Transcriptome sequencing and metabolic pathways of astaxanthin accumulated in Haematococcus pluvialis mutant under 15% CO2

The production of FAHFA is enhanced when Haematococcus pluvialis is grown in CO2

Microalgae are considered as a potential source of bioactive compounds to be used in different fields including food and pharmaceutical industry. In this context, fatty acid esters of hydroxy-fatty acids (FAHFA) are emerging as a new class of compounds with anti-inflammatory and anti-diabetic properties. An existing gap in the field of algal research is the limited knowledge regarding the production of these compounds. Our research questions aimed to determine whether the microalga H. pluvialis can synthesize FAHFA and whether the production levels of these compounds are increased when cultivated in a CO2-rich environment. To answer these questions, we used a LC-QTOF/MS method for the characterization of FAHFA produced by H. pluvialis while an LC-MS/MS method was used for their quantitation. The cultivation conditions of H. pluvialis, which include the utilization of CO2, can result in a 10-50-fold increase in FAHFA production.

Extraction of Astaxanthin from Haematococcus pluvialis and Preparation of Astaxanthin Liposomes

Astaxanthin has 550 times more antioxidant activity than vitamin E, so it can scavenge free radicals in vivo and improve body immunity. However, the poor stability of astaxanthin becomes a bottleneck problem that limits its application. Herein, Haematococcus pluvialis (H. pluvialis) as a raw material was used to extract astaxanthin, and the optimal extraction conditions included the extraction solvent (EA:EtOH = 1:6, v/v), extraction temperature (60 °C), and extraction time (70 min). The extracted astaxanthin was then loaded using lecithin to form corresponding liposomes via the ethanol injection method. The results showed that the particle size and zeta potential of the prepared liposomes were 105.8 ± 1.2 nm and -38.0 ± 1.7 mV, respectively, and the encapsulation efficiency of astaxanthin in liposomes was 88.83%. More importantly, the stability of astaxanthin was significantly improved after being embedded in the prepared liposomes.

Astaxanthin: Past, Present, and Future

Astaxanthin (AX), a lipid-soluble pigment belonging to the xanthophyll carotenoids family, has recently garnered significant attention due to its unique physical properties, biochemical attributes, and physiological effects. Originally recognized primarily for its role in imparting the characteristic red-pink color to various organisms, AX is currently experiencing a surge in interest and research. The growing body of literature in this field predominantly focuses on AXs distinctive bioactivities and properties. However, the potential of algae-derived AX as a solution to various global environmental and societal challenges that threaten life on our planet has not received extensive attention. Furthermore, the historical context and the role of AX in nature, as well as its significance in diverse cultures and traditional health practices, have not been comprehensively explored in previous works. This review article embarks on a comprehensive journey through the history leading up to the present, offering insights into the discovery of AX, its chemical and physical attributes, distribution in organisms, and biosynthesis. Additionally, it delves into the intricate realm of health benefits, biofunctional characteristics, and the current market status of AX. By encompassing these multifaceted aspects, this review aims to provide readers with a more profound understanding and a robust foundation for future scientific endeavors directed at addressing societal needs for sustainable nutritional and medicinal solutions. An updated summary of AXs health benefits, its present market status, and potential future applications are also included for a well-rounded perspective.

Metabolite analysis of soybean oil on promoting astaxanthin production of Phaffia rhodozyma

BACKGROUND

Astaxanthin is a carotenoid with strong antioxidant property. In addition, it has anti-cancer, anti-tumor, anti-inflammatory and many other functions. The micro-organisms that mainly produce astaxanthin are Haematococcus pluvialis and Phaffia rhodozyma. Compared with H. pluvialis, P. rhodozyma has shorter fermentation cycle and easier to control culture conditions, but the yield of astaxanthin in P. rhodozyma is low. This article studied how to improve the astaxanthin production of P. rhodozyma.

RESULTS

The results showed that when 10 mL L^-1 soybean oil was added to the culture medium, astaxanthin production increased significantly, reaching 7.35 mg L^-1 , which was 1.4 times that of the control group, and lycopene and β-carotene contents also increased significantly. Through targeted metabolite analysis, the fatty acids in P. rhodozyma significantly increased under the soybean oil stimulation, especially the fatty acids closely related to the formation of astaxanthin esters, included palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1n9), linoleic acid (C18:2n6), α-linolenic acid (C18:3n3) and γ-linolenic acid (C18:3n6), thereby increasing the astaxanthin esters content.

CONCLUSION

It showed that the addition of soybean oil can promote the accumulation of astaxanthin by promoting the increase of astaxanthin ester content. © 2022 Society of Chemical Industry.

Astaxanthin as a King of Ketocarotenoids: Structure, Synthesis, Accumulation, Bioavailability and Antioxidant Properties

Astaxanthin (3,3-dihydroxy-β, β-carotene-4,4-dione) is a ketocarotenoid synthesized by Haematococcus pluvialis/lacustris, Chromochloris zofingiensis, Chlorococcum, Bracteacoccus aggregatus, Coelastrella rubescence, Phaffia rhodozyma, some bacteria (Paracoccus carotinifaciens), yeasts, and lobsters, among others However, it is majorly synthesized by Haematococcus lacustris alone (about 4%). The richness of natural astaxanthin over synthetic astaxanthin has drawn the attention of industrialists to cultivate and extract it via two stage cultivation process. However, the cultivation in photobioreactors is expensive, and converting it in soluble form so that it can be easily assimilated by our digestive system requires downstream processing techniques which are not cost-effective. This has made the cost of astaxanthin expensive, prompting pharmaceutical and nutraceutical companies to switch over to synthetic astaxanthin. This review discusses the chemical character of astaxanthin, more inexpensive cultivating techniques, and its bioavailability. Additionally, the antioxidant character of this microalgal product against many diseases is discussed, which can make this natural compound an excellent drug to minimize inflammation and its consequences.

Combined analysis of chromatin accessibility and gene expression profiles provide insight into Fucoxanthin biosynthesis in Isochrysis galbana under green light

Isochrysis galbana, as a potential accumulator of fucoxanthin, has become a valuable material to develop functional foods for humans. Our previous research revealed that green light effectively promotes the accumulation of fucoxanthin in I. galbana, but there is little research on chromatin accessibility in the process of transcriptional regulation. This study was conducted to reveal the mechanism of fucoxanthin biosynthesis in I. galbana under green light by analyzing promoter accessibility and gene expression profiles. Differentially accessible chromatin regions (DARs)-associated genes were enriched in carotenoid biosynthesis and photosynthesis-antenna protein formation, including IgLHCA1, IgLHCA4, IgPDS, IgZ-ISO, IglcyB, IgZEP, and IgVDE. The motifs for the MYB family were also identified as candidates controlling metabolic regulation responses to green light culture of I. galbana, including IgMYB1, IgMYB2, IgMYB33, IgMYB42, IgMYB98, IgMYB118, and IgMYB119. The results of differential expression analysis and WGCNA showed that several genes or transcription factors (TFs) related to carotenoid metabolism and photosynthesis exhibited a higher expression level and were significantly upregulated in A-G5d compared with A-0d and A-W5d, including IgMYB98, IgLHCA1, IgLHCX2, IgLHCB4, and IgLHCB5. This suggests that upregulation of these genes by green light may be the key factor leading to fucoxanthin accumulation by regulating the photosynthesis-antenna protein pathway. An integrated analysis of ATAC-seq and RNA-seq showed that 3 (IgphoA, IgPKN1, IgOTC) of 34 DARs-associated genes displayed obvious changes in their chromatin regions in ATAC-seq data, suggesting that these genes specific for green light may play a key role in fucoxanthin biosynthesis in I. galbana through a complex regulatory network of multiple metabolic pathways interacting with each other. These findings will facilitate in-depth understanding the molecular regulation mechanisms of fucoxanthin in I. galbana and its role in response to green light regulation, providing technical support for the construction of high fucoxanthin content strains.

Bioprocess Strategy of Haematococcus lacustris for Biomass and Astaxanthin Production Keys to Commercialization: Perspective and Future Direction

Haematococcus lacustris (formerly called Haematococcus pluvialis) is regarded as the most promising microalgae for the production of natural astaxanthin, which is secondary metabolism used as a dietary supplement, also for cosmetic applications, due to its high anti-oxidant activity. Astaxanthin has a wide range of biological activities and high economic potential, and currently dominates the market in its synthetic form. Furthermore, because of the difficulty of bioprocess and the high cost of cultivation, astaxanthin extracted from this microalga is still expensive due to its low biomass and pigment productivities. Large-scale biomass production in biotechnological production necessitates the processing of a large number of cultures as well as the use of both indoor and outdoor systems, such as open pond raceway systems and photo-bioreactors (PBR). The photo-bioreactors systems are suitable for mass production because growth conditions can be controlled, and the risk of contamination can be reduced to a certain extent and under specific culture parameters. This review discusses current technologies being developed to improve cultivation and operation efficiency and profitability, as well as the effect of parameter factors associated with H. lacustris cultivation on biomass and astaxanthin bioproduction, and even strategies for increasing bioproduction and market potential for H. lacustris astaxanthin.

Multifaceted strategies for economic production of microalgae Haematococcus pluvialis-derived astaxanthin via direct conversion of CO2

Owing to its strong antioxidant properties, astaxanthin has a high market price in the nutraceutical and pharmaceutical fields, and its demand is increasing. Furthermore, with an increase in the demand for green technology, astaxanthin production through direct CO₂ conversion using the autotrophic green microalga Haematococcus pluvialis as a bio-platform has received much attention. Large-scale outdoor cultivation of H. pluvialis using waste CO₂ sources and sunlight can secure sustainability and improve economic efficiency. However, low strain performance, reduced light utilization because of increased cell density, and inefficient transfer of gaseous CO₂ into liquid culture broth hinder its large-scale commercialization of astaxanthin. Herein, we presented a multifaceted strategy, including the development of high-efficiency strains, a culture system for astaxanthin accumulation, and astaxanthin extraction from biomass, for economically producing astaxanthin from H. pluvialis through direct CO₂ conversion. Future perspectives were presented by comparing and analyzing various previous studies conducted using the latest technology.

Metabolism and strategies for enhanced supply of acetyl-CoA in Saccharomyces cerevisiae

Acetyl-CoA is a kind of important cofactor that is involved in many metabolic pathways. It serves as the precursor for many interesting commercial products, such as terpenes, flavonoids and anthraquinones. However, the insufficient supply of acetyl-CoA limits biosynthesis of its derived compounds in the intracellular. In this review, we outlined metabolic pathways involved in the catabolism and anabolism of acetyl-CoA, as well as some important derived products. We examined several strategies for the enhanced supply of acetyl-CoA, and provided insight into pathways that generate acetyl-CoA to balance metabolism, which can be harnessed to improve the titer, yield and productivities of interesting products in Saccharomyces cerevisiae and other eukaryotic microorganisms. We believe that peroxisomal fatty acid β-oxidation could be an attractive strategy for enhancing the supply of acetyl-CoA.

Using green alga Haematococcus pluvialis for astaxanthin and lipid co-production: Advances and outlook

Astaxanthin is one of the secondary carotenoids involved in mediating abiotic stress of microalgae. As an important antioxidant and nutraceutical compound, astaxanthin is widely applied in dietary supplements and cosmetic ingredients. However, most astaxanthin in the market is chemically synthesized, which are structurally heterogeneous and inefficient for biological uptake. Astaxanthin refinery from Haematococcus pluvialis is now a growing industrial sector. H. pluvialis can accumulate astaxanthin to ∼5% of dry weight. As productivity is a key metric to evaluate the production feasibility, understanding the biological mechanisms of astaxanthin accumulation is beneficial for further production optimization. In this review, the biosynthesis mechanism of astaxanthin and production strategies are summarized. The current research on enhancing astaxanthin accumulation and the potential joint-production of astaxanthin with lipids was also discussed. It is conceivable that with further improvement on the productivity of astaxanthin and by-products, the algal-derived astaxanthin would be more accessible to low-profit applications.

Carotenoids from fungi and microalgae: A review on their recent production, extraction, and developments

The demand for carotenoids from natural sources obtained by biological extraction methods is increasing with the development of biotechnology and the continued awareness of food safety. Natural plant-derived carotenoids have a relatively high production cost and are affected by the season, while microbial-derived carotenoids are favored due to their natural, high-efficiency, low production cost, and ease of industrialization. This article reviewed the following aspects of natural carotenoids derived from microorganisms: (1) the structures and properties of main carotenoids; (2) fungal and microalgal sources of the main carotenoids; (3) influencing factors and modes of improvement for carotenoids production; (4) efficient extraction methods for carotenoids; and (5) the commercial value of carotenoids. This review provided a reference and guidance for the development of natural carotenoids derived from microorganisms.

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

BACKGROUND

The 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.

RESULTS

Based 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.

CONCLUSION

The 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.

Integrating Transcriptomics and Metabolomics to Characterize Metabolic Regulation to Elevated CO2 in Chlamydomonas Reinhardtii

With atmospheric CO₂ increasing, a large amount of CO₂ is absorbed by oceans and lakes, which changes the carbonate system and affects the survival of aquatic plants, especially microalgae. The main aim of our study was to explore the responses of Chlamydomonas reinhardtii (Chlorophyceae) to elevated CO₂ by combined transcriptome and metabolome analysis under three different scenarios: control (CK, 400 ppm), short-term elevated CO₂ (ST, 1000 ppm), and long-term elevated CO₂ (LT, 1000 ppm). The transcriptomic data showed moderate changes between ST and CK. However, metabolic analysis indicated that fatty acids (FAs) and partial amino acids (AAs) were increased under ST. There was a global downregulation of genes involved in photosynthesis, glycolysis, lipid metabolism, and nitrogen metabolism but increase in the TCA cycle and β-oxidation under LT. Integrated transcriptome and metabolome analyses demonstrated that the nutritional constituents (FAs, AAs) under LT were poor compared with CK, and most genes and metabolites involved in C and N metabolism were significantly downregulated. However, the growth and photosynthesis of cells under LT increased significantly. Thus, C. reinhardtii could form a specific adaptive evolution to elevated CO₂, affecting future biogeochemical cycles.

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.

Simultaneous promotion of photosynthesis and astaxanthin accumulation during two stages of Haematococcus pluvialis with ammonium ferric citrate

To simultaneously promote biomass yield and astaxanthin content of Haematococcus pluvialis, ammonium ferric citrate (AFC) was employed to stimulate light harvest in photosynthesis during the green stage and oxidation induction in astaxanthin accumulation during the red stage. AFC not only improved chlorophyll synthesis by 22.5% to provide more electrochemical potential energy in the green stage, but also alleviated photosystem II damage to maintain a high level of effective quantum yield by enhancing carotenoid production. The citrate derived from AFC stimulated acetyl-CoA and NADPH production through citric acid cycle and transaminase cycle during the red stage, resulting in an increased lipid content by 1.77-fold. The astaxanthin content in H. pluvialis cells cultivated with 5 μM AFC was 12.5% higher than that without AFC, which was attributed to severe oxidative stress caused by AFC through Haber-Weiss reaction. These results provided a new approach to reduce emission of greenhouse gasses with producing high-value products.

An Integrated Strategy for Nutraceuticals from Haematoccus pluvialis: From Cultivation to Extraction

The aim of this study was to develop an effective integrated cultivation system for Haematococcus pluvialis as a source of bioactive compounds such as astaxanthin, lutein, proteins, and fatty acids (FAs). The Chlorophyta H. pluvialis was cultivated in a vertical bubble column photobioreactor (VBC-PBR) under batch mode, allowing switching from green to red phase for astaxanthin induction. The combined effect of light intensity and nutrients on bioactive compound formation was investigated. Results showed that growth under lower nutrients availability and light intensity led to a higher concentration of biomass. Growth under high light intensity with an appropriate concentration of nitrate, sulfate, phosphate and magnesium led to ~85% and ~58% higher production of total carotenoids and fatty acids, respectively. Under high stress conditions, ~90% nitrate and phosphate consumption were observed.

Astaxanthin accumulation difference between non-motile cells and akinetes of Haematococcus pluvialis was affected by pyruvate metabolism

Background

Haematococcus pluvialis is the best source of natural astaxanthin, known as the king of antioxidants. H. pluvialis have four cell forms: spore, motile cell, non-motile cell and akinete. Spores and motile cells are susceptible to photoinhibition and would die under photoinduction conditions. Photoinduction using non-motile cells as seeds could result in a higher astaxanthin production than that using akinetes. However, the mechanism of this phenomenon has not been clarified.

Results

Transcriptome was sequenced and annotated to illustrate the mechanism of this phenomenon. All differentially expressed genes involved in astaxanthin biosynthesis were up-regulated. Particularly, chyb gene was up-regulated by 16-fold, improving the conversion of β-carotene into astaxanthin. Pyruvate was the precursor of carotenoids biosynthesis. Pyruvate kinase gene expression level was increased by 2.0-fold at the early stage of akinetes formation. More changes of gene transcription occurred at the early stage of akinetes formation, 52.7% and 51.9% of total DEGs in control group and treatment group, respectively.

Conclusions

Genes transcription network was constructed and the synthesis mechanism of astaxanthin was clarified. The results are expected to further guide the in-depth optimization of the astaxanthin production process in H. pluvialis by improving pyruvate metabolism.

Transcriptome Analysis Reveals the Molecular Mechanisms Underlying Adenosine Biosynthesis in Anamorph Strain of Caterpillar Fungus

Caterpillar fungus is a well-known fungal Chinese medicine. To reveal molecular changes during early and late stages of adenosine biosynthesis, transcriptome analysis was performed with the anamorph strain of caterpillar fungus. A total of 2,764 differentially expressed genes (DEGs) were identified (p ≤ 0.05, |log₂ Ratio| ≥ 1), of which 1,737 were up-regulated and 1,027 were down-regulated. Gene expression profiling on 4–10 d revealed a distinct shift in expression of the purine metabolism pathway. Differential expression of 17 selected DEGs which involved in purine metabolism (map00230) were validated by qPCR, and the expression trends were consistent with the RNA-Seq results. Subsequently, the predicted adenosine biosynthesis pathway combined with qPCR and gene expression data of RNA-Seq indicated that the increased adenosine accumulation is a result of down-regulation of ndk, ADK, and APRT genes combined with up-regulation of AK gene. This study will be valuable for understanding the molecular mechanisms of the adenosine biosynthesis in caterpillar fungus.

Biocompatible liquid-type carbon nanodots (C-paints) as light delivery materials for cell growth and astaxanthin induction of Haematococcus pluvialis

In this study, we aimed to demonstrate the feasibility of the application of biocompatible liquid type fluorescent carbon nanodots (C-paints) to microalgae by improving microalgae productivity. C-paints were prepared by a simple process of ultrasound irradiation using polyethylene glycol (PEG) as a passivation agent. The resulting C-paints exhibited a carbonyl-rich surface with good uniformity of particle size, excellent water solubility, photo-stability, fluorescence efficiency, and good biocompatibility (<10.0 mg mL^-1 of C-paints concentration). In the practical application of C-paints to microalgae culture, the most effective and optimized condition leading to growth promoting effect was observed at a C-paints concentration of 1.0 mg mL^-1 (>20% higher than the control cell content). A C-paints concentration of 1-10.0 mg mL^-1 induced an approximately >1.8 times higher astaxanthin content than the control cells. The high light delivery effect of non-cytotoxic C-paints was applied as a stress condition for H. pluvialis growth and was found to play a major role in enhancing productivity. Notably, the results from this study are an essential approach to improve astaxanthin production, which can be used in various applications because of its therapeutic effects such as cancer prevention, anti-inflammation, immune stimulation, and treatment of muscle-soreness.

Molecular cloning and functional analysis of a Δ12-fatty acid desaturase from the Antarctic microalga Chlamydomonas sp. ICE-L

Chlamydomonas sp. ICE-L, which can thrive in extreme environments of the Antarctic, could represent a promising alternative for polyunsaturated fatty acid (PUFA) production. A new Δ¹²-fatty acid desaturase (FAD)-encoding gene (Δ ¹² CiFAD), 1269 bp in size, was cloned from Chlamydomonas sp. ICE-L. Bioinformatics analysis showed that Δ ¹² CiFAD-encoded protein was homologous to known FADs with conserved histidine motifs, and localized to the chloroplast. Functional analysis of Δ ¹² CiFAD indicated that recombinant Synechococcus 6803 expressing Δ¹²CiFAD could accumulate C18:2, whereas recombinant Saccharomyces cerevisiae expressing this enzyme could not accumulate C18:2 or any other new fatty acids. These results indicate that Δ¹²CiFAD is a functional enzyme in the chloroplast that can adjust Chlamydomonas sp. ICE-L cell membrane fluidity to adapt to Antarctic extreme low-temperature environments, which give us insights into the frigostable and cold-resistant mechanisms of hypothermic organisms.

Bench-Scale Cultivation of Microalgae Scenedesmus almeriensis for CO2 Capture and Lutein Production

In this study, Scenedesmus almeriensis as green microalga was cultivated on bench-scale for carbon dioxide (CO2) capture and lutein production. The autotrophic cultivation of S. almeriensis was carried out by using a vertical bubble column photo-bioreactor (VBC-PBR) with a continuous flow of a gaseous mixture of oxygen (O2), nitrogen (N2), and CO2, the latter in content of 0.0–3.0 %v/v. The liquid phase was batch. S. almeriensis growth was optimized. In addition, lutein extraction was carried out by using accelerated solvent extraction with ethanol as Generally Recognized as Safe (GRAS) solvent at 67 °C and 10 MPa. Upon optimization of CO2 concentration, the maximum biomass productivity, equal to 129.24 mg·L−1·d−1, was achieved during the cultivation by using a content of CO2 equal to 3.0 %v/v and it allowed to obtain a lutein content of 8.54 mg·g−1, which was 5.6-fold higher in comparison to the analogous process carried out without CO2 addition. The ion chemical analysis in the growth medium showed that by gradually increasing CO2 content, the nutrient consumption during the growth phase also increased. This study may be of potential interest for lutein extraction at industrial scale, since it is focused on pigment production from a natural source with a concomitantly CO2 capture.

Transcriptional Analysis of Chlorella Pyrenoidosa Exposed to Bisphenol A

Bisphenol A (BPA) is the raw material of 71% of polycarbonate-based resins and 27% of epoxy-based resins which are used for coating metal-based food and beverage cans. Meanwhile, it is taken into account as a typical environmental pollutant. Hormesis may occur in algae exposed to BPA. In this study, the effects of BPA on Chlorella pyrenoidosa were assessed based on growth inhibition and transcriptome analysis. We have focused on two exposure scenarios as follows: (1) exposure to a low stimulation concentration (0.1 mg.L⁻¹, 19.35% promotion in cell density on the 3rd day); (2) exposure to a high inhibition concentration (10 mg.L⁻¹, 64.71% inhibition in cell density on the 3rd day). Transcriptome analysis showed enrichment in nucleotide transport, single-organism transport, cellular respiration. Among them, adenosine triphosphate (ATP) synthase and Nicotinamide adenine dinucleotide (NADH) dehydrogenase were upregulated under 0.1 mg.L⁻¹ BPA treatment. These changes enhanced the physiological and energy metabolic pathways of C. pyrenoidosa, thereby stimulating cell proliferation. At exposure to the high BPA, severe inhibited changes in the expression levels of several pathways were observed, which were related to tricarboxylic acid (TCA) cycle, glycolysis, fatty acid metabolism, oxidative phosphorylation, and photosynthesis. Therefore, BPA could negatively affect growth inhibition through the multiple energy metabolism processes. These results may result in a deeper insight into BPA-induced biphasic responses in algae, and provide vital information to assess the potential ecological risks of exposure to BPA in an aquatic ecosystem.

Comparative transcriptome analysis of a long-time span two-step culture process reveals a potential mechanism for astaxanthin and biomass hyper-accumulation in Haematococcus pluvialis JNU35

BACKGROUND

Among all organisms tested, Haematococcus pluvialis can accumulate the highest levels of natural astaxanthin. Nitrogen starvation and high irradiance promote the accumulation of starch, lipid, and astaxanthin in H. pluvialis, yet their cell division is significantly retarded. Accordingly, adaptive regulatory mechanisms are very important and necessary to optimize the cultivation conditions enabling an increase in biomass; as well as promoting astaxanthin accumulation by H. pluvialis. To clarify the intrinsic mechanism of high-level astaxanthin and biomass accumulation in the newly isolated strain, H. pluvialis JNU35, nitrogen-sufficiency and nitrogen-depletion conditions were employed. Time-resolved comparative transcriptome analysis was also conducted by crossing the two-step culture process.

RESULTS

In the present study, we report the overall growth and physiological, biochemical, and transcriptomic characteristics of H. pluvialis JNU35 in response to nitrogen variation. From eight sampling time-points (2 days, 4 days, 8 days, 10 days, 12 days, 14 days, 16 days, and 20 days), 25,480 differentially expressed genes were found. These genes included the significantly responsive unigenes associated with photosynthesis, astaxanthin biosynthesis, and nitrogen metabolic pathways. The expressions of all key and rate-limiting genes involved in astaxanthin synthesis were significantly upregulated. The photosynthetic pathway was found to be attenuated, whereas the ferredoxin gene was upregulated, which might activate the cyclic electron-transport chain as compensation. Moreover, the expressions of genes related to nitrogen transport and assimilation were upregulated. The expressions of genes in the proteasome pathway were also upregulated. In contrast, the chloroplasts and nonessential proteins were gradually degraded, activating the specific ornithine-urea cycle pathway. These changes may promote the sustained accumulation of astaxanthin and biomass.

CONCLUSIONS

To the best of our knowledge, this paper is the first to investigate the long-term differences of gene expression from two-step culture process in the astaxanthin producer, H. pluvialis JNU35. According to our results, β-carotene ketolase (bkt1 and bkt2) serves as the key enzyme regulating astaxanthin accumulation in H. pluvialis JNU35. The cyclic electron-transport chain and novel nitrogen metabolic process were used adaptively as the regulatory mechanism compensating for different levels of stress. The in-depth study of these metabolic pathways and related key genes can reveal the underlying relationship between cell growth and astaxanthin accumulation in H. pluvialis JNU35.

High-value bioproducts from microalgae: Strategies and progress

Microalgae have been considered as alternative sustainable resources for high-value bioproducts such as lipids (especially triacylglycerides [TAGs]), polyunsaturated fatty acids (PUFAs), and carotenoids, due to their relatively high photosynthetic efficiency, no arable land requirement, and ease of scale-up. It is of great significance to exploit microalgae for the production of high-value bioproducts. How to improve the content or productivity of specific bioproducts has become one of the most urgent challenges. In this review, we will describe high-value bioproducts from microalgae and their biosynthetic pathways (mainly for lipids, PUFAs, and carotenoids). Recent progress and strategies for the enhanced production of bioproducts from microalgae are also described in detail, and these strategies take advantages of optimized cultivation conditions with abiotic stress, chemical stress (addition of metabolic precursors, phytohormones, chemical inhibitors, and chemicals inducing oxidative stress response), and molecular approaches such as metabolic engineering, transcriptional engineering, and gene disruption strategies (mainly RNAi, antisense RNA, miRNA-based knockdown, and CRISPR/Cas9).

Molecular characterization of CO2 sequestration and assimilation in microalgae and its biotechnological applications

Microalgae are renewable feedstock for sustainable biofuel production, cell factory for valuable chemicals and promising in alleviation of greenhouse gas CO₂. However, the carbon assimilation capacity is still the bottleneck for higher productivity. Molecular characterization of CO₂ sequestration and assimilation in microalgae has advanced in the past few years and are reviewed here. In some cyanobacteria, genes for 2-oxoglytarate dehydrogenase was replaced by four alternative mechanisms to fulfill TCA cycle. In green algae Coccomyxa subellipsoidea C-169, alternative carbon assimilation pathway was upregulated under high CO₂ conditions. These advances thus provide new insights and new targets for accelerating CO₂ sequestration rate and enhancing bioproduct synthesis in microalgae. When integrated with conventional parameter optimization, molecular approach for microalgae modification targeting at different levels is promising in generating value-added chemicals from green algae and cyanobacteria efficiently in the near future.

In vivo kinetics of lipids and astaxanthin evolution in Haematococcus pluvialis mutant under 15% CO2 using Raman microspectroscopy

In vivo spatiotemporal dynamics of lipids and astaxanthin evolution in Haematococcus pluvialis mutant induced with 15% CO₂ and high light intensity were monitored with high spatial resolution in a non-destructive and label-free manner using single-cell Raman imaging. Astaxanthin intensity increased by 3.5 times within 12h under 15% CO₂, and the accumulation rate was 5.8 times higher than that under air. Lipids intensity under 15% CO₂ was 27% higher than that under air. The lipids initially concentrated in chloroplast under 15% CO₂ due to an increase of directly photosynthetic fatty acid, which was different from the whole-cell dispersed lipids under air. Astaxanthin produced in chloroplast first accumulated around nucleus and then spread in cytoplasmic lipids under both air and 15% CO₂. The calculation results of kinetic models for lipids and astaxanthin evolutions showed that accumulation rate of lipids was much higher than that of astaxanthin in cells.

Transcriptome-based analysis on carbon metabolism of Haematococcus pluvialis mutant under 15% CO2

To elucidate the mechanism underlying the enhanced growth rate in the Haematococcus pluvialis mutated with ⁶⁰Co-γ rays and domesticated with 15% CO₂, transcriptome sequencing was conducted to clarify the carbon metabolic pathways of mutant cells. The CO₂ fixation rate of mutant cells increased to 2.57gL^-1d^-1 under 15% CO₂ due to the enhanced photosynthesis, carbon fixation, glycolysis pathways. The upregulation of PetH, ATPF0A and PetJ related to photosynthetic electron transport, ATP synthase and NADPH generation promoted the photosynthesis. The upregulation of genes related to Calvin cycle and ppdK promoted carbon fixation in both C3 and C4 photosynthetic pathways. The reallocation of carbon was also enhanced under 15% CO₂. The 19-, 14- and 3.5-fold upregulation of FBA, TPI and PK genes, respectively, remarkably promoted the glycolysis pathways. This accelerated the conversion of photosynthetic carbon to pyruvate, which was an essential precursor for astaxanthin and lipids biosynthesis.