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In the present research, the freshwater green alga Ankistrodesmus sp. EHY was isolated from Erhai Lake. To obtain high productivity of biomass and lipids, a joint strategy comprising salinity stress and chemical induction and two-stage cultivation was developed to improve the production of lipids. Coupling salinity stress and gamma-aminobutyric acid (GABA) or CaCl2 addition increased the lipid content to 56.47 ± 1.56% or 59.52 ± 0.00%, which was 1.24 or 1.31 times greater than that of the control, respectively (p

Microalgae-based wastewater treatment can fulfill the dual roles for biofuel production and wastewater bioremediation. The present study focused on the effects of myo-inositol (MI) on the lipid production of Monoraphidium sp. QLY-1 and nutrients removal of molasses wastewater (MW). The highest microalgal lipid productivity (134.70 mg L−1 d−1) was achieved under 0.5 g L−1 MI treatment, which was 1.35-fold higher than the pure MW group. Besides, the nutrients and metals removal from MW were significantly elevated after applying MI. Mechanism exploration revealed that the algal intracellular protein content, glutathione (GSH) level, and the expressions of key lipogenic genes were significantly elevated, while the carbohydrate content and reactive oxygen species (ROS) level were suppressed in the presence of MI, which provided more precursors and a balanced redox condition for lipid biosynthesis. More importantly, the phytohormones abscisic acid (ABA) and ethylene (ET) contents were significantly improved by MI, which was likely responsible for the promotion of lipid accumulation. This research suggests that MI is a promising stimulant for enhancing microalgal lipid production and wastewater nutrients removal, and contributes to regulating the microalgal lipid accumulation in wastewater using exogenous hormones.

To obtain higher yield of natural astaxanthin, the present study aims to develop a viable and economic induction strategy for astaxanthin production comprising succinic acid (SA) combined with sodium hydrosulfide (NaHS). The biomass (1.33 g L

Vascular abnormalities are directly related to the tumor immunosuppressive microenvironment, which is an important obstacle to effective immunotherapy. The combination of antiangiogenesis therapy and immunotherapy may promote a mutually reinforcing cycle of immune reprogramming and vascular normalization to increase the effectiveness of immunotherapy. Herein, a hydrogel/nanosystem-mediated antiangiogenesis combined immunotherapy strategy was used to regulate the tumor microenvironment by the controlled release of apatinib, CD47 antibody (aCD47), and CpG. The combination of hydrogel with nanoparticles protected drug activity and maintained a long-term slow release of the drug for maximum synergistic efficacy. Apatinib promotes vascular normalization in tumors and enhances the efficacy of aCD47-based immunotherapy. The addition of immunoadjuvant CpG further enhanced antigen presentation and stimulated the anti-tumor activity of macrophages to strengthen the efficacy of antiangiogenesis combined immunotherapy. The main effector immune cells, including CD4

This paper aims to explore the role of proline (Pro) in the production of biomass and astaxanthin (AST) in stress-induced Haematococcus pluvialis. The astaxanthin content and productivity were 24.02 mg g

In the present study, exogenous myo-inositol (MI) was applied to induce natural astaxanthin and biolipid accumulation in Haematococcus pluvialis. Under 200 μM MI, algal cells exhibited 62.11 % and 34.67 % increases in astaxanthin and lipid content, respectively, compared to the control. The carotenogenesis and lipogenesis genes were upregulated by induction of MI. Interestingly, MI addition elevated the ethylene (ETH) content and activated antioxidant enzyme-associated gene levels, which could be involved in alleviating oxidative stress. Further data showed that the ETH signal played a positive function in stimulating astaxanthin biosynthesis under MI induction. Supplementation with ethephon plus MI boosted the astaxanthin content to 33.08 ± 0.03 mg g

With the goal of cost-effective and high-efficient microalgae-based biodiesel production, this study evaluated the feasibility of the joint strategy concerning myo-inositol (MI) and salinity stress on lipid productivity of Monoraphidium sp. QLY-1 in molasses wastewater (MW). The maximal lipid productivity (147.79 mg L−1 d−1) was obtained under combined 0.5 g L−1 MI and 10 g L−1 NaCl treatment, which was 1.40-fold higher than the control. Meanwhile, the nutrients removal from MW was markedly increased under MI-NaCl treatment. Moreover, exogenous MI upregulated key lipogenic genes' expressions, activated autophagic activity and ethylene (ET) signaling, and ultimately alleviated the salinity-induced damage via reactive oxygen species (ROS) signaling. Further pharmacologic experiment confirmed the indispensable role of ET in the lipogenesis progress under the combined treatment. These data demonstrated the combined salinity stress and MI treatment to be capable for lipid hyperproduction and wastewater nutrients removal, which contributes to practically integrating the microalgae cultivation with wastewater treatment.

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.

Melatonin (MT) plays important roles in plant disease response, but the mechanisms are largely unknown. Here, we show that MT functions in stomatal immunity in Panax notoginseng and Arabidopsis thaliana. Biochemical analyses showed that MT-induced stomatal closure plays a prominent role in preventing invasion of bacteria Pseudomonas syringe pv. tomato (Pst) DC3000 via activation of mitogen-activated protein kinase (MAPK) and NADPH oxidase-mediated reactive oxygen species production in P. notoginseng. The first putative phytomelatonin receptor 1 (PMTR1) is a plasma membrane protein required for perceiving MT signaling in stomatal closure and activation of MAPK. Biochemical and genetic tests found PMTR1 is essential for flg22- and MT-induced MAPK activation in a heterotrimeric GTP-binding protein Gα subunit GPA1-independent manner. GPA1 functions in the same genetic pathways of FLS2/BAK1 (Flagellin Sensing 2/Brassinosteroid Insensitive 1-associated kinase 1)- as well as PMTR1-mediated flg22 and MT signaling in stomatal closure. The stomata in pmtr1 are insensitive to MT and flg22, but the application of MT induces stomatal closure and reduces the bacterial growth in fls2 and bak1 plants, indicating that PMTR1 might be a downstream signaling component in FLS2- and BAK1-mediated stomatal immunity. In summary, our results (i) demonstrate that phytomelatonin functions in the priming of stomatal immunity and (ii) provide insights into the phytomelatonin signaling transduction pathway.

In the current research, a novel microalgae strain was isolated from Yajiageng Red Rock Beach and identified as Acutodesmus sp. HLGY. To obtain high-efficiency production of lutein from algae, the feasibility of using melatonin (MT) to increase lutein yield of Acutodesmus sp. HLGY was evaluated. Under the 7.5 μM MT treatment, the lutein content and lutein productivity were 17.44 mg g

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.

In this study, it was found that fulvic acid (FA) enhanced the contents of astaxanthin and lipids in Haematococcus pluvialis under high light and nitrogen starvation conditions by 2- and 1.2-fold, respectively. Meanwhile, the carbohydrate and chlorophyll contents were decreased by FA induction, whereas the levels of reactive oxygen species (ROS) and glutathione (GSH) as well as the expression of astaxanthin and lipid biosynthetic genes were increased. To further explore the interrelation between FA and the biosynthesis of astaxanthin and lipids, a metabolomics analysis of H. pluvialis by combined FA and abiotic stress exposure was conducted by using LC-MS/MS. The contents of some cytoprotective metabolites and signal molecules, including d-maltose, succinate, malic acid, melatonin (MT), and some amino acids, were increased under FA induction and abiotic stress conditions. These metabolites are intermediates in the TCA cycle and Calvin cycle, providing more precursors for the synthesis of astaxanthin and lipids. Moreover, the signal molecules might contribute to enhancing the abiotic stress tolerance. This study provided new insights into the regulatory mechanism of FA on astaxanthin and lipid accumulation in H. pluvialis.

Microalgae are potential candidates for the production of third-generation biofuels, but the high-energy consumption of harvesting biomass makes current commercial microalgal biodiesel production economically unfeasible. Self-flocculation is regarded as a cost-efficient strategy for harvesting microalgae. In this work, a coculture system was developed by the mixotrophic cultivation of two poorly flocculating microalgae, Desmodesmus sp. ZFY and Monoraphidium sp. QLY-1, for self-flocculation of microalgal cells. The flocculation efficiency for harvesting was enhanced to 85.33% with settling for 4 h in coculture, which was better than that in mono-culture (57.98% and 32.45%). Coculture resulted in changes in both the concentration and composition of extracellular polymeric substances (EPS) in the culture, which partially contributed to cell aggregation. The concentration of polysaccharides in LB-EPS ranged from 27.01% and 27.35% in mono-culture to 46.53% in coculture. Moreover, the fine properties of lipid productivity (93.99 mg L−1 d −1) and fatty acid composition in coculture indicated the potential to produce biodiesel within most biodiesel standards.

Molasses wastewater can be utilized for microalgae cultivation, and the harvested algal biomass is suitable for biofuel production. In this study, the effects of melatonin on the metabolism and activities of three key lipid biosynthetic enzymes of Monoraphidium sp. FXY-10 cultured in molasses wastewater were evaluated. The maximal biomass (1.21 g L−1) and lipid content (68.69%) were obtained during melatonin treatment and were approximately 1.42- and 1.15-fold, respectively, of the group not treated with melatonin. The carbohydrate and protein contents of melatonin-treated cells were also increased. MT treatment upregulated the activities of malic enzyme and acetyl-CoA carboxylase but downregulated phosphoenolpyruvate carboxylase activity, which was correlated with lipid accumulation in FXY-10. MT-treated samples showed greater potentials for removing organic carbon (92.33% chemical oxygen demand removal) and nutrients (90.07% nitrogen removal, 86.04% phosphorus removal) than the non-treated samples. Additionally, the economic feasibility of melatonin treatment demonstrated that a combined strategy is ideal for molasses wastewater treatment and biofuel production.

Cadmium (Cd) stress is a cost-effective, clean, and environmentally friendly approach for microalgal lipid production and wastewater treatment. To study the regulatory networks involved in the growth and development of microalgae, the relationship between calcium and reactive oxygen species (ROS) signalling in the regulation of cell growth and lipid biosynthesis and Cd biosorption after Cd treatment were analysed. The highest lipid content (52.78%) and productivity (96.75 mg L−1 d−1) were enhanced by 1.59- and 1.39-fold in algal cells treated with 80 μM and 40 μM Cd, respectively, compared to the control. The protein content, ROS, antioxidases, Ca2+ and lipid biosynthetic gene transcription levels were upregulated by Cd induction. Additionally, the results indicated that increased endogenous Ca2+ could alleviate oxidative stress by activating antioxidases and mediating Cd biosorption, resulting in cell growth modulation and lipid accumulation. These results demonstrated that Cd stress is a useful strategy for lipid production and Cd bioaccumulation in microalgae and that there is vital interaction between ROS and Ca2+ levels in the regulation of lipid accumulation under Cd stress.

In the present study, the effects of four different culture media on the growth, astaxanthin production and morphology of Haematococcus pluvialis LUGU were studied under two-step cultivation. The interactions between astaxanthin synthesis and secondary messengers, reactive oxygen species (ROS) and mitogen-activated protein kinases (MAPK) were also investigated. In the first green vegetative cell stage, maximal biomass productivity (86.54 mg L−1 day−1) was obtained in BBM medium. In the induction stage, the highest astaxanthin content (21.5 mg g−1) occurred in BG-11 medium, which was higher than in any other media. The expressions of MAPK and astaxanthin biosynthetic genes in BG-11 were higher than in any other media, whereas the ROS content was lower. Biochemical and physiological analyses suggested that the ROS, MAPK and astaxanthin biosynthetic gene expression was involved in astaxanthin biosynthesis in H. pluvialis under different culture media conditions. This study proposes a two-step cultivation strategy to efficiently produce astaxanthin using microalgae.

Induction of copper ion (Cu

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.

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.

In this study, melatonin (MT) promoted lipid accumulation in Monoraphidium sp. QLY-1 under nitrogen deficiency conditions. The lipid accumulation increased 1.22- and 1.36-fold compared with a nitrogen-starved medium and a normal BG-11 medium, respectively. The maximum lipid content was 51.38%. The reactive oxygen species (ROS) level in the presence of melatonin was lower than that in the control group, likely because of the high antioxidant activities. The application of melatonin upregulated the gibberellin acid (GA) production and rbcL and accD expression levels but downregulated the abscisic acid (ABA) content and pepc expression levels. These findings demonstrated that exogenous melatonin could further improve the lipid production in Monoraphidium sp. QLY-1 by regulating antioxidant systems, signalling molecules, and lipid biosynthesis-related gene expression under nitrogen deficiency conditions.

Melatonin (N‑acetyl‑5‑methoxytryptamine) is widely regarded as an important messenger in higher plants and mammals in their resistance to various biotic and abiotic stresses. However, the role of melatonin in microalgae has been rarely investigated. In this study, melatonin was first used to promote astaxanthin biosynthesis under limited nitrogen and high light conditions. The interactions between melatonin and the secondary messengers, namely, nitric oxide (NO) and salicylic acid (SA), during the stress response were also investigated. Moreover, fatty acid biosynthesis was explored. Finally, the expression levels of astaxanthin biosynthesis genes in Haematococcus pluvialis LUGU supplemented with melatonin were simultaneously monitored through quantitative real-time PCR. The astaxanthin content of the microalgae increased 2.36-fold after treatment with 10 μM of melatonin. The maximal astaxanthin content achieved was 31.32 mg g−1. The increased NO and SA production caused by melatonin occurred in parallel with the up-regulation of the expression of astaxanthin biosynthesis genes and the enhancement in astaxanthin biosynthesis in H. pluvialis LUGU. The analysis of fatty acid composition showed that melatonin stimulated the production of C16:0 (palmitic acid); thus, the percentage of astaxanthin esters was higher than that observed in the control algae. Furthermore, the inhibitors, namely, carboxy-PTIO, paclobutrazol, and N‑acetyl‑ l ‑cysteine, significantly suppressed the levels of secondary messengers. Meanwhile, the role of melatonin in mediating the rise in astaxanthin content was prevented when the SA- and NO-dependent pathways were inhibited. These findings indicate that melatonin plays a physiological role in direct and indirect responses to abiotic stresses in H. pluvialis.

Haematococcus pluvialis is a commercial microalgae used for natural astaxanthin production. This study aims to investigate the roles of melatonin (MT) and 3-methyladenine (3-MA) in regulating the cell growth and biosynthesis of astaxanthin and fatty acids under adverse conditions by H. pluvialis. Upon the dual treatments, the maximum astaxanthin concentration (46.78 mg L-1) was 2.39- and 1.35-fold higher compared with the control and MT treatments, respectively. Concomitantly, the combined application of MT and 3-MA suppressed autophagy but promoted the production of biomass and lipids and upregulated carotenogenesis, lipogenesis and antioxidant enzyme-related genes at the transcriptional level, which were linked with astaxanthin and lipid biosynthesis and oxidative stress. Additionally, astaxanthin exhibited a noticeable increase under MT coupled with 3-MA in the other two strains of H. pluvialis. This study proposed a potential method for astaxanthin induction and provided insights into the function of autophagy in modulating cell growth and astaxanthin synthesis.

This study investigated the effects of copper (Cu) on astaxanthin and lipid biological synthesis in unicellular alga Haematococcus pluvialis under high-light (HL) and nitrogen-deficiency (ND) conditions. During a 15-day cultivation period, the astaxanthin and lipid contents reached the peak values (3.32% and 47.72%) under 6 μM Cu treatment, which were increased by 66.87% and 34.99% compared to nontreated group, respectively. The application of Cu also increased the transcriptional expression of biosynthesis genes and antioxidant enzyme-related genes, as well as increased the intracellular calcium (Ca2+) level but led to a decrease in reactive oxygen species (ROS) levels. Additionally, Cu treatment induced the activation of calcium-dependent protein kinases (CDPKs) and mitogen-activated protein kinases (MAPKs). This approach simultaneously facilitated astaxanthin and lipid production, and the role of Cu were elucidated on the regulation of signal transduction (e.g., Ca2+, CDPK, MAPK and ROS) in the carotenogenesis and lipogenesis in H. pluvialis.

In this study, it was found that fulvic acid (FA) enhanced the contents of astaxanthin and lipids in

The effects of the combined treatment of fulvic acid (FA) and salinity stress on lipid production in Monoraphidium sp. QLY-1 at multiple levels was investigated in this study. The results indicated that the highest lipid content (59.53%) in QLY-1 was achieved by combining FA treatment and salinity stress. Compared with the control group and FA addition alone, the group treated with both FA and salinity stress had increased contents of reactive oxygen species (ROS), antioxidases, and nitric oxide (NO). Additionally, the addition of FA enhanced the expression levels of mitogen-activated protein kinases (MAPKs) and key genes related to lipid biosynthesis in QLY-1 under salinity stress. Collectively, biochemical analyses indicated that ROS, NO, MAPK, expression of lipid biosynthesis-related genes and antioxidant systems were involved in the lipid biosynthesis pathways of QLY-1 under the combined treatment of FA and salinity stress.

The major goal of this study was to explore the functions of butylated hydroxyanisole (BHA) combined with abiotic stress on the cultivation of the microalga Haematococcus pluvialis for astaxanthin and lipid production. Here, the effect of BHA on astaxanthin and lipid accumulation and physiological and metabolomic profiles was investigated. These results suggested that astaxanthin content was increased by 2.17-fold compared to the control. The lipid content was enhanced by 1.22-fold. BHA treatment simultaneously reduced carbohydrates and protein and delayed the decay of chlorophyll. Furthermore, metabolomic analysis demonstrated that BHA upregulated and activated the bioprocesses involved in cellular basal metabolism and signalling systems, such as glycolysis, the TCA cycle, amino acid metabolism and the phosphatidylinositol signalling system, thus enhancing astaxanthin and lipid accumulation. Altogether, this research shows the dramatic effects of BHA on algal metabolism in the regulation of key metabolic nodes and provides novel insights into microalgal regulation and metabolism.

In this study, the effects of strigolactone (SL) on the biomass, lipid content, biochemical properties, and gene transcription of Monoraphidium sp. QLY-1 were examined. The lipid content and lipid productivity increased by 61% and 55% in QLY-1 under 1 μM SL induction compared to the control group, respectively. SL also upregulated the levels of endogenous NO and Ca2+ and lipid biosynthesis gene transcription. Subsequently, the relationship between Ca2+ and nitric oxide (NO) in the regulation of cell growth and lipid accumulation of QLY-1 under SL induction conditions was analysed. An increase in endogenous Ca2+ regulated cell growth and lipid biosynthesis by modulating the levels of NO and lipid biosynthesis-related gene expression. Collectively, this study provided a valuable approach for biofuel production from microalgae under SL induction and demonstrated that there is crucial crosstalk between the Ca2+ and NO signalling in the manipulation of lipid biosynthesis in microalgae under SL treatment.

Microalgae are a promising biofuel resource, but their high cost and low productivity hinder their commercial applications. In the present study, Monoraphidium sp. QLZ-3 was cultivated in walnut shell extracts (WSE) supplemented with carbon dioxide (CO2). Biomass was enhanced from 0.40 g L−1 to 1.18 g L−1, and lipid content reached 49.54% in WSE-12% CO2 media. Biomass and lipid productivity reached 196.88 and 97.52 mg L−1 d−1, which were 1.33- and 1.57-fold higher than those of the control, respectively. The amount of carbohydrates increased, but the protein contents decreased. Furthermore, the application of CO2 promoted nutrient and polyphenol absorption and upregulated the expression levels of lipid biosynthetic genes of this WSE-cultivated alga. These results indicated that coupling WSE and CO2 could be an efficient strategy to enhance biofuel production by microalgae.

The primary goal of this study was to assess the roles of chemical factors and bioprocess strategies on a mixotrophic culture of the microalga Haematococcus pluvialis during γ-aminobutyric acid (GABA) production. A two-stage strategy was used to increase the biomass and GABA accumulation of H. pluvialis. In stage I, mixotrophic growth of H. pluvialis in the presence of fulvic acid (FA) produced a high biomass (1.84 g L−1) with a GABA content of 25.45 mg g−1. Furthermore, a maximum GABA content of 38.57 mg g−1 was obtained when cells were cultured with 0.4 g L−1 NaCl under photoautotrophic conditions in stage II, whereas the carbohydrate content of cells sharply decreased from 26.68 to 18.22%. In addition, salt stress upregulated the expression of the gad and cam genes. The results of this study demonstrate an efficient strategy to produce GABA from the microalga H. pluvialis.

In this study, crude lipids, including pigment accumulation, in Heveochlorella sp. Yu were evaluated after the addition of four antioxidants (melatonin, fulvic acid, butylhydroxytoluene and butylated hydroxyanisole). Melatonin enhanced crude lipid accumulation in this alga, with crude lipid productivity increasing by 31.83% (from 185.57 mg L−1 d−1 to 244.63 mg L−1 d−1). Moreover, the level of reactive oxygen species was reduced and the concentration of nitric oxide increased with melatonin addition. Such changes in reactive oxygen species and nitric oxide concentration might regulate signaling pathways related to lipid biosynthesis. Furthermore, the expression levels of key genes (accD, ME and rbcL) associated with the lipid biosynthesis pathway were analyzed, and melatonin addition was found to enhance expression of these genes. Collectively, biochemical analyses suggest that nitric oxide and reactive oxygen species levels and expression of key genes involved in lipid biosynthesis are closely connected with lipid accumulation under melatonin treatment.

The current study examined the effect of walnut shell extracts (WSE) on simultaneous astaxanthin and lipid production by Haematococcus pluvialis under abiotic stress. After supplementation with 15% WSE, the contents of astaxanthin and lipids were enhanced by 77.57% and 23.39%, respectively, compared with the control. The chlorophyll, carbohydrate and protein contents decreased, and the transcription levels of carotenogenic and lipogenic genes, as well as the levels of the reactive oxygen species (ROS), glutathione (GSH) and light‑oxygen-voltage histidine kinase (LOV-HK), all increased. Further investigation showed that LOV-HK regulated astaxanthin and lipid biosynthesis by mediating carotenogenesis, lipogenesis and ROS signalling. Overall, the present study demonstrated that microalgae cultivation coupled with walnut shell recycling is an effective strategy for hyper-accumulation of astaxanthin and lipids and that there is a pivotal interaction between LOV-HK and ROS in WSE-induced regulation of astaxanthin and lipid synthesis in H. pluvialis.

The aim of this study was to explore the effect of melatonin (MT) on lipid accumulation in Monoraphidium sp. QLY-1 under salinity stress. The interconnection between MT, oxidative stress and autophagy in the regulation of lipid synthesis was also clarified. MT application significantly promoted the lipid content, peaking at 51.74%, which was 1.16- and 1.44-fold higher than those induced by salinity stress alone (44.77%) and control group (35.96%), respectively. Exogenous MT also upregulated the transcription levels of lipogenesis-related genes and decreased salinity stress-induced oxidative stress. Interestingly, the application of MT upregulated autophagic activity, which led to the 1.47-fold upregulation of autophagy-related atg8 gene expression. Additional results demonstrated that autophagy promotion led to further increases in lipid accumulation by involving lipogenesis gene expression and ROS signalling in cells treated with MT under salinity stress conditions. The maximum lipid content (54.26%) was achieved by treatment with MT and rapamycin (RAPA), an activator of autophagy. Taken together, these findings indicate that combination of MT and salinity stress is a valuable strategy for enhancing lipid production in microalgae and provides novel insights into the regulatory mechanisms underlying the effects of MT and autophagy on the promotion of lipid synthesis and improvements to algal tolerance.

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.

The combination of combined fulvic acid (FA) and two-step cultivation was proposed as a novel strategy to enhance biomass and oil accumulation of Monoraphidium sp. FXY-10 compared with the traditional monoculture of microalgae. In this study, the effect of FA on the growth and lipid accumulation of Monoraphidium sp. FXY-10 was investigated using one- and two-stage cultivation. In the heterotrophic one-stage cultivation, 1.5-fold biomass was enhanced with 80 mg L−1 FA (6.41 g L−1) compared with the control biomass (4.21 g L−1). In the heterotrophic–photoautotrophic two-stage cultivation, different FA concentrations were used to stimulate lipid accumulation. The lipid content increased from 30.78% to 54.65% when 25 mg L−1 FA was added to the culture. The main fatty acids in the FA-triggered Monoraphidium sp. FXY-10 were C16/C18 fatty acids (97%), which are appropriate for biodiesel production. Results showed that the proposed strategy is useful for biomass and oil production of Monoraphidium sp. FXY-10.

Haematococcus pluvialis is a promising natural source of high-value antioxidant astaxanthin under stress conditions. Biotic or abiotic elicitors are effective strategies for improving astaxanthin production in H. pluvialis. Butylated hydroxyanisole (BHA) was identified as an effective inducer for H. pluvialis LUGU. Under a treatment of 2mgL(-1) BHA (BHA2), astaxanthin content reached a maximum of 29.03mgg(-1) dry weight (DW) (2.03-fold of that in the control) after 12day of the mid-exponential growth phase. Subsequently, H. pluvialis LUGU was subjected to BHA2 at different growth phases because an appropriate time node for adding elicitors is vital for the entire production to succeed. As a result, the highest astaxanthin content (29.3mgg(-1) DW) was obtained in cells on day 14 (BHA2 14) of the late-exponential growth phase. Furthermore, the samples treated with BHA2 14 and the control group were compared in terms of the transcriptional expression of seven carotenogenesis genes, fatty acid composition, and total accumulated astaxanthin. All selected genes exhibited up-regulated expression profiles, with chy, crtO, and bkt exhibiting higher maximum transcriptional levels than the rest. Oleic acid content increased 33.15-fold, with acp, fad, and kas expression being enhanced on the day when astaxanthin was produced rapidly.

A two-step strategy comprising heterotrophic cultivation and photo-chemical modulator induction was developed to enhance biomass and lipid accumulation in the oleaginous Monoraphidium sp. QLY-1, which was isolated from Qilu Lake in Yunnan Plateau. The algae were first cultivated heterotrophically to achieve high biomass concentration (5.54gL(-1)) with a lipid content of 22.47%. The cultivated algae were diluted, transferred to light environment, and treated with different chemical elicitors. Results showed that the lipid content increased to 36.68% after 3-day of photoinduction. The lipid content was further enhanced by 1.21, 1.32, and 1.29 folds in algal cells treated with nitrogen deficiency, 20gL(-1) NaCl, and 5mM glycine betaine, respectively. The maximum lipid content (48.54%) and lipid productivity (121.27mgL(-1)d(-1)) were obtained in treatments with 20gL(-1) NaCl and 5mM GB, respectively. This study proposes a strategy to efficiently produce lipids by using microalgae.

Haematococcus pluvialis is one of the most promising natural sources of astaxanthin. Morphological and genomic characterization of a novel strain of H. pluvialis LUGU, which was isolated from Lugu Lake in Yunnan Plateau, was described in this study. The algae can accumulate astaxanthin under abiotic stress. Thus, the influences of fulvic acid (FA) on biomass and astaxanthin were investigated, and transcriptional levels of three astaxanthin biosynthetic genes in H. pluvialis, namely, phytoene desaturase gene (PDS), lycopene cyclase gene (LCY), and β-carotene hydroxylase gene (CHY), were measured. The astaxanthin content increased by 86.89% and 9.78% in 5 and 10 mg L−1 FA-treated algal cells, respectively. The transcriptional levels of PDS and CHY genes were also enhanced under FA induction. Results suggested that high level of astaxanthin accumulation may correlate astaxanthin biosynthetic genes with FA induction. This research demonstrated a useful strategy to efficiently produce astaxanthin by using H. pluvialis.

In this study, the cross-talk between gama-aminobutyric acid (GABA) and calcium ion (Ca2+) signalling in the regulation of lipid production and cell growth in microalgae under fulvic acid and salinity stress (FA-salinity treatment) was investigated. GABA enhanced the lipid content and lipid productivity rate considerably, which were 1.27 and 1.29 times higher than those of the control, respectively. The levels of biosynthetic gene transcription, GSH, Ca2+ and cellular GABA were promoted by GABA addition, but decreased the ROS levels. Furthermore, the application of Ca2+ also increased lipid synthesis by regulating ROS and GABA signalling and lipogenesis-related genes. These results indicated that cytosolic GABA and Ca2+ levels exert crucial cross-talk in the modulation of cell growth and lipid accumulation induced by FA-salinity treatment. Collectively, this study demonstrated the beneficial effects caused by induction of the combination of chemical compounds on lipid production and provided new insights into lipid synthesis in microalgae.