Effect of fulvic acid induction on the physiology, metabolism, and lipid biosynthesis-related gene transcription of Monoraphidium sp. FXY-10

ROS and Ca2+ signaling involved in important lipid changes of Chlorella pyrenoidosa under nitrogen stress conditions

MAIN CONCLUSION

Nitrogen stress altered important lipid parameters and related genes in Chlorella pyrenoidosa via ROS and Ca2+ signaling. The mutual interference between ROS and Ca2+ signaling was also uncovered. The changed mechanisms of lipid parameters (especially lipid classes and unsaturation of fatty acids) in microalgae are not completely well known under nitrogen stress. Therefore, Chlorella pyrenoidosa was exposed to 0, 0.5, 1 and 1.5 g L-1 NaNO3 for 4 days. Then, the physiological and biochemical changes were measured. It was shown that the total lipid contents, neutral lipid ratios as well as their related genes (accD and DGAT) increased obviously while the polar lipid ratios, degrees of unsaturation as well as their related genes (PGP and desC) decreased significantly in nitrogen stress groups. The obvious correlations supported that gene expressions should be the necessary pathways to regulate the lipid changes in C. pyrenoidosa under nitrogen stress. The changes in ROS and Ca2+ signaling as well as their significant correlations with corresponding genes and lipid parameters were analyzed. The results suggested that ROS and Ca2+ may regulate these gene expressions and lipid changes in C. pyrenoidosa under nitrogen stress conditions. This was verified by the subordinate tests with an ROS inhibitor and calcium reagents. It also uncovered the clues of mutual interference between ROS and Ca2+ signaling. To summarize, this study revealed the signaling pathways of important lipid changes in microalgae under N stress.

Remediation of the microecological environment of heavy metal-contaminated soil with fulvic acid, improves the quality and yield of apple

The excessive application of chemical fertilizers and pesticides in apple orchards is responsible for high levels of manganese and copper in soil, and this poses a serious threat to soil health. We conducted a three-year field experiment to study the remediation effect and mechanism of fulvic acid on soil with excess manganese and copper. The exogenous application of fulvic acid significantly reduced the content of manganese and copper in soil and plants; increased the content of calcium; promoted the growth of apple plants; improved the fruit quality and yield of apple; increased the content of chlorophyll; increased the activity of superoxide dismutase, peroxidase, and catalase; and reduced the content of malondialdehyde. The number of soil culturable microorganisms, soil enzyme activity, soil microbial community diversity, and relative abundance of functional bacteria were increased, and the detoxification of the glutathione metabolism function was enhanced. The results of this study provide new insights that will aid the remediation of soil with excess manganese and copper using fulvic acid.

Lipid production characteristics of a newly isolated microalga Asterarcys quadricellulare R-56 as biodiesel feedstock

In this study, a new microalgal strain, Asterarcys quadricellulare R-56, was isolated for biomass and lipid production. The effects of carbon and nitrogen sources and initial pH on the cell growth and lipid accumulation of strain R-56 were investigated. At 10 g L^-1 glucose, 0.6 g L^-1 sodium nitrate, and pH 7, the highest biomass of 4.18 g L^-1 and lipid content of 43.66% were obtained. Microalgae had a broad pH tolerance in the range of 5-11, and the pH of the culture medium was close to neutral at the end of cultivation. The maximum contents of chlorophyll, carbohydrate, and protein under the recommended culture conditions were 19.47 mg mL^-1, 21.80%, and 29.94%, respectively. Palmitic and palmitoleic acid contents in strain R-56 accounted for as high as 83.73% of total fatty acids. This study suggested that strain R-56 was a promising lipid producer for high-quality biodiesel production.

A low-cost technique for biodiesel production in Ankistrodesmus sp. EHY by using harvested microalgal effluent

The present study aims to use Ankistrodesmus sp. EHY to develop a viable and economic lipid production strategy using recycling of harvested microalgal effluent. In comparison to the control, the highest lipid content (52.4 %) and productivity (250.72 mg L^-1 d^-1) were achieved when 40 % recycled medium was used. Consistent with the trend of lipid accumulation, the six key lipogenetic genes were upregulated, as well as reactive oxygen species (ROS), glutathione (GSH) and genes encoding antioxidant enzymes during cultivation in recycled medium. Moreover, the consumption of dissolved organic carbon (DOC) and the increased humic acid (HA) in the recycled medium might also be associated with lipid biosynthesis. The biodiesel parameters of alga biomass-derived lipids were fitted to the standard of commercial biodiesel. In conclusion, this study offers an economically viable strategy for microalgal biofuel production and wastewater treatment using recycling of harvested microalgal effluent.

Myo-inositol facilitates astaxanthin and lipid coproduction in Haematococcus pluvialis by regulating oxidative stress and ethylene signalling

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^-1 by further upregulating astaxanthin biosynthesis genes and blocking reactive oxidative species (ROS) levels, and vice versa under ETH inhibition. This study provides a potential induction approach for natural astaxanthin production and explains the role of ethylene signalling in regulating astaxanthin synthesis by H. pluvialis.

Focus on the role of synthetic phytohormone for mixotrophic growth and lipid accumulation by Chlorella pyrenoidosa

Synthetic phytohormone (SP) is regarded as an attractive candidate for microalgae cultivation due to its potential for high-value microalgae biomass production. Herein, α-naphthylacetic acid (NAA), indomethacin (IN) and 2,4-dichlorophenoxyacetic acid (2,4-D) were used for the mixotrophic cultivation of Chlorella pyrenoidosa with mariculture wastewater (MW) acidogenic fermentation effluent. The growth and lipid accumulation of Chlorella pyrenoidosa added with SP were enhanced, given their high bioavailability of the nutrients. Among these three SPs, IN was optimal for Chlorella pyrenoidosa growth, with the maximum optical density of 1.81. NAA exhibited the best performance for lipid production and the proportion of lipid reached 50.24%. Furthermore, the energy of Chlorella pyrenoidosa cultured with SP preferentially allocated to lipogenesis. To understand the mechanism of lipid accumulation in Chlorella pyrenoidosa in response to SP, the enzyme activities involved in carbon metabolism were determined. The malic enzyme (ME) and acetyl-CoA carboxylase (ACCase) were positively correlated with lipid accumulation. Phosphoenolpyruvate carboxylase (PEPC) was the negative feedback enzyme for lipid synthesis. The findings could provide valuable information for regulation mechanism of lipid accumulation and value-added products recovery by microalgae.

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

Humic substances (HS) act as biostimulants for terrestrial photosynthetic organisms. Their effects on plants are related to specific HS features: pH and redox buffering activities, (pseudo)emulsifying and surfactant characteristics, capacity to bind metallic ions and to encapsulate labile hydrophobic molecules, ability to adsorb to the wall structures of cells. The specific properties of HS result from the complexity of their supramolecular structure. This structure is more dynamic in aqueous solutions/suspensions than in soil, which enhances the specific characteristics of HS. Therefore, HS effects on microalgae are more pronounced than on terrestrial plants. The reported HS effects on microalgae include increased ionic nutrient availability, improved protection against abiotic stress, including against various chemical pollutants and ionic species of potentially toxic elements, higher accumulation of value-added ingredients, and enhanced bio-flocculation. These HS effects are similar to those on terrestrial plants and could be considered microalgal biostimulant effects. Such biostimulant effects are underutilized in current microalgal biotechnology. This review presents knowledge related to interactions between microalgae and humic substances and analyzes the potential of HS to enhance the productivity and profitability of microalgal biotechnology.

Gamma-aminobutyric acid coupled with copper ion stress stimulates lipid production of green microalga Monoraphidium sp. QLY-1 through multiple mechanisms

Induction of copper ion (Cu²⁺) stress is a method used to increase lipid accumulation in microalgae, but it decreases cell growth. In this work, the impacts of gamma-aminobutyric acid (GABA) coupled with Cu²⁺ stress on the biomass and oil yield in Monoraphidium sp. QLY-1 were investigated. Results suggested that the combined treatment of GABA and Cu²⁺ resulted in a higher lipid content (55.13%) than Cu²⁺ treatment (48.43%). Furthermore, GABA addition upregulated the levels of lipid-relevant genes, cellular GABA, ethylene (ETH), and antioxidant enzyme activities and alleviated oxidative damage caused by Cu²⁺ stress. The autophagy-relevant gene atg8 was also upregulated by GABA treatment. Further exploration indicated that cell autophagy induced the lipid content up to 58.09% with GABA and Cu²⁺ stress treatment. This investigation demonstrates that the coupling strategy can stimulate lipid production and shed light on the underlying mechanisms in lipid biosynthesis, cell autophagy, and stress response of microalgae.

Coupling of myo-inositol with salinity regulates ethylene-induced microalgal lipid hyperproduction in molasses wastewater

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.

Ginger Extract and Fulvic Acid Foliar Applications as Novel Practical Approaches to Improve the Growth and Productivity of Damask Rose

Plant biostimulants (BIOs) have been identified as among the best agricultural practices over the past few decades. Ginger extract (GE) and fulvic acid (FA) are a new family of multifunctional BIOs that positively affect development processes in plants. However, the underlying mechanisms that influence these development processes are still unknown. The objective of this study was to determine how GE and FA affect the plant growth and productivity in damask rose. Furthermore, the mechanisms of these BIOs that regulate the performance of this plant were investigated. Damask rose plants were foliar-sprayed with GE (5, 10 and 15 mg L^-1) or FA (1, 3 and 5 g L^-1), while control plants were sprayed with tap water. The results showed that GE or FA foliar applications enhanced plant height and branch number much more than the control; however, FA treatment was more effective than GE. Intriguingly, flower number, flower yield, relative water content, and total chlorophyll content were all improved by either GE or FA, paying attention to reducing the blind shoot number per plant. Relative to the control, foliar application with 15 mg L^-1 GE or 3 mg L^-1FA increased the flower number by 16.11% and 19.83% and the flower yield per hectare by 40.53% and 52.75%, respectively. Substantial enhancements in volatile oil content and oil yield were observed due to GE and FA treatments, especially with the highest concentrations of both BIOs. The treatments of GE and FA considerably improved the total soluble sugars, total phenolic content, total anthocyanin content, and total carotenoid content, more so with FA. Additionally, the contents of N, P, K, Mg, Fe, and Zn elements were also enhanced by applying either GE or FA, especially at higher levels of both BIOs. In sum, our findings illuminate the potential functions of exogenous application of GE and FA in improving the growth, flower yield, and volatile oil yield in damask rose through enhancing the phytochemical and nutrient profiles. Applications of GE and FA can, thus, be a promising approach for enhancing the productivity of damask rose.

Long-term effect of fulvic acid amendment on the anammox biofilm system at 15 ℃: performance, microbial community and metagenomics analysis

The role of fulvic acid (FA) on the anammox system at 15 ℃ was investigated. After operation for 113 days, total inorganic nitrogen removal efficiency in FA amendment reactor achieved to 58.6% on average, higher than that of control group (42.1%). Anammox-related functional genes, i.e., hzo and hzs, also demonstrated higher expression level after introduction of FA. It was observed that Candidatus Kuenenia became more competitive than Candidatus Brocadia with the existence of FA at 15 ℃. Also, co-occurrence analysis showed that FA stimulated the complexity and interactive relationship of microbial communities in the anammox system. Metagenomics analysis revealed that FA introduction stimulated relative abundances of genes in central pathway of tricarboxylic acid cycle such as ACO, IDH, OGDH, SCS, FUM, and MDH. Meanwhile, metabolomics analysis revealed that metabolites related to amino sugar metabolic pathways (glucose 1-phosphate, UDP-D-glucuronate, UDP) and redox reactions (NAD⁺ and NADH) improved in the FA amendment reactor.

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.

Effects of potassium fulvic acid and potassium humate on microbial biodiversity in bulk soil and rhizosphere soil of Panax ginseng

Potassium fulvic acid (BSFA) and potassium humate (KHM), as organic fertilizers, can improve soil structure, increase soil nutrient levels and prevent plant diseases. However, knowledge is limited regarding how BSFA and KHM influence soil microbial communities and the interrelationships between community members associated with Panax ginseng. Soil pH and nutrient content increased significantly as a result of the addition of BSFA and KHM. The pH, NH₄⁺-N, NO₃^--N, AP and AK increased by 1.72 %-5.55 %, 70.09 %-108.39 %, 35.38 %-216.20 %, 1.21 %-14.19 % and 3.40 %-5.94 %, respectively, in the BSFA and KHM treatments. The soil nutrient increase may be related to Micrococcaceae and arbuscular mycorrhizal fungi (AMF). The structure of the microbial community also changed radically from that of the control group, and Chloroflexi (2.69 %-3.15 %), Actinobacteria (4.33 %-7.53 %) and Acidobacteria (9.44 %-11.62 %) were the dominant microorganisms at the phylum level in bacteria. In contrast, the dominant fungi at the phylum level were Ascomycota (77.39 %-78.08 %), Glomeromycota (0.36 %-2.68), Olpidiomycota (0.02 %-3.78 %) and Basidiomycota (0.80 %-1.17 %). Fusarium oxysporum and Ascomycota were biomarkers for BSFA and KHM, which may be related to pathogenic bacteria. Network analysis revealed that the association among members of the soil microbial community was more positive than negative following application of KHM, and more positive (62.5 %) than negative (37.5 %) correlations were observed between bacteria, whereas the fungal community exhibited more positive (97.3 %) than negative (2.7 %) correlations. PICRUST predicted the microbial function of adding KHM and BSFA to the soil, and these pathways mainly belong to the degradation and metabolism of organic matter, saprophytic organisms and plant pathogens. In summary, our study demonstrated that the addition of BSFA and KHM increased the nutrients in the ginseng soil and reshaped the microbial function in soils, providing a theoretical foundation for soil improvement and biological control of ginseng diseases. However, due to the limitations of greenhouse cultivation, additional long-term experiments on farmland with different climate changes are recommended.

Enhancement of Lipid Productivity and Self-flocculation by Cocultivating Monoraphidium sp. FXY-10 and Heveochlorella sp. Yu Under Mixotrophic Mode

To maintain high microalgae lipid productivity and flocculation efficiency simultaneously and reduce the production cost of microalgae lipids, Monoraphidium sp. FXY-10 with high lipid-producing capacity and Heveochlorella sp. Yu with strong self-flocculation ability were cocultivated and studied. Cocultivated microalgae lipid productivity and flocculation efficiency were increased to 203.8 mg L-1 day-1 and 70.55%, respectively, which is potentially related to the excessive competitive depletion of nitrogen sources and the upregulation of correlative key genes in lipid anabolic metabolism. Under cocultivation conditions, microalgae cells could enter the stationary phase 2 days earlier than that under monocultivation conditions, thus reducing the culture time. Relative expression of the accD, ME, and rbcL genes was upregulated to varying degrees, and the enzyme activities of ACCase, ME, and RuBisCO were also significantly increased compared with those in monocultivation. Moreover, fatty acid composition showed that microalgae lipids in cocultivation exhibited potential as a feedstock for biodiesel.

Exogenous Antioxidants Improve the Accumulation of Saturated and Polyunsaturated Fatty Acids in Schizochytrium sp. PKU#Mn4

Species of Schizochytrium are well known for their remarkable ability to produce lipids intracellularly. However, during their lipid accumulation, reactive oxygen species (ROS) are generated inevitably as byproducts, which if in excess results in lipid peroxidation. To alleviate such ROS-induced damage, seven different natural antioxidants (ascorbic acid, α-tocopherol, tea extract, melatonin, mannitol, sesamol, and butylated hydroxytoluene) were evaluated for their effects on the lipid accumulation in Schizochytrium sp. PKU#Mn4 using a fractional factorial design. Among the tested antioxidants, mannitol showed the best increment (44.98%) in total fatty acids concentration. However, the interaction effects of mannitol (1 g/L) and ascorbic acid (1 g/L) resulted in 2.26 ± 0.27 g/L and 1.45 ± 0.04 g/L of saturated and polyunsaturated fatty acids (SFA and PUFA), respectively, in batch fermentation. These concentrations were further increased to 7.68 ± 0.37 g/L (SFA) and 5.86 ± 0.03 g/L (PUFA) through fed-batch fermentation. Notably, the interaction effects yielded 103.7% and 49.6% increment in SFA and PUFA concentrations in batch fermentation. The possible mechanisms underlining those increments were an increased maximum growth rate of strain PKU#Mn4, alleviated ROS level, and the differential expression of lipid biosynthetic genes andupregulated catalase gene. This study provides an applicable strategy for improving the accumulation of SFA and PUFA in thraustochytrids by exogenous antioxidants and the underlying mechanisms.

Role of copper in the enhancement of astaxanthin and lipid coaccumulation in Haematococcus pluvialis exposed to abiotic stress conditions

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.

Recent Advances in the Molecular Effects of Biostimulants in Plants: An Overview

As the world develops and population increases, so too does the demand for higher agricultural output with lower resources. Plant biostimulants appear to be one of the more prominent sustainable solutions, given their natural origin and their potential to substitute conventional methods in agriculture. Classified based on their source rather than constitution, biostimulants such as humic substances (HS), protein hydrolysates (PHs), seaweed extracts (SWE) and microorganisms have a proven potential in improving plant growth, increasing crop production and quality, as well as ameliorating stress effects. However, the multi-molecular nature and varying composition of commercially available biostimulants presents challenges when attempting to elucidate their underlying mechanisms. While most research has focused on the broad effects of biostimulants in crops, recent studies at the molecular level have started to unravel the pathways triggered by certain products at the cellular and gene level. Understanding the molecular influences involved could lead to further refinement of these treatments. This review comprises the most recent findings regarding the use of biostimulants in plants, with particular focus on reports of their molecular influence.

Current perspectives on integrated approaches to enhance lipid accumulation in microalgae

In recent years, research initiatives on renewable bioenergy or biofuels have been gaining momentum, not only due to fast depletion of finite reserves of fossil fuels but also because of the associated concerns for the environment and future energy security. In the last few decades, interest is growing concerning microalgae as the third-generation biofuel feedstock. The CO₂ fixation ability and conversion of it into value-added compounds, devoid of challenging food and feed crops, make these photosynthetic microorganisms an optimistic producer of biofuel from an environmental point of view. Microalgal-derived fuels are currently being considered as clean, renewable, and promising sustainable biofuel. Therefore, most research targets to obtain strains with the highest lipid productivity and a high growth rate at the lowest cultivation costs. Different methods and strategies to attain higher biomass and lipid accumulation in microalgae have been extensively reported in the previous research, but there are fewer inclusive reports that summarize the conventional methods with the modern techniques for lipid enhancement and biodiesel production from microalgae. Therefore, the current review focuses on the latest techniques and advances in different cultivation conditions, the effect of different abiotic and heavy metal stress, and the role of nanoparticles (NPs) in the stimulation of lipid accumulation in microalgae. Techniques such as genetic engineering, where particular genes associated with lipid metabolism, are modified to boost lipid synthesis within the microalgae, the contribution of "Omics" in metabolic pathway studies. Further, the contribution of CRISPR/Cas9 system technique to the production of microalgae biofuel is also briefly described.

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.

Cross-talk between gama-aminobutyric acid and calcium ion regulates lipid biosynthesis in Monoraphidium sp. QLY-1 in response to combined treatment of fulvic acid and salinity stress

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.

Strigolactone mediates jasmonic acid-induced lipid production in microalga Monoraphidium sp. QLY-1 under nitrogen deficiency conditions

The roles of jasmonic acid (JA) in the regulation of cell growth and lipid biosynthesis under the combination of strigolactone (SL) treatment and nitrogen deficiency (ND) were investigated. In this work, the optimised ND condition (46.18%) and ND combined with SL treatment (53.71%) showed 1.11- and 1.29-fold increases in lipid content in Monoraphidium sp. QLY-1 compared with the control condition (41.57%). The levels of JA, glutathione (GSH), and γ-aminobutyric acid (GABA) and lipogenic genes expression were upregulated by the combination of SL and ND, but the ROS level was decreased. Furthermore, exogenous JA supplementation induced the highest lipid content (57.12%) and productivity (312.35 mg L-1 d-1) under ND combined with SL treatment. This study provided a combined strategy for enhancing lipid production and supplied novel insights into the role of JA signalling in regulating lipid synthesis and oxidative stress in microalgae by combining SL treatment with ND.

Exogenous addition of indole acetic acid and kinetin under nitrogen-limited medium enhances lipid yield and expression of glycerol-3-phosphate acyltransferase & diacylglycerol acyltransferase genes in indigenous microalgae: A potential approach for biodiesel production

In the present study, a combination of phytohormones (indole acetic acid and kinetin) was augmented in nitrogen-limited medium to achieve higher biomass and lipid yield in Graesiella emersonii NC-M1 and Chlorophyta sp. NC-M5. This condition was recorded with a 2.3- and 2.5-fold increase in biomass and lipid yield for Graesiella emersonii NC-M1 compared to the nitrogen-limited condition. Also, this condition showed a 1.6- and 1.08-fold increase in lipid yield and neutral lipid compared to the standard condition. Phytohormones addition also reduced oxidative damage caused by nitrogen-limitation and enhanced monounsaturated fatty acid content. Further, a 5.2- and 3.17-fold enhance in expression level of GPAT and DGAT genes were noticed under nitrogen-limited medium supplemented with phytohormones compared to control.

γ-Aminobutyric acid (GABA) regulates lipid production and cadmium uptake by Monoraphidium sp. QLY-1 under cadmium stress

This study explored the effects of γ-aminobutyric acid (GABA) on the production of biomass and lipids and on the uptake of Cd²⁺ by microalgae under cadmium (Cd) stress. Compared with the control and Cd stress alone, 2.5 mM GABA increased the maximum lipid content (55.37%) by 49.37% and 9.42%, respectively. GABA application resulted in increased contents of protein and glutathione (GSH) and in upregulated activity of α-amylase but decreased contents of starch, reactive oxygen species (ROS) and Cd²⁺, with no effect on subsequent biodiesel quality. Additional analysis of GABA further indicated that increased cellular GABA contents could promote lipid synthesis and reduce Cd accumulation by regulating the expression levels of lipogenesis genes, ROS signalling and mineral nutrient uptake under Cd stress. Collectively, these findings show that GABA not only increases lipid production in microalgae but also is involved in the mechanisms by which microalgae respond to Cd stress.

Plant hormone induced enrichment of Chlorella sp. omega-3 fatty acids

BACKGROUND

Omega-3 fatty acids have various health benefits in combating against neurological problems, cancers, cardiac problems and hypertriglyceridemia. The main dietary omega-3 fatty acids are obtained from marine fish. Due to the pollution of marine environment, recently microalgae are considered as the promising source for the omega-3 fatty acid production. However, the demand and high production cost associated with microalgal biomass make it necessary to implement novel strategies in improving the biomass and omega-3 fatty acids from microalgae.

RESULTS

Four plant hormones zeatin, indole acetic acid (IAA), gibberellic acid (GBA) and abscisic acid (ABA) were investigated for their effect on the production of biomass and lipid in isolated Chlorella sp. The cells showed an increase of the biomass and lipid content after treatments with the plant hormones where the highest stimulatory effect was observed in ABA-treated cells. On the other hand, IAA showed the highest stimulatory effect on the omega-3 fatty acids content, eicosapentaenoic acid (EPA) (23.25%) and docosahexaenoic acid (DHA) (26.06%). On the other hand, cells treated with ABA had highest lipid content suitable for the biodiesel applications. The determination of ROS markers, antioxidant enzymes, and fatty acid biosynthesis genes after plant hormones treatment helped elucidate the mechanism underlying the improvement in biomass, lipid content and omega-3 fatty acids. All four plant hormones upregulated the fatty acid biosynthesis genes, whereas IAA particularly increased omega-3-fatty acids as a result of the upregulation of omega-3 fatty acid desaturase.

CONCLUSIONS

The contents of omega-3 fatty acids, the clinically important compounds, were considerably improved in IAA-treated cells. The highest lipid content obtained from ABA-treated biomass can be used for biodiesel application according to its biodiesel properties. The EPA and DHA enriched ethyl esters are an approved form of omega-3 fatty acids by US Food and Drug Administration (FDA) which can be utilized as the therapeutic treatment for the severe hypertriglyceridemia.

Characterization of coal-based fulvic acid and the construction of a fulvic acid molecular model

Fulvic acid (FA) is composed of many molecular units with similar characteristic structures. The characterization and molecular model construction of coal-based FA is the key for the scientific basis and applied science of FA.

Application of chemicals for enhancing lipid production in microalgae-a short review

Microalgae have attracted great attention as a promising sustainable resource for biofuel production. In studies aiming to improve lipid accumulation, many key enzymes involved in lipid biosynthesis were identified and confirmed, but genetic engineering remains a challenge in most species of microalgae. In an alternative approach, various chemical modulators can be used to directly regulate the lipid biosynthesis pathway, with similar effects to gene overexpression and interference approaches, including improving the precursor supply and blocking competing pathways. The produced lipid can be protected from being converted into other metabolites by the chemicals such as lipase inhibitors. In addition, a few chemicals were also demonstrated to greatly influence cell growth and lipid accumulation by indirect regulation of the lipid biosynthesis pathway, such as increasing cell permeability or regulating oxidative stress. Thus, adding chemical modulators can be a useful alternative strategy for improving lipid accumulation in large-scale cultivation of microalgae.

Improvement in lipid production in Monoraphidium sp. QLY-1 by combining fulvic acid treatment and salinity stress

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.

Physiological and Metabolomics Analyses Reveal the Roles of Fulvic Acid in Enhancing the Production of Astaxanthin and Lipids in Haematococcus pluvialis under Abiotic Stress Conditions

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.

Enhancing biomass, lipid production, and nutrient utilization of the microalga Monoraphidium sp. QLZ-3 in walnut shell extracts supplemented with carbon dioxide

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.

Enhancement of lipid accumulation in Monoraphidium sp. QLY-1 by induction of strigolactone

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.

Enhancing Haematococcus pluvialis biomass and γ-aminobutyric acid accumulation by two-step cultivation and salt supplementation

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.

Adaptive evolution of microalgal strains empowered by fulvic acid for enhanced polyunsaturated fatty acid production

Microalgae have emerged as the potential source for value-added products such as polyunsaturated fatty acids (PUFAs). Metabolic engineering of multiple metabolic pathways has promoted eicosapentaenoic acid (EPA) production in microalgae, however, further improvement is warranted owing to the burgeoning demand. Here we improved the microalgal strains by adaptive evolution under hyposalinity treatment, which showed that 70% salinity potentiated the algae to enhance PUFAs. To exploit the maximal PUFA production potential of evolved strains, we subjected evolved algae to light, temperature and fulvic acid treatment. Amongst, fulvic acid (15 mg/L) enhanced growth and achieved the highest EPA content (13.9%) in the evolved diatom. Fulvic acid enhanced antioxidant potential and unprecedently governed the expression of PUFA and lipid biosynthetic genes. Collectively, this investigation demonstrates the efficacy of adaptive evolution empowered by fulvic acid and exemplifies a feasible strain improving strategy to harness the biotechnological potential of microalgae.

Coupling of abiotic stresses and phytohormones for the production of lipids and high-value by-products by microalgae: A review

Microalgae can produce lipids and high-value by-products under abiotic stress conditions, including nutrient starvation, high light intensity, extreme temperature, high salinity and the presence of heavy metals. However, the growth and development of microalgae and the accumulation of metabolites may be inhibited by adverse stresses. In recent years, phytohormones have emerged as a topic of intense focus in microalgae research. Phytohormones could sustain the growth of microalgae under abiotic stress conditions. In addition, the combination of plant hormones and abiotic stresses could further promote the biosynthesis of metabolites and improve the ability of microalgae to tolerate abiotic stresses. This review primarily focuses on the regulatory effects of exogenous phytohormones on the biosynthesis of metabolites by microalgae under adverse environmental conditions and discusses the mechanisms of phytohormone-mediated cell growth, stress tolerance and lipid biosynthesis in microalgae under abiotic stress conditions.

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).

Butylated hydroxytoluene induces astaxanthin and lipid production in Haematococcus pluvialis under high-light and nitrogen-deficiency conditions

The aim of this study was to investigate the effects of butylated hydroxytoluene (BHT) on the production of astaxanthin and lipids in Haematococcus pluvialis LUGU under high-light and nitrogen-deficiency conditions. Astaxanthin and lipid contents were increased by 71.13% and 10.71%, respectively, in algal cells treated with 2 mg L^-1 BHT. The maximal contents of astaxanthin and lipids were 3.17% and 46%, respectively. The levels of reactive oxygen species (ROS) in the presence of BHT were lower than in the control, and this effect involved strong activation of several antioxidases. Additionally, BHT application upregulated endogenous nitric oxide (NO) production. These results showed that this approach is useful for stimulating production of astaxanthin and lipids in H. pluvialis and that exogenous BHT induces astaxanthin and lipid production, which is responsible for the signalling molecule responses against abiotic stress conditions in H. pluvialis.

Enhancing biomass and lipid accumulation in the microalgae Schizochytrium sp. by addition of fulvic acid and EDTA

Enhancing lipid productivity and reducing oxidative damage is essential for lipid overproduction in microalgae. In this study, addition of 20 mg/L fulvic acid (FA) resulted a 34.4% increase of lipid yield in Schizochytrium sp. Furthermore, the cooperative effect of FA and EDTA on cell growth and lipid production was investigated. The combined addition of 20 mg/L FA and 1.0 g/L EDTA yielded a maximal cell dry weight of 130.7 g/L and lipid productivity of 1.16 g/L/h, representing 36.4% and threefold increase over the non-supplemented group, respectively. Moreover, compared with the non-supplemented group, the combined addition strategy exhibited overall lower levels of reactive oxygen species and malondialdehyde, which accompanied with 66.7% and 81.9% higher superoxide dismutase and catalase activity, respectively. Furthermore, a 24.1–37.1% increase of malic enzyme and 19.4–25.2% decrease of phosphoenolpyruvate carboxylase activity was observed during the entire fermentation stage (0–108 h). Results suggested that the combined addition strategy not only enhanced lipid accumulation, but also prevented the lipid peroxidation.

Melatonin: A Multifunctional Molecule That Triggers Defense Responses against High Light and Nitrogen Starvation Stress in Haematococcus pluvialis

Melatonin (MLT), a ubiquitously distributed small molecule, functions in plant responses to various biotic and abiotic stresses. However, the interactions between melatonin and other important molecules in Haematococcus pluvialis response stresses are largely unknown. In the present study, exogenous melatonin improved H. pluvialis resistance to nitrogen starvation and high light. We concluded that exogenous melatonin treatment prevented the reactive oxygen species (ROS) burst and limited cell damage induced by abiotic stress through activation of antioxidant enzymes and antioxidants. Astaxanthin, a major antioxidant in H. pluvialis cells, exhibited a 2.25-fold increase in content after treatment with melatonin. The maximal astaxanthin content was 32.4 mg g^-1. The functional roles of the nitric oxide (NO)-mediated mitogen activated protein kinase (MAPK) signaling pathway and cyclic adenosine monophosphate (cAMP) signaling pathway induced by melatonin were also evaluated. The results clearly indicate that cAMP signaling pathways are positively associated with microalgal astaxanthin biosynthesis. Additionally, the NO-dependent MAPK signaling cascade is activated in response to astaxanthin accumulation induced by melatonin, confirming that MAPK is a target of NO action in physiological processes. This work is the first to use H. pluvialis as in vivo model and documents the influence of melatonin on the physiological response to abiotic stress in this microalgae.

Melatonin enhances lipid production in Monoraphidium sp. QLY-1 under nitrogen deficiency conditions via a multi-level mechanism

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.

Effects of fulvic acids on goat sperm

The effects of adding fulvic acids (FAs) to semen extenders on the quality parameters of frozen–thawed goat buck spermatozoa remain undetermined. Buck semen samples collected from six mature goat bucks once a week were diluted with Tris–egg yolk-based extenders. The diluted semen samples were supplemented with FAs (0.2, 0.4 and 0.6%, w/w), cryopreserved, and evaluated for sperm-quality parameters. Addition of FAs to the extender increased progressive motility, acrosome integrity, membrane integrity, and superoxide dismutase and catalase activities and decreased percentage abnormality and sperm malondialdehyde level compared with the control group. However, excessive FA addition (>0.4%, w/w) to semen extenders did not improve the efficiency. The results indicated that FAs could be a promising cryoprotectant for goat buck sperm.

Comprehensive analysis of transcriptional and proteomic profiling reveals silver nanoparticles-induced toxicity to bacterial denitrification

Although the toxicity of silver nanoparticles (Ag NPs or nanosilver) to model bacteria has been reported, the effects of Ag NPs on microbial denitrification under anoxic conditions and the mechanism of Ag NPs induced-toxicity to denitrification remain unclear. In this study, the effects of Ag NPs on Paracoccus denitrificans under anoxic conditions were investigated, and the mechanism was explored by analyzing the transcriptional and proteomic responses of bacteria to Ag NPs. The presence of 5mg/L Ag NPs led to excessive nitrate accumulation (232.5 versus 5.3mg/L) and increased nitrous oxide emission. Transcriptional analysis indicated that Ag NPs restrained the expression of key genes related to denitrification. Specifically, the genes involved in denitrifying catalytic reduction and electron transfer were significantly down-regulated. Moreover, the expression of the genes responsible for polyhydroxybutyrate synthesis was enhanced, which was adverse to denitrification. Proteomic profiling revealed that the syntheses of the proteins involved in catalytic process, electron transfer, and metabolic process were inhibited by Ag NPs. The activities of nitrate reductase and nitrite reductase in the presence of 5mg/L Ag NPs were only 42% and 61% of those in the control, respectively, indicating the inhibition of denitrifying enzymes. These results improve understanding of the inhibitory mechanism of Ag NPs toward bacterial denitrification.

A strategy for promoting lipid production in green microalgae Monoraphidium sp. QLY-1 by combined melatonin and photoinduction

Microalgae lipids are potential candidates for the production of renewable biodiesel. The combination of plant hormones and two-step cultivation regulates lipid production in microalgae. A strategy for promoting lipid accumulation in Monoraphidium sp. QLY-1 by combining exogenous melatonin (MT) and photoinduction was developed. The effects of melatonin on the lipid content, reactive oxygen species (ROS), and activities of three key fatty acid biosynthetic enzyme in Monoraphidium sp. QLY-1 were investigated. The lipid content increased by 1.32-fold under 1μM melatonin treatment. The maximum lipid content achieved was 49.6%. However, the protein and carbohydrate contents decreased rapidly from 57.21% to 47.96% and from 53.4% to 37.71%, respectively. Biochemical and physiological analyses suggested that the ROS and lipid biosynthesis-related enzyme activities correlated with increased lipid accumulation under photo-melatonin induction conditions.