Comparative physiological and metabolomic analyses of the hyper-accumulation of astaxanthin and lipids in Haematococcus pluvialis upon treatment with butylated hydroxyanisole

Regulatory mechanisms of autophagy on DHA and carotenoid accumulation in Crypthecodinium sp. SUN

BACKGROUND

Autophagy is a crucial process of cellular self-destruction and component reutilization that can affect the accumulation of total fatty acids (TFAs) and carotenoids in microalgae. The regulatory effects of autophagy process in a docosahexaenoic acid (DHA) and carotenoids simultaneously producing microalga, Crypthecodinium sp. SUN, has not been studied. Thus, the autophagy inhibitor (3-methyladenine (MA)) and activator (rapamycin) were used to regulate autophagy in Crypthecodinium sp. SUN.

RESULTS

The inhibition of autophagy by 3-MA was verified by transmission electron microscopy, with fewer autophagy vacuoles observed. Besides, 3-MA reduced the glucose absorption and intracellular acetyl-CoA level, which resulting in the decrease of TFA and DHA levels by 15.83 and 26.73% respectively; Surprisingly, 3-MA increased intracellular reactive oxygen species level but decreased the carotenoids level. Comparative transcriptome analysis showed that the downregulation of the glycolysis, pentose phosphate pathway and tricarboxylic acid cycle may underlie the decrease of acetyl-CoA, NADPH and ATP supply for fatty acid biosynthesis; the downregulation of PSY and HMGCR may underlie the decreased carotenoids level. In addition, the class I PI3K-AKT signaling pathway may be crucial for the regulation of carbon and energy metabolism. At last, rapamycin was used to activate autophagy, which significantly enhanced the cell growth and TFA level and eventually resulted in 1.70-fold increase in DHA content.

CONCLUSIONS

Our findings indicate the mechanisms of autophagy in Crypthecodinium sp. SUN and highlight a way to manipulate cell metabolism by regulating autophagy. Overall, this study provides valuable insights to guide further research on autophagy-regulated TFA and carotenoids accumulation in Crypthecodinium sp. SUN.

Dual Roles of Two Malic Enzymes in Lipid Biosynthesis and Salt Stress Response in Dunaliella salina

Triacylglycerols (TAG) from microalgae can be used as feedstocks for biofuel production to address fuel shortages. Most of the current research has focused on the enzymes involved in TAG biosynthesis. In this study, the effects of malic enzyme (ME), which provides precursor and reducing power for TAG biosynthesis, on biomass and lipid accumulation and its response to salt stress in Dunaliella salina were investigated. The overexpression of DsME1 and DsME2 improved the lipid production, which reached 0.243 and 0.253 g/L and were 30.5 and 36.3% higher than wild type, respectively. The transcript levels of DsME1 and DsME2 increased with increasing salt concentration (0, 1, 2, 3, and 4.5 mol/L NaCl), indicating that DsMEs participated in the salt stress response in D. salina. It was found that cis-acting elements associated with the salt stress response were present on the promoters of two DsMEs. The deletion of the MYB binding site (MBS) on the DsME2 promoter confirmed that MBS drives the expression of DsME2 to participate in osmotic regulation in D. salina. In conclusion, MEs are the critical enzymes that play pivotal roles in lipid accumulation and osmotic regulation.

Comprehensive understanding of the regulatory mechanism by which selenium nanoparticles boost CO2 fixation and cadmium tolerance in lipid-producing green algae under recycled medium

Recycled medium plus cadmium is a promising technique for reducing the cultivation cost and enhancing the yield of microalgae lipids. However, oxidative stress and cadmium toxicity significantly hinder the resulting photosynthetic efficiency, cell growth and cell activity. Herein, selenium nanoparticles (SeNPs) were used to increase the total biomass, biolipid productivity, and tolerance to cadmium. Wide-ranging analyses of photosynthesis, energy yield, fatty acid profiles, cellular ultrastructure, and oxidative stress biomarkers were conducted to examine the function of SeNPs in CO2 fixation and cadmium resistance in Ankistrodesmus sp. EHY. The application of 15 μM cadmium and 2 mg L-1 SeNPs further enhanced the algal biomass productivity and lipid productivity to 500.64 mg L-1 d-1 and 301.14 mg L-1 d-1, respectively. Moreover, the rates of CO2 fixation, chlorophyll synthesis and total nitrogen removal were similarly increased by the application of SeNPs. Exogenous SeNPs strengthened cell growth and cadmium tolerance by upregulating photosynthesis, the TCA cycle and the antioxidant system, reducing the uptake and translocation of cadmium, and decreasing the levels of reactive oxidative stress (ROS), extracellular polymeric substances (EPSs) and cellular Cd2+ level in EHY under recycled medium and cadmium stress conditions. Additionally, a maximum energy yield of 127.40 KJ L-1 and a lipid content of 60.15% were achieved in the presence of both SeNPs and cadmium stress. This study may inspire the efficient disposal of recycled medium and biolipid production while also filling the knowledge gaps regarding the mechanisms of SeNP functions in carbon fixation and cadmium tolerance in microalgae.

Metabolomic analysis reveals astaxanthin biosynthesis in heterotrophic microalga Chromochloris zofingiensis

The utilization of gibberellic acid-3, high carbon/nitrogen ratio and salinity concentration can effectively enhance astaxanthin biosynthesis in Chromochloris zofingiensis under the heterotrophic conditions, but the underlying mechanisms remained yet to be investigated. The metabolomics analysis revealed that enhancement of the glycolysis, pentose phosphate pathways (PPP), and tricarboxylic acid (TCA) cycle led to astaxanthin accumulation under the induction conditions. The increased fatty acids can significantly increase astaxanthin esterification. The addition of appropriate concentrations of glycine (Gly) and γ-aminobutyric acid (GABA) promoted astaxanthin biosynthesis in C. zofingiensis, as well as benefiting for biomass yield. With the addition of 0.5 mM GABA, the astaxanthin yield increased to 0.35 g·L^-1, which was 1.97-fold higher than that of the control. This study advanced understanding about astaxanthin biosynthesis in heterotrophic microalga, and provided novel strategies for enhanced astaxanthin production in C. zofingiensis.

Improving the growth of Spirulina in CO2 absorption and microalgae conversion (CAMC) system through mixotrophic cultivation: Reveal of metabolomics

Mixotrophic cultivation was proposed to enhance the biomass and carbon sequestration efficiency of Spirulina in CO₂ absorption and microalgae conversion (CAMC) system, and the underlying metabolic mechanism was also explored. The result showed that mixotrophic enhanced the performance of CAMC system, the maximum biomass, total carbon conversion capacity and efficiency was obtained at 0.5 g/L acetate group, which was 60.47 %, 63.06 % and 59.77 % higher than control. Adding 0.5 g/L acetate enhanced the activities of Rubisco and Acetyl-CoA, arrived at 89.59 U/g and 5.16 nmol/g, respectively. Metabolomics analyses suggested that mixotrophic changed metabolic flux and affected intracellular composition. Mixotrophic up-regulated Calvin cycle, glycolysis, and tricarboxylic acid (TCA) cycle, induced more carbon fluxes into central carbon metabolism for the growth of Spirulina. These results suggested that mixotrophic could supply effective energy and carbon skeleton for rapid growth of Spirulina, and provided a theoretical basis for large-scale application of CAMC system.

Impacts of a bacterial algicide on metabolic pathways in Chlorella vulgaris

Chlorella is a dominant species during harmful algal blooms (HABs) worldwide, which bring about great environmental problems and are also a serious threat to drinking water safety. Application of bacterial algicides is a promising way to control HABs. However, the identified bacterial algicides against Chlorella and the understanding of their effects on algal metabolism are very limited. Here, we isolated a novel bacterium Microbacterium paraoxydans strain M1 that has significant algicidal activities against Chlorella vulgaris (algicidal rate 64.38 %, at 120 h). Atrazine-desethyl (AD) was then identified from strain M1 as an effective bacterial algicide, with inhibition or algae-lysing concentration values (EC50) of 1.64 μg/mL and 1.38 μg/mL, at 72 h and 120 h, respectively. LAD (2 μg/mL AD) or HAD (20 μg/mL AD) causes morphology alteration and ultrastructure damage, chlorophyll a reduction, gene expression regulation (for example, psbA, 0.05 fold at 24 h, 2.97 fold at 72 h, and 0.23 fold of the control in HAD), oxidative stress, lipid oxidation (MDA, 2.09 and 3.08 fold of the control in LAD and HAD, respectively, at 120 h) and DNA damage (average percentage of tail DNA 6.23 % at 120 h in HAD, slight damage: 5∼20 %) in the algal cells. The impacts of AD on algal metabolites and metabolic pathways, as well as the algal response to the adverse effects were investigated. The results revealed that amino acids, amines, glycosides and urea decreased significantly compared to the control after 24 h exposure to AD (p < 0.05). The main up-regulated metabolic pathways implied metabonomic resistance and defense against osmotic pressure, oxidative stress, photosynthesis inhibition or partial cellular structure damage, such as phenylalanine metabolism, arginine biosynthesis. The down-regulated glycine, serine and threonine metabolism is a major lead in the algicidal mechanism according to the value of pathway impact. The down-regulated glycine, and serine are responsible for the downregulation of glyoxylate and dicarboxylate metabolism, aminoacyl-tRNA biosynthesis, glutathione metabolism, and sulfur metabolism, which strengthen the algae-lysing effect. It is the first time to highlight the pivotal role of glycine, serine and threonine metabolism in algicidal activities, which provided a new perspective for understanding the mechanism of bacterial algicides exerting on algal cells at the metabolic level.

Sodium chloride stimulates the biomass and astaxanthin production by Haematococcus pluvialis via a two-stage cultivation strategy

A major challenge facing by astaxanthin industrialization is the low productivity and high production costs. This study established a two-stage cultivation strategy based on the application of NaCl to improve the production of biomass and astaxanthin by Haematococcus pluvialis. During the first growth stage, 12.5 mg L^-1 NaCl led to a remarkable enhancement in biomass, which was 1.28 times compared with the control. Moreover, 2 g L^-1 NaCl stimulated the astaxanthin content from 12.18 mg g^-1 to 25.92 mg g^-1 during the second induction stage. Simultaneously, salinity stress application increased the lipids and GABA contents, as well as the levels of Ca²⁺ and carotenogenic genes' expression, but suppressed the contents of carbohydrate and protein and high-light induced-ROS. This study proposed a simple and convenient strategy for efficient coproduction of biomass and astaxanthin and provides insights into the underlying mechanism of astaxanthin biosynthesis in H. pluvialis induced by salinity stress.

Enhancing astaxanthin and lipid coproduction in Haematococcus pluvialis by the combined induction of plant growth regulators and multiple stresses

Coupling chemical induction and abiotic stresses is a beneficial strategy for astaxanthin (Asta) induction in Haematococcus pluvialis. The combined application of melatonin (MT) and putrescine (Put) induced Asta and lipid biosynthesis in H. pluvialis under adverse conditions. Under MT and Put inductions, the highest Asta and lipid contents were 3.64% and 55.84%, which were 1.71- and 1.17-times higher than the control group, respectively. The combination of MT and Put also enhanced the expression of carotenogenic, lipogenic and antioxidant enzyme genes. Additionally, this combined treatment increased the endogenous Put content while decreasing the reactive oxygen species (ROS) and γ-aminobutyric acid (GABA) levels. Further results proved that endogenous Put promoted Asta production and alleviated oxidative stress by regulating carotenogenesis and GABA and ROS signaling. This study describes a potential process for stimulating Asta and lipid coproduction and highlights the connections among MT, Put, signaling molecules, Asta and lipid synthesis in H. pluvialis.

An integrated approach for the efficient separation of specialty compounds from biomass of the marine microalgae Amphidinium carterae

An amphidinol-prioritized fractioning approach was for the first time developed to isolate multiple specialty metabolites such as amphidinols, carotenoids and fatty acids using the biomass of the marine microalgae Amphidinium carterae. The biomass was produced in a raceway photobioreactor and the exhausted culture media were reused, thus fulfilling sustainability criteria employing a circular economy concept. The integrated bioactive compounds-targeted approach presented here consisted of four steps with which recovery percentages of carotenoids, fatty acids and amphidinols of 97%, 82% and 99 %, respectively, were achieved. The proposed process was proved to be a better extraction system for this microalga than another based on a sequential gradient partition with water and four water-immiscible organic solvents (hexane, carbon tetrachloride, dichloromethane and n-butanol). The proposed process could be scaled-up as a commercial solid-phase extraction technology well-established for industrial bioprocesses.

A fed-batch feeding with succinic acid strategy for astaxanthin and lipid hyper-production in Haematococcus pluviualis

Effects of succinic acid (SA) in fed-batch feeding mode on astaxanthin and lipids biopoduction of Haematococcus pluvialis against abiotic stresses were explored. By comparison with the control, the initial addition of SA on day 0 increased the production of astaxanthin by 71.61%. More importantly, the maximum values of astaxanthin (35.88 mg g^-1) and lipid (54.79%) contents were obtained after supplementation of SA on day 7. Meanwhile, under SA treatment, the chlorophyll, carbohydrate, and protein levels were reduced, but the intracellular levels of SA and reactive oxygen species (ROS), the transcription levels of astaxanthin and fatty acids biosynthesis-, and antioxidant system-related genes were increased. Furthermore, scaling-up cultivation in bioreactor further enhanced the astaxanthin productivity from H. pluvialis. Generally, this study proved the intermittent SA feeding method in fed-batch culture as a potent strategy that facilitated massive astaxanthin and lipids production in algae.

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.

New paradigm in diatom omics and genetic manipulation

Diatoms are one of the most heterogeneous eukaryotic plankton known for regulating earth's biogeochemical cycles and maintaining the marine ecosystems ever since the late Eocene epoch. The advent of multidisciplinary omics approach has both epitomized and revolutionized the nature of their chimeric genetic toolkit, ecophysiology, and metabolic adaptability as well as their interaction with other communities. In addition, advanced functional annotation of transcriptomic and proteomic data using cutting edge bioinformatics tools together with high-resolution genome-scale mathematical modeling has effectively proven as the catapult in solving genetic bottlenecks in microbial as well as diatom exploration. In this review, a corroborative summation of the robust work done in manipulating, engineering, and sequencing of the diatom genomes besides underpinning the holistic application of omics in transcription and translation has been discussed in order to shrewd their multifarious novel potential in the field of biotechnology and provide an insight into their dynamic evolutionary relevance.

Gamma-aminobutyric acid facilitates the simultaneous production of biomass, astaxanthin and lipids in Haematococcus pluvialis under salinity and high-light stress conditions

The effects of γ-aminobutyric acid (GABA) on the biomass and astaxanthin and lipids production in Haematococcus pluvialis under combined salinity stress and high-light stresses were investigated. The results showed that the highest biomass (1.65 g L^-1), astaxanthin production (3.86 mg L^-1 d^-1) and lipids content (55.11%) in H. pluvialis LUGU were observed under the 0.25 mM GABA treatment. Moreover, compared with salinity and high-light stress, GABA treatment also increased the transcript levels of biosynthesis genes, the contents of endogenous GABA and carbohydrates but decreased reactive oxygen species (ROS) levels. Further evidence revealed that intracellular GABA could regulate cell growth, astaxanthin production and lipids synthesis by mediating carotenogenesis, lipogenesis and ROS signalling. Collectively, this study provides a combined strategy for promoting the coproduction of astaxanthin and lipids and sheds light on the regulatory mechanism through which GABA affects cell growth, astaxanthin production and lipids biosynthesis in H. pluvialis under unfavourable conditions.

Integration of physiological and metabolomic profiles to elucidate the regulatory mechanisms underlying the stimulatory effect of melatonin on astaxanthin and lipids coproduction in Haematococcus pluvialis under inductive stress conditions

The effect of melatonin (MT) on the coproduction of astaxanthin and lipids was studied in Haematococcus pluvialis under inductive stress conditions. The contents of astaxanthin and lipids were enhanced by 1.78- and 1.3-fold, respectively. MT treatment upregulated the transcription levels of carotenogenic, lipogenic and antioxidant system-related genes and decreased the levels of abiotic stress-induced reactive oxidative species (ROS). Further metabolomic analysis suggested that the intermediates in glycolysis and TCA cycle facilitate the accumulation of astaxanthin and lipids in algae treated with MT. Meanwhile, MT treatment upregulated the metabolite levels of the γ-aminobutyric acid (GABA) shunt, which might regulate the carbon-nitrogen balance and the antioxidant system. After MT treatment, exogenous linoleic acid, succinate, and GABA further increased the astaxanthin content. This study may help to elucidate the specific responses to MT induction in H. pluvialis and to identify novel biomarkers that may be employed to further promote astaxanthin and lipids coproduction.

Reprogramming microorganisms for the biosynthesis of astaxanthin via metabolic engineering

There is an increasing demand for astaxanthin in food, feed, cosmetics and pharmaceutical applications because of its superior anti-oxidative and coloring properties. However, naturally produced astaxanthin is expensive, mainly due to low productivity and limited sources. Reprogramming of microorganisms for astaxanthin production via metabolic engineering is a promising strategy. We primarily focus on the application of synthetic biology, enzyme engineering and metabolic engineering in enhancing the synthesis and accumulation of astaxanthin in microorganisms in this review. We also discuss the biosynthetic pathways of astaxanthin within natural producers, and summarize the achievements and challenges in reprogramming microorganisms for enhancing astaxanthin production. This review illuminates recent biotechnological advances in microbial production of astaxanthin. Future perspectives on utilization of new technologies for boosting microbial astaxanthin production are also discussed.

Biotechnological production of astaxanthin from the microalga Haematococcus pluvialis

Although biotechnologies for astaxanthin production from Haematococcus pluvialis have been developed for decades and many production facilities have been established throughout the world, the production cost is still high. This paper is to evaluate the current production processes and production facilities, to analyze the R&D strategies for process improvement, and to review the recent research advances shedding light on production cost reduction. With these efforts being made, we intent to conclude that the production cost of astaxanthin from Haematococcus might be substantially reduced to the levels comparable to that of chemical astaxanthin through further R&D and the future research might need to focus on strain selection and improvement, cultivation process optimization, innovation of cultivation methodologies, and revolution of extraction technologies.

Melatonin and calcium act synergistically to enhance the coproduction of astaxanthin and lipids in Haematococcus pluvialis under nitrogen deficiency and high light conditions

This study focused on the influence of integrating melatonin (MT) and calcium (Ca2+) on the simultaneous accumulation of astaxanthin and lipids in Haematococcus pluvialis under abiotic stress conditions. Compared with the control condition, MT induction enhanced astaxanthin and lipid contents by 65.89% and 27.38%, respectively. The highest contents of astaxanthin and lipids under combined exposure to MT and Ca2+ were 3.8% and 49.53%, respectively, which were 1.13- and 1.21-fold higher than those of cells treated with MT alone. The application of MT and Ca2+ also promoted the expression of carotenogenic and lipogenic genes and increased the levels of Ca2+ and γ-aminobutyric acid (GABA) but decreased reactive oxygen species (ROS) levels. Further evidence indicated that the increased cellular Ca2+ could promote astaxanthin biosynthesis under MT induction by regulating carotenogenic gene levels and GABA and ROS signalling. The integrated strategy efficiently improved the coproduction of astaxanthin and lipids in H. pluvialis.

Morphological Change and Cell Disruption of Haematococcus pluvialis Cyst during High-Pressure Homogenization for Astaxanthin Recovery

Haematococcus pluvialis accumulates astaxanthin, which is a high-value antioxidant, during the red cyst stage of its lifecycle. The development of a rigid cell wall in the cysts hinders the recovery of astaxanthin. We investigated morphological changes and cell disruption of mature H. pluvialis cyst cells while using high-pressure homogenization for astaxanthin extraction. When treated with French-press-cell (pressure, 10,000–30,000 psi; passage, 1–3), the intact cyst cells were significantly broken or fully ruptured, releasing cytoplasmic components, thereby facilitating the separation of astaxanthin by ethyl acetate. Fluorescence microscopy observations using three different fluorescent dyes revealed that a greater degree of cell breakage caused greater external dispersion of astaxanthin, chlorophyll, lipids, proteins, and carbohydrates. The mechanical treatment resulted in a high cell disruption rate of up to 91% based on microscopic cell typing and Coulter methods. After the ethyl acetate extraction, the astaxanthin concentration significantly increased by 15.2 mg/L in proportion to the increase in cell disruption rate, which indicates that cell disruption is a critical factor for solvent-based astaxanthin recovery. Furthermore, this study recommends a synergistic combination of the fast instrumental particle-volume-distribution analysis and microscope-based morphologic phenotyping for the development of practical H. pluvialis biorefinery processes that co-produce various biological products, including lipids, proteins, carbohydrates, chlorophyll, and astaxanthin.