 3 phosphoric acids, 3 sugars, 2 amines, and 8 other compounds. Multivariate statistical analyses (principal component analysis, partial least squares-discriminant analysis), and hierarchical cluster analysis revealed the clustering of the metabolites. Glycine and glycolic acid had accumulated substantially at both the brown and red phases, indicating that photorespiration was inhibited by CM. In presence of CM, the TCA cycle was restricted at the brown phase due to decreased intermediates, specifically, decreased levels of fructose and glucose. However, inhibiting photorespiration enhanced the levels of many intracellular cytoprotective metabolites, such as amino acids, polyamines, polyols and sucrose. A hypothetical metabolic regulation model of the photorespiratory pathway affecting astaxanthin accumulation of H. pluvialis is proposed. This study provides the first metabolomic evidence that photorespiration enhances astaxanthin accumulation.

Comparative transcriptome analysis at seven time points during Haematococcus pluvialis motile cell growth and astaxanthin accumulation

Although the molecular basis of stress-induced astaxanthin accumulation in Haematococcus pluvialis has been closely studied, the reasons for astaxanthin accumulation, rather than other carotenoids, have not been thoroughly investigated to date. With the aim of improving our understanding of the complex metabolic changes, especially astaxanthin biosynthesis, occurring during fast-growing vegetative cell proliferation, transformation of intermediate cells and formation of resting non-motile cells, seven specified developmental points with perceptible color difference were sampled and sequenced using an Illumina NextSeq500 Sequencer. Comparative transcriptome profiling was employed to identify the differentially expressed genes. RT-qPCR experiments were employed to validate some important DEGs identified by the profiling. After pairwise comparisons, 2674 DEGs were identified. Seventeen unigenes related to carotenoid biosynthesis were actively transcribed according to KEGG pathway analysis. Compared with these thirteen DEGs involved in carotenoid synthesis, four unigenes including lut1, vde, nced and crtR-3 had low expression and were not found to be differentially expressed during the whole process. This is probably one of the reasons for astaxanthin accumulation in H. pluvialis. In phase 4, the up-regulation of lcyB was accompanied by the down-regulation of lcyE, which appears to be a good strategy for promoting the accumulation of β-carotene, rather than δ-carotene. Bkt came into focus for its most extensive change, followed by crtR-1 and crtR-2. BKT and CrtR were further confirmed to play an essential role in the stress-dependent initiation of astaxanthin synthesis. RT-qPCR results were consistent with the transcriptomic data.

Enhancing Astaxanthin Accumulation in Haematococcus pluvialis by Coupled Light Intensity and Nitrogen Starvation in Column Photobioreactors.

Natural astaxanthin mainly derives from a microalgae producer, Haematococcus pluvialis. The induction of nitrogen starvation and high light intensity is particularly significant for boosting astaxanthin production. However, the different responses to light intensity and nitrogen starvation needed to be analyzed for biomass growth and astaxanthin accumulation. The results showed that the highest level of astaxanthin production was achieved in nitrogen starvation, and was 1.64 times higher than the control group at 11 days. With regard to the optimization of light intensity utilization, it was at 200 μmo/m²/s under nitrogen starvation that the highest astaxanthin productivity per light intensity was achieved. In addition, both high light intensity and a nitrogen source had significant effects on multiple indicators. For example, high light intensity had a greater significant effect than a nitrogen source on biomass dry weight, astaxanthin yield and astaxanthin productivity; in contrast, nitrogen starvation was more beneficial for enhancing astaxanthin content per dry weight biomass. The data indicate that high light intensity synergizes with nitrogen starvation to stimulate the biosynthesis of astaxanthin.

Enhanced astaxanthin accumulation in Haematococcus pluvialis using high carbon dioxide concentration and light illumination

In this study, an economical two-stage method was proposed for the production of natural astaxanthin from Haematococcus pluvialis without a medium replacement step. In stage 1, H. pluvialis were grown under low light illumination until they reached optimal biomass. In stage 2, cells were switched to astaxanthin induction conditions utilizing the combination of high light illumination and elevated carbon dioxide levels (5 or 15%). The introduction of CO2 altered the C/N balance creating a nutrient deficiency without a change of media. The resulting astaxanthin yield was 2–3 times that of using either stressor alone. This astaxanthin induction method has many advantages over current methods including no medium replacement and a short induction time of less than four days.

A strategy for promoting astaxanthin accumulation in Haematococcus pluvialis by 1-aminocyclopropane-1-carboxylic acid application

The green algae Haematococcus pluvialis is a freshwater unicellular microalga belonging to Chlorophyceae. It is one of the best natural sources of astaxanthin, a secondary metabolite commonly used as an antioxidant and anti-inflammatory agent. Due to the importance of astaxanthin, various efforts have been made to increase its production. In this study, we attempted to develop a strategy for promoting astaxanthin accumulation in H. pluvialis using 1-aminocyclopropane-1-carboxylic acid (ACC), a precursor of ethylene (normally known as an aging hormone in plants). Our results demonstrated that ACC could enhance the growth of H. pluvialis, thereby promoting astaxanthin accumulation. Therefore, ACC has an indirect influence on astaxanthin production. We further verified the effect of ACC with a direct treatment of ethylene originated from banana peels. These results indicate that ethylene could be applied as an indirect method for enhancing growth and astaxanthin biosynthesis in H. pluvialis.

The role of photorespiration during astaxanthin accumulation in Haematococcus pluvialis (Chlorophyceae)

Most previous studies on Haematococcus pluvialis have been focused on growth and astaxanthin accumulation. However, the relationships between photorespiration and astaxanthin accumulation have not been clarified. The purpose of this study was to examine the role of photorespiration during the process of astaxanthin accumulation in H. pluvialis. During astaxanthin accumulation, the astaxanthin content was reduced significantly when photorespiration was inhibited by its specific inhibitor, carboxymethoxylamine. The inhibition of photorespiration did not change the dry weight, chlorophyll content and OJIP transients during the incubation; however, the inhibition of photorespiration significantly decreased the photochemistry of photosystem II and total photosynthetic O2 evolution capacity. Moreover, the restriction in photorespiration was synchronized with a decrease of astaxanthin accumulation. These results suggest that the photorespiratory pathway in H. pluvialis can accelerate astaxanthin accumulation. We speculate that photorespiration can enhance astaxanthin accumulation in the following ways: (i) photorespiration directly affects the glycerate-3-phosphate (PGA) level, which is intrinsically related to the accumulation of astaxanthin in H. pluvialis; (ii) the photorespiratory pathway indirectly affects the PGA level by effecting the dark reactions of photosynthesis, which then results in the enhancement of astaxanthin accumulation in H. pluvialis.

Identification and mechanism analysis of chemical modulators enhancing astaxanthin accumulation in Haematococcus pluvialis

Haematococcus pluvialis is one natural source for high-value anti-oxidant astaxanthin. In this study, we evaluated 23 chemicals of 8 groups (antioxidant, oxidant, signal transducer, metal ion, auxin, gibberellin, cytokinin and amine) for their effects on astaxanthin accumulation in H. pluvialis. The results showed that oxidant and signal transduction chemicals increased astaxanthin accumulation in H. pluvialis significantly by more than 20%. To reveal possible regulatory mechanism, a gas chromatography–mass spectrometry (GC–MS) and a liquid chromatography–mass spectrometry (LC–MS) based metabolomic analysis were applied to determine metabolic profiles of H. pluvialis treated with chemical modulators, and then the metabolomic data was analyzed by a partial least squares-discriminate analysis (PLS-DA) and a weighted correlation network analysis (WGCNA) to identify possible metabolic modules and metabolites relevant to the increased astaxanthin accumulation. The analyses showed six metabolic modules and seven hub metabolites were possibly related to the stimulatory roles of oxidant or signal transduction in H. pluvialis. The study provided new insights to the regulatory mechanism of oxidant and signal transduction chemicals on astaxanthin accumulation in H. pluvialis.

Enhanced autotrophic astaxanthin production from Haematococcus pluvialis under high temperature via heat stress-driven Haber–Weiss reaction

High temperatures (30-36 °C) inhibited astaxanthin accumulation in Haematococcus pluvialis under photoautotrophic conditions. The depression of carotenogenesis was primarily attributed to excess intracellular less reactive oxygen species (LROS; O2 (-) and H2O2) levels generated under high temperature conditions. Here, we show that the heat stress-driven inefficient astaxanthin production was improved by accelerating the iron-catalyzed Haber-Weiss reaction to convert LROS into more reactive oxygen species (MROS; O2 and OH·), thereby facilitating lipid peroxidation. As a result, during 18 days of photoautotrophic induction, the astaxanthin concentration of cells cultured in high temperatures in the presence of iron (450 μM) was dramatically increased by 75 % (30 °C) and 133 % (36 °C) compared to that of cells exposed to heat stress alone. The heat stress-driven Haber-Weiss reaction will be useful for economically producing astaxanthin by reducing energy cost and enhancing photoautotrophic astaxanthin production, particularly outdoors utilizing natural solar radiation including heat and light for photo-induction of H. pluvialis.