The effect of nitrogen limitation on the physiology and metabolism of chlorella vulgaris var L3

Soil health improvement by inoculation of indigenous microalgae in saline soil

Using biological methods to improve saline soils is recognized as an eco-friendly and sustainable way. In this study, two indigenous algae YJ-1 and YJ-2 screened from salinized farmland were inoculated into saline soils with different salinization levels to investigate their potential in enhancing soil health by laboratory microcosm experiment. The results showed that individual inoculation of the two algae quickly resulted in the formation of algal crusts, and the chlorophyll content in the saline soils gradually increased with the incubation time. The soil pH decreased significantly from the initial 8.15-9.45 to 6.97-7.56 after 60-day incubation. The exopolysaccharides secretion and the activities of catalase, sucrase, and urease in saline soils also increased. Microalgal inoculation increased soil organic matter storage, while decreasing the available nutrient contents possibly due to the depletion of microalgal growth. PCA and PCC results identified that microalgal biomass as the predominant variable affecting soil quality. Overall, these data revealed the great potential of microalgae in the amelioration of saline soils, especially in pH reduction and enzyme activity enhancement. This study will provide the theoretical foundation for improving saline soils via algalization.

Iron and nitrogen regulate carbon transformation in a methanotroph-microalgae system

Nutrient supply is important for maintaining a methanotroph and microalgae (MOB-MG) system for biogas valorization. However, there is a lack of understanding regarding how key elements regulate the growth of a MOB-MG coculture. In this study, a MOB-MG coculture with high protein content (0.47 g/g biomass) was established from waste activated sludge using synthetic biogas. An increase in iron availability substantially stimulated the specific growth rate (from 0.18 to 0.62 day-1) and biogas conversion rate (from 26.81 to 106.57 mg-C L-1 day-1) of the coculture. Moreover, the protein content remained high (0.51 g/g biomass), and the total lipid content increased (from 0.09 to 0.14 g/g biomass). Nitrogen limitation apparently constrained the specific growth rate (from 0.64 to 0.28 day-1) and largely reduced the protein content (from 0.51 to 0.31 g/g biomass) of the coculture. Intriguingly, the lipid content remained unchanged after nitrogen was depleted. The eukaryotic community was consistently dominated by MG belonging to Chlorella, while the populations of MOB shifted from Methylococcus/Methylosinus to Methylocystis due to iron and nitrogen amendment. In addition, diverse non-methanotrophic heterotrophs were present in the community. Their presence neither compromised the performance of the coculture system nor affected the protein content of the biomass. However, these heterotrophs may contribute to high carbon conversion efficiency by utilizing the dissolved organic carbon released by MOB and MG. Overall, the findings highlight the vital roles of iron and nitrogen in achieving efficient conversion of biogas, fast growth of cells, and optimal biomass composition in a MOB-MG coculture system.

Potential of algal oil production from secondary treated sewage: a study using Chlorella vulgaris and synthetic wastewater

To assess the potential of using the secondary treated wastewater for the production of algal biofuel, batch experiments were carried out in photobioreactors using indigenous Chlorella vulgaris isolated from the natural freshwater body. Secondary treated wastewater with partial nitrification was simulated using various proportions of NO3-N and NH4-N while keeping the total nitrogen the same. Experiments with similar concentrations of nitrate without the NH4-N were used for comparison. In the presence of only NO3-N in the concentration range of 9-37 mg/L, the growth and fatty acid methyl ester (FAME) production was similar to the literature reports. When NH4-N was present along with NO3-N, the biomass growth was adversely affected, indicating an impact on the metabolic activity. For the same initial concentrations of nitrate in the culture, the maximum biomass concentration was reduced by 50-60% in the presence of NH4-N. The FAME profile changed significantly and a new FAME was identified, suggesting an impact on the lipid synthesis pathway. Comparison and analysis with the help of existing literature indicated that the adverse effect due to NH4-N was a function of pH. The growth, biomass yield, and FAME production were unaffected by a wide range of phosphorus concentrations. Maximum fatty acid methyl ester (FAME) suitable for biodiesel production (fatty acid carbon chain length C16 to C18) was 381.01 mg/L (224.58 mg/g of dry biomass), produced at NO3-N concentration of 18.5 mg/L and initial nitrogen loading per unit biomass of 0.37 g NO3-N/g of dry biomass.

Biodiesel Production from the Marine Alga Nannochloropsis oceanica Grown on Yeast Wastewater and the Effect on Its Biochemical Composition and Gene Expression

Microalgae-based biodiesel synthesis is currently not commercially viable due to the high costs of culture realizations and low lipid yields. The main objective of the current study was to determine the possibility of growing Nannochloropsis oceanica on Saccharomyces cerevisiae yeast wastewater for biodiesel generation at an economical rate. N. oceanica was grown in Guillard F/2 synthetic medium and three dilutions of yeast wastewater (1, 1.25, and 1.5%). Biodiesel properties, in addition to carbohydrate, protein, lipid, dry weight, biomass, lipid productivity, amino acids, and fatty acid methyl ester (FAMEs) content, were analyzed and the quality of the produced biodiesel is assessed. The data revealed the response of N. oceanica to nitrogen-deficiency in the three dilutions of yeast wastewater. N. oceanica in Y2 (1.25%) yeast wastewater dilution exhibited the highest total carbohydrate and lipid percentages (21.19% and 41.97%, respectively), and the highest lipid productivity (52.46 mg L-1 day -1) under nitrogen deficiency in yeast wastewater. The fatty acids profile shows that N. oceanica cultivated in Y2 (1.25%) wastewater dilution provides a significant level of TSFA (47.42%) and can be used as a feedstock for biodiesel synthesis. In addition, N. oceanica responded to nitrogen shortage in wastewater dilutions by upregulating the gene encoding delta-9 fatty acid desaturase (Δ9FAD). As a result, the oleic and palmitoleic acid levels increased in the fatty acid profile of Y2 yeast wastewater dilution, highlighting the increased activity of Δ9FAD enzyme in transforming stearic acid and palmitic acid into oleic acid and palmitoleic acid. This study proved that the Y2 (1.25%) yeast wastewater dilution can be utilized as a growth medium for improving the quantity of specific fatty acids and lipid productivity in N. oceanica that affect biodiesel quality to satisfy global biodiesel requirements.

Supplementation of PUFA extracted from microalgae for the development of chicken patties

In recent years, there has been a growing interest in development of a diverse range of foods that are enriched with omega-3 fatty acids. It is widely recognized that through dietary interventions, the lipid fraction of food can be modified to enhance its nutritional content. This study is aimed to develop chicken patties enriched with poly unstaurated fatty acids (PUFAs) extracted from microalgae aurintricarboxylic acid (ATA) concentration of 0% (T0), 1% (T1), 2% (T2), and 3% (T3). All treatments were stored at -18 °C for one month and analysed at an interval of 0, 10, 20, and 30 days to assess the effect of PUFAs supplementation on physicochemical, oxidative, microbiological and organoleptic properties of chicken patties. The results revealed that moisture content was significantly increased during the storage; the maximum moisture was observed in T0 (67.25% ± 0.03) on day 0, while the minimun was found in T3 (64.69% ± 0.04) on day 30. Supplemenatation of PUFAs in chicken patties significantly enhanced the fat content of the product the highest fat content was observed for T3 (9.7% ± 0.06. An increase in PUFAs concentration led to a significant increase in thiobarbituric acid reactive substances (TBARS). TBARS were increased from 1.22 ± 0.43 at 0 days to 1.48 ± 0.39 at 30 days of storage. The PUFAs incorporation negatively effected sensory acceptance of the product ranging from (8.41 ± 0.17 to 7.28 ± 0.12). However, the sensory scores were in acceptable range for supplemented patties as compared to control sample. Treatment T3 depicted the highest nutritional content. The sensory and physiochemical analysis of supplemented patties suggested that PUFAs extracted from microalgae can be used as a functional ingredient in the preparation various meat products particularly chicken meta patties. However, antioxidants should be added to to prevent lipid oxidation in the product.

Regionalized Life-Cycle Water Impacts of Microalgal-Based Biofuels in the United States

While algal biofuels have the potential to reduce the national reliance on fossil fuels, high water consumption associated with algal biomass cultivation represents a major concern potentially compromising the sustainable commercialization of this technology. This study focuses on quantifying the water footprint (WF) and water scarcity footprint (WSF) of renewable diesel derived from algal biomass and provides insights into where algal cultivation is less water-intensive than traditional ethanol and biodiesel feedstocks. Results are generated with an engineering process model developed to predict the life-cycle water consumption, considering green, blue, and gray water, of algae facilities across the United States at a high spatiotemporal resolution. The total WFs for Florida and Arizona are determined to be 13.1 and 17.6 m³ GJ^-1, respectively. The blue WF in Arizona is shown to be 8.5 times larger than in Florida, while the green WF is 4.5 times smaller, but when combined into a total WF, there is just a 26% difference between the two locations. The analysis reveals that the total life-cycle WFs of algal renewable diesel are smaller than the optimal WFs of corn ethanol and soybean biodiesel. Algal systems benefit from higher growth rates and offer the opportunity to manage wastewater streams, therefore generating smaller green and gray WFs than those of conventional biofuels. The WSF analysis identifies the Gulf Coast as the most suitable region for algal cultivation, with cultivation in the western US shown to exacerbate local water stress levels.

Metabolic pathways for biosynthesis and degradation of starch in Tetraselmis chui during nitrogen deprivation and recovery

Tetraselmis chui is known to accumulate starch when subjected to stress. This phenomenon is widely studied for the purpose of industrial production and process development. Yet, knowledge about the metabolic pathways involved is still immature. Hence, in this study, transcription of 27 starch-related genes was monitored under nitrogen deprivation and resupply in 25 L tubular photobioreactors. T. chui proved to be an efficient starch producer under nitrogen deprivation, accumulating starch up to 56% of relative biomass content. The prolonged absence of nitrogen led to an overall down-regulation of the tested genes, in most instances maintained even after nitrogen replenishment when starch was actively degraded. These gene expression patterns suggest post-transcriptional regulatory mechanisms play a key role in T. chui under nutrient stress. Finally, the high productivity combined with an efficient recovery after nitrogen restitution makes this species a suitable candidate for industrial production of high-starch biomass.

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.

Growth and Biochemical Composition Characteristics of Arthrospira platensis Induced by Simultaneous Nitrogen Deficiency and Seawater-Supplemented Medium in an Outdoor Raceway Pond in Winter

Arthrospira platensis, a well-known cyanobacterium, is widely applied not only in human and animal nutrition but also in cosmetics for its high amounts of active products. The biochemical composition plays a key role in the application performance of the Arthrospira biomass. The present study aimed to evaluate the growth and biochemical composition characteristics of A. platensis, cultured with a nitrogen-free and seawater-supplemented medium in an outdoor raceway pond in winter. The results showed that the biomass yield could achieve 222.42 g m⁻², and the carbohydrate content increased by 247% at the end of the culture period (26 d), compared with that of the starter culture. The daily and annual areal productivities were 3.96 g m⁻² d⁻¹ and 14.44 ton ha⁻¹ yr⁻¹ for biomass and 2.88 g m⁻² d⁻¹ and 10.53 ton ha⁻¹ yr⁻¹ for carbohydrates, respectively. On the contrary, a profound reduction was observed in protein, lipid, and pigment contents. Glucose, the main monosaccharide in the A. platensis biomass, increased from 77.81% to 93.75% of total monosaccharides. Based on these results, large-scale production of carbohydrate-rich A. platensis biomass was achieved via a low-cost culture, involving simultaneous nitrogen deficiency and supplementary seawater in winter.

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.

Transcriptome responses of the dinoflagellate Karenia mikimotoi driven by nitrogen deficiency

The availability of ambient N nutrient is often correlated with the occurrences of harmful algal bloom formed by certain dinoflagellates, making it important to understand how these species might be responding to such conditions. Here, transcriptome sequencing of Karenia mikimotoi was conducted to understand the underlying molecular mechanisms by which this dinoflagellate copes with nitrogen (N) deficiency. Transcriptomic analysis revealed 8802 unigenes (3.56%) that were differentially expressed with ≥ 2-fold change. Under N-depleted conditions, genes involved in glycolysis, fatty acid metabolism, and the tricarboxylic acid (TCA) cycle as well as lipid accumulation were significantly upregulated. The elevated expression of enzymes used in protein degradation and turnover suggests possible metabolic reconfiguration towards accelerated N recycling. Moreover, a significant increase in urea transporter was observed, indicating increased assimilation of organic nitrogen resources as an alternative in N-depleted cultures of K. mikimotoi. The down-regulated glutamate synthase genes were also identified under N deficiency, suggesting suppression of primary amino acid synthesis to save N resource. Taken together, results of this study show enhanced multiple N resource acquisition and reuse of multiple N resources constitute a comprehensive strategy to cope with N deficiency in a dinoflagellate.

Optimal influent N-to-P ratio for stable microalgal cultivation in water treatment and nutrient recovery

Species specific nitrogen-to-phosphorus molar ratio (NPR) has been suggested for green microalgae. Algae can store nitrogen and phosphorus, suggesting that the optimum feed concentration dynamically changes as function of the nutrient storage. We assessed the effect of varying influent NPR on microalgal cultivation in terms of microbial community stability, effluent quality and biokinetics. Mixed green microalgae (Chlorella sorokiniana and Scenedesmus sp.) and a monoculture of Chlorella sp. were cultivated in continuous laboratory-scale reactors treating used water. An innovative image analysis tool, developed in this study, was used to track microbial community changes. Diatoms proliferated as influent NPR decreased, and were outcompeted once cultivation conditions were restored to the optimal NPR range. Low NPR operation resulted in decrease in phosphorus removal, biomass concentration and effluent nitrogen concentration. ASM-A kinetic model simulation results agreed well with operational data in the absence of diatoms. The failure to predict operational data in the presence of diatoms suggest differences in microbial activity that can significantly influence nutrient recovery in photobioreactors (PBR). No contamination occurred during Chlorella sp. monoculture cultivation with varying NPRs. Low NPR operation resulted in decrease in biomass concentration, effluent nitrogen concentration and nitrogen quota. The ASM-A model was calibrated for the monoculture and the simulations could predict the experimental data in continuous operation using a single parameter subset, suggesting stable biokinetics under the different NPR conditions. Results show that controlling the influent NPR is effective to maintain the algal community composition in PBR, thereby ensuring effective nutrients uptake.

Stress induced modifications in photosystem II electron transport, oxidative status, and expression pattern of acc D and rbc L genes in an oleaginous microalga Desmodesmus sp

The study aimed at understanding the biochemical and molecular level modifications in Desmodesmus sp. under lipid inducing stress conditions. The low-temperature (5 °C) incubation and nitrogen starvation reduced the PS II electron transport in microalga with a maximum reduction of 50-57% in ET₀/ABS values. The PS II electron transport recovered in UV treated cultures after an initial reduction of 87-93% in ET₀/ABS values. A 2.7-4.4 fold increase in ROS and MDA levels was observed under low-temperature incubation, and nitrogen starvation. The UV treatment caused 1.3-2.4 fold higher ROS and MDA levels than control. The low-temperature incubated, nitrogen starved, and UV treated cultures showed 2.4-4 fold higher acc D gene expression. A higher rbc L gene expression was observed under low-temperature stress. The study showed modifications in PS II electron transport, oxidative status, and expression of acc D and rbc L genes under stress conditions.

Development of a single helical baffle to increase CO2 gas and microalgal solution mixing and Chlorella PY-ZU1 biomass yield

A single helical baffle (SHB), consisting of twisted turns, was developed to convert straight flow into spiral flow in a Chlorella PY-ZU1 open raceway pond (ORWP) bubbled with 15% CO₂. Microalgal solution flowing through the SHB alternative helical interspaces generated whirling flow both vertically and horizontally, which decreased mixing and increased mass transfer rates. The optimized SHB had a pitch length to total SHB length ratio of 0.13 and SHB diameter to ORWP single channel width ratio of 0.30, which decreased mixing times and increased mass transfer coefficients by 41.1% and 38.4% respectively. SHB moved Chlorella PY-ZU1 from the ORWP bottom to the top, increasing light exposure for photosynthesis. Cellular electron transfer rates and photochemical efficiency (φPSII) increased by 18%, chlorophyll a content increased by 16% and variable to maximum fluorescence ratio increased by 13%. The microalgal biomass of SHB ORWP was 23% higher than that of conventional ORWP.

Storage of starch and lipids in microalgae: Biosynthesis and manipulation by nutrients

Microalgae accumulate starch and lipid as storage metabolites under nutrient depletion, which can be used as sustainable feedstock for biorefinery. Omics analysis coupled with enzymatic and genetic verifications uncovered a partial picture of pathways and important enzymes or regulators related to starch and lipid biosynthesis as well as the carbon partitioning between them under nutrient depletion conditions. Depletion of macronutrients (N, P, and S) resulted in considerable enhancement of starch and/or lipid content in microalgae, but the accompanying declined photosynthesis hampered the achievements of high concentrations. This review summarized the current knowledge on the pathways and the committed steps as well as their carbon allocation involved in starch and lipid biosynthesis, and focused on the manipulation of different nutrients and the alleviation of oxidative stress for enhanced storage metabolites production. The biological and engineering approaches to cope with the conflict between biomass production and storage metabolites accumulation are proposed.

Regulatory carbon metabolism underlying seawater-based promotion of triacylglycerol accumulation in Chlorella kessleri

Chlorella kessleri accumulates triacylglycerol usable for biodiesel-fuel production to >20% dry cell weight in three days when cultured in three-fold diluted seawater, which imposes the combinatory stress of hyperosmosis and nutrients limitation. The quantitative behavior of major C-compounds, and related-gene expression patterns were investigated in Chlorella cells stressed with hyperosmosis, nutrients limitation, or their combination, to elucidate the C-metabolism for economical seawater-based triacylglycerol accumulation. Combinatory-stress cells showed repressed protein synthesis with initially accumulated starch being degraded later, the C-metabolic flow thereby being diverted to fatty acid and subsequent triacylglycerol accumulation. This C-flow diversion was induced by cooperative actions of nutrients-limitation and hyperosmosis. Semi-quantitative PCR analysis implied positive rewiring of the diverted C-flow into triacylglycerol in combinatory-stress cells through upregulation of gene expression concerning fatty acid and triacylglycerol synthesis, and starch synthesis and degradation. The information of regulatory C-metabolism will help reinforce the seawater-based triacylglycerol accumulation ability in algae including Chlorella.

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.

Response of lipid biosynthesis in Chlorella pyrenoidosa to intracellular reactive oxygen species level under stress conditions

An increase in the total lipid content in algal cells under stress conditions is often accompanied by an increase in reactive oxygen species (ROS). However, the link between these two events is unclear. In this study, the regulatory mechanism of ROS formation on lipid accumulation in C. pyrenoidosa was investigated using a Fenton-like reaction. A high Spearman correlation coefficient of 0.901 was obtained between the Hydroxyl radical (OH) level and lipid content. Importantly, the increase in the total lipid content was clearly coupled with a significant increase in the intracellular OH concentration rather than increases in the H₂O₂ and O₂^- concentrations. Transcriptome data confirms that most of the differential expression genes (DEGs) involved in fatty acid and glycerolipid biosynthesis were up-regulated by the increased OH under stress conditions. These results reveal that lipid accumulation in algal cells was promoted by OH.

Why Algae Release Volatile Organic Compounds-The Emission and Roles

A wide spectrum of volatile organic compounds (VOCs) are released from algae in aquatic ecosystems. Environmental factors such as light, temperature, nutrition conditions and abiotic stresses affect their emission. These VOCs can enhance the resistance to abiotic stresses, transfer information between algae, play allelopathic roles, and protect against predators. For homogeneous algae, the VOCs released from algal cells under stress conditions transfer stress information to other cells, and induce the acceptors to make a preparation for the upcoming stresses. For heterogeneous algae and aquatic macrophytes, the VOCs show allelopathic effects on the heterogeneous neighbors, which benefit to the emitter growth and competing for nutrients. In cyanobacterial VOCs, some compounds such as limonene, eucalyptol, β-cyclocitral, α-ionone, β-ionone and geranylacetone have been detected as the allelopathic agents. In addition, VOCs can protect the emitters from predation by predators. It can be speculated that the emission of VOCs is critical for algae coping with the complicated and changeable aquatic ecosystems.

Effect of ammonium nitrogen on microalgal growth, biochemical composition and photosynthetic performance in mixotrophic cultivation

To enhance microalgal growth and optimize ammonium utilization, the effect of ammonium on microalgal growth, biochemical composition and photosynthetic performance were investigated by mixotrophic cultivation of microalga Spirulina platensis comparing with autotrophic cultivation. The results indicated that elevated ammonium significantly affected the microalgal growth, but the microalga in mixotrophic cultivation showed better growth and stronger tolerance to higher ammonium. The microalgal proteins were increased by increasing nitrogen concentration. The synthesis of microalgal carbohydrates was inhibited by higher ammonium, especially in mixotrophic cultivation. The addition of ammonium decreased the microalgal lipids in autotrophic cultivation but increased microalgal lipids in mixotrophic cultivation. Ammonium negatively affected the microalgal photosynthetic performance. The inhibition was intensified by elevated ammonium, inducing stronger photosystem protection mechanism, particularly in mixotrophic cultivation. The rate of ammonium inhibition to the microalgal photosystem was quick in the early stage by decreasing electron transport rate of PS II.

Production of primary metabolites in Microcystis aeruginosa in regulation of nitrogen limitation

The aim of this work was to study the regulatory effect of nitrogen (N) deficiency on primary metabolites in Microcystis aeruginosa, and promote the utilization of the alga. Low-N and Non-N conditions, especially Non-N, reduced the cell growth and photosynthetic abilities compared to Normal-N, as N deficiency triggered the down-regulation of genes involving in the photosynthetic process. Non-N not changed lipid content, due to no up-regulation of genes that promoted lipid synthesis. Soluble protein content significantly decreased under Non-N, which may result from the declined expression of genes relating to amino acid and histidyl-transfer RNA synthesis. Soluble and insoluble carbohydrate content significantly increased under Non-N, as the expression variation of genes blocked sugar degradation and promoted lipopolysaccharide synthesis. Therefore, M. aeruginosa can be used as the feedstock to produce carbohydrates under N deficiency for bioethanol production, and the remainder lipids after carbohydrate extraction can be used to produce biodiesel.

Variation of fatty acid desaturation in response to different nitrate levels in Auxenochlorella pyrenoidosa

Microalgae are promising feedstocks for biodiesel, where the high proportion of monounsaturated fatty acid such as oleic acid (C18:1) is preferred. To regulate fatty acid desaturation in microalgae, the relationship among nitrate concentration, fatty acid composition and the expression levels of desaturase genes was explored. Dynamic variations of fatty acid profiles suggested nitrate could induce desaturation of C18 fatty acids. The content of C18:1 in Auxenochlorella pyrenoidosa was 30.88% at 0 g l^-1 nitrate concentration compared with 0.48% at 1.5 g l^-1. The expressions of relative delta-9, 12 and 15 fatty acid desaturase genes (Δ9, Δ12 and Δ15FADs) were further investigated. The 330% upregulated expression of Δ9FAD in logarithmic phase at 0 g l^-1 resulted in C18:1 accumulation. Moreover, nitrate replenishment caused a sharp reduction of C18:1 from 34.79% to 0.22% and downregulation of Δ9FAD expression to 1% of the nitrate absence level, indicating the pivotal role of Δ9FAD in C18:1 accumulation. Finally, overexpression of Δ9FAD in Escherichia coli and Saccharomyces cerevisiae resulted in an increase of C18:1, confirming its ability of desaturating C18:0. The results could provide a new approach and scientific guidance for the improvement of biodiesel quality and industrialization of high-valued chemicals by means of metabolic engineering.

Effects of nitrogen nutrients on the volatile organic compound emissions from Microcystis aeruginosa

Cyanobacteria release abundant volatile organic compounds (VOCs), which can poison other algae and cause water odor. To uncover the effects of nitrogen (N) nutrients on the formation of cyanobacteria VOCs, the cell growth, VOC emission and the expression of genes involving in VOC formation in Microcystis aeruginosa were investigated under different N conditions. With the supplement of NaNO₃, NaNO₂, NH₄Cl, urea, Serine (Ser) and Arginine (Arg) as the sole N source, NaNO₃, urea and Arg showed the best effects on M. aeruginosa cell growth, and limited N supply inhibited the cell growth. M. aeruginosa released 26, 25, 23, 27, 23 and 25 compounds, respectively, in response to different N forms, including furans, sulfocompounds, terpenoids, benzenes, hydrocarbons, aldehydes, and esters. Low-N especially Non-N condition markedly promoted the VOC emission. Under Non-N condition, four up-regulated genes involving in VOC precursor formation were identified, including the genes of pyruvate kinase, malic enzyme and phosphotransacetylase for terpenoids, the gene of aspartate aminotransferase for benzenes and sulfocompounds. In eutrophic water, cyanobacteria release different VOC blends using various N forms, and the reduction of N amount caused by cyanobacteria massive growth can promote algal VOC emission by up-regulating the gene expression.

Effect of nitrogen limitation on biochemical composition and photosynthetic performance for fed-batch mixotrophic cultivation of microalga Spirulina platensis

In this study, the effect of nitrogen limitation on microalgal growth, biochemical composition and photosynthetic performance was investigated in fed-batch mixotrophic cultivation of microalga Spirulina platensis, compared with that in autotrophic cultivation. The microalgal biomass productivity was greatly enhanced by mixotrophic cultivation. With nitrogen limitation, the mixotrophic culture accelerated the degradation of microalgal pigments and proteins to supply intracellular nitrogen for maintaining higher biomass productivity, simultaneously accumulating more carbohydrates. The mixotrophic cultivation amplified the adverse effect of nitrogen limitation on the microalgal photosynthetic performance in comparison with autotrophic cultivation. This fed-batch mixotrophic cultivation is an effective strategy for enhancing biomass productivity and total carbohydrates yield under nitrogen limited conditions.

Cultivation, characterization, and properties of Chlorella vulgaris microalgae with different lipid contents and effect on fast pyrolysis oil composition

A systematic study of the effect of nitrogen levels in the cultivation medium of Chlorella vulgaris microalgae grown in photobioreactor (PBR) on biomass productivity, biochemical and elemental composition, fatty acid profile, heating value (HHV), and composition of the algae-derived fast pyrolysis (bio-oil) is presented in this work. A relatively high biomass productivity and cell concentration (1.5 g of dry biomass per liter of cultivation medium and 120 × 10⁶ cells/ml, respectively) were achieved after 30 h of cultivation under N-rich medium. On the other hand, the highest lipid content (ca. 36 wt.% on dry biomass) was obtained under N-depletion cultivation conditions. The medium and low N levels favored also the increased concentration of the saturated and mono-unsaturated C16:0 and C18:1(n-9) fatty acids (FA) in the lipid/oil fraction, thus providing a raw lipid feedstock that can be more efficiently converted to high-quality biodiesel or green diesel (via hydrotreatment). In terms of overall lipid productivity, taking in consideration both the biomass concentration in the medium and the content of lipids on dry biomass, the most effective system was the N-rich one. The thermal (non-catalytic) pyrolysis of Chlorella vulgaris microalgae produced a highly complex bio-oil composition, including fatty acids, phenolics, ethers, ketones, etc., as well as aromatics, alkanes, and nitrogen compounds (pyrroles and amides), originating from the lipid, protein, and carbohydrate fractions of the microalgae. However, the catalytic fast pyrolysis using a highly acidic ZSM-5 zeolite, afforded a bio-oil enriched in mono-aromatics (BTX), reducing at the same time significantly oxygenated compounds such as phenolics, acids, ethers, and ketones. These effects were even more pronounced in the catalytic fast pyrolysis of Chlorella vulgaris residual biomass (after extraction of lipids), thus showing for the first time the potential of transforming this low value by-product towards high added value platform chemicals.

Leveraging algal omics to reveal potential targets for augmenting TAG accumulation

Ongoing global efforts to commercialize microalgal biofuels have expedited the use of multi-omics techniques to gain insights into lipid biosynthetic pathways. Functional genomics analyses have recently been employed to complement existing sequence-level omics studies, shedding light on the dynamics of lipid synthesis and its interplay with other cellular metabolic pathways, thus revealing possible targets for metabolic engineering. Here, we review the current status of algal omics studies to reveal potential targets to augment TAG accumulation in various microalgae. This review specifically aims to examine and catalog systems level data related to stress-induced TAG accumulation in oleaginous microalgae and inform future metabolic engineering strategies to develop strains with enhanced bioproductivity, which could pave a path for sustainable green energy.

Growth kinetics, fatty acid composition and metabolic activity changes of Crypthecodinium cohnii under different nitrogen source and concentration

The effect of varying concentrations of the nitrogen source on the growth kinetics, lipid accumulation, lipid and DHA productivity, and fatty acid composition of C. cohnii was elucidated. Growth of C. cohnii was in three distinct growth stages: cell growth, lipid accumulation and a final lipid turnover stage. Most of lipids were accumulated in lipid accumulation stage (48-120 h) though, slow growth rate was observed during this stage. NaNO₃ supported significantly higher lipid content (26.9% of DCW), DHA content (0.99 g/L) and DHA yield (44.2 mg/g glucose) which were 2.5 to 3.3-folds higher than other N-sources. The maximum level of C16-C18 content (% TFA) was calculated as 43, 54 and 43% in lipid accumulation stage under low nitrogen (LN, 0.2 g/L), medium nitrogen (MN, 0.8 g/L) and high nitrogen (HN, 1.6 g/L) treatments, respectively. Cultures with LN, by down-regulating cell metabolism, trigger onset of lipogenic enzymes. Conversely, NAD⁺/NADP⁺-dependent isocitrate dehydrogenase (NAD⁺/NADP⁺-ICDH) were less active in LN than HN treatments which resulted in retardation of Kreb's Cycle and thereby divert citrate into cytoplasm as substrate for ATP-citrate lyase (ACL). Thereby, ACL and fatty acid synthase (FAS) were most active in lipid accumulation stage at LN treatments. Glucose-6-phosphate dehydrogenase (G6PDH) was more active than malic enzyme (ME) in lipid accumulation stage and showed higher activities in NaNO₃ than other N-sources. This represents that G6PDH contributes more NADPH than ME in C. cohnii. However, G6PDH and ME together seems to play a dual role in offering NADPH for lipid biosynthesis. This concept of ME together with G6PD in offering NADPH for lipogenesis might be novel in this alga and needed to be explored.

Effects of twenty standard amino acids on biochemical constituents, docosahexaenoic acid production and metabolic activity changes of Crypthecodinium cohnii

The influence of 20 standard amino acids was investigated on growth, lipid accumulation, docosahexaenoic acid (DHA) production and cell biochemical composition of Crypthecodinium cohnii. C. cohnii efficiently utilize organic nitrogen (predominantly threonine and to a lesser extent tyrosine and serine) as compared to inorganic nitrogen (NH₄)₂SO₄. However, No significant effect was observed on major biochemical composition of C. cohnii (lipids, carbohydrates and proteins) under N limitation or supplementation with different N-sources. Key lipogenic enzymes glucose-6-phosphate dehydrogenase, ATP-citrate lyase, fatty acid synthase, malic enzyme, citrate synthase (CS), NAD⁺ and NADP⁺ dependent isocitrate dehydrogenase were shown to be vital in lipogenesis of C. cohnii. Our results indicated that the process of lipid accumulation in C. cohnii is growth-associated and does not depend upon the trigger of nitrogen depletion. This unusual behavior would suggest that the metabolism of the cells may not be entirely the same as in other lipid-accumulating microorganisms.

Maximization of cell growth and lipid production of freshwater microalga Chlorella vulgaris by enrichment technique for biodiesel production

Chlorella vulgaris was cultivated under limitation and starvation and under controlled conditions using different concentrations of nitrate (NaNO₃) and phosphate (K₂HPO₄ and KH₂PO₄) chemicals in modified Bold basal medium (BBM). The biomass and lipid production responses to different media were examined in terms of optical density, cell density, dry biomass, and lipid productivity. In the 12-day batch culture period, the highest biomass productivity obtained was 72.083 mg L^-1 day^-1 under BBM - N_controlP_limited condition. The highest lipid content, lipid concentration, and lipid productivity obtained were 53.202 %, 287.291 mg/L, and 23.449 mg L^-1 day^-1 under BBM - N_ControlP_Deprivation condition, respectively. Nitrogen had a major effect in the biomass concentration of C. vulgaris, while no significant effect was found for phosphorus. Nitrogen and phosphorus starvation was found to be the strategy affecting the lipid accumulation and affected the lipid composition of C. vulgaris cultures.

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

Fulvic acid (FA) triggers lipid accumulation in Monoraphidium sp. FXY-10, which can produce biofuels. Therefore, the metabolism shift and gene expression changes influenced by fulvic acid should be investigated. In this study, lipid and protein contents increased rapidly from 44.6% to 54.3% and from 31.4% to 39.7% under FA treatment, respectively. By contrast, carbohydrate content sharply declined from 49.5% to 32.5%. The correlation between lipid content and gene expression was also analyzed. Results revealed that accD, ME, and GPAT genes were significantly correlated with lipid accumulation. These genes could likely influence lipid accumulation and could be selected as modification candidates. These results demonstrated that FA significantly increased microalgal lipid accumulation by changing the intracellular reactive oxygen species, gene expression, and enzyme activities of acetyl-CoA carboxylase, malic enzyme, and phosphoenolpyruvate carboxylase.

Improving carbohydrate and starch accumulation in Chlorella sp. AE10 by a novel two-stage process with cell dilution

BACKGROUND

Microalgae are highly efficient cellular factories that capture CO₂ and are also alternative feedstock for biofuel production. Carbohydrates, proteins, and lipids are major biochemical components in microalgae. Carbohydrates or starch in microalgae are possible substrates in yeast fermentation for biofuel production. The carbon partitioning in microalgae could be regulated through environmental stresses, such as high concentration of CO₂, high light intensity, and nitrogen starvation conditions. It is essential to obtain carbohydrate-rich microalgae via an optimal bioprocess strategy.

RESULTS

The carbohydrate accumulation in a CO₂ tolerance strain, Chlorella sp. AE10, was investigated with a two-stage process. The CO₂ concentration, light intensity, and initial nitrogen concentration were changed drastically in both stages. During the first stage, it was cultivated over 3 days under 1% CO₂, a photon flux of 100 μmol m^-2 s^-1, and 1.5 g L^-1 NaNO₃. It was cultivated under 10% CO₂, 1000 μmol m^-2 s^-1, and 0.375 g L^-1 NaNO₃ during the second stage. In addition, two operation modes were compared. At the beginning of the second stage of mode 2, cells were diluted to 0.1 g L^-1 and there was no cell dilution in mode 1. The total carbohydrate productivity of mode 2 was increased about 42% compared with that of mode 1. The highest total carbohydrate content and the highest starch content of mode 2 were 77.6% (DW) and 60.3% (DW) at day 5, respectively. The starch productivity was 0.311 g L^-1 day^-1 and the total carbohydrate productivity was 0.421 g L^-1 day^-1 in 6 days.

CONCLUSIONS

In this study, a novel two-stage process was proposed for improving carbohydrate and starch accumulation in Chlorella sp. AE10. Despite cell dilution at the beginning of the second stage, environmental stress conditions of high concentration of CO₂, high light intensity, and limited nitrogen concentration at the second stage were critical for carbohydrate and starch accumulation. Although the cells were diluted, the growths were not inhibited and the carbohydrate productivity was improved. These results were helpful to establish an integrated approach from CO₂ capture to biofuel production by microalgae.

Carbon and Acyl Chain Flux during Stress-induced Triglyceride Accumulation by Stable Isotopic Labeling of the Polar Microalga Coccomyxa subellipsoidea C169

Deriving biofuels and other lipoid products from algae is a promising future technology directly addressing global issues of atmospheric CO₂ balance. To better understand the metabolism of triglyceride synthesis in algae, we examined their metabolic origins in the model species, Coccomyxa subellipsoidea C169, using stable isotopic labeling. Labeling patterns arising from [U-¹³C]glucose, ¹³CO₂, or D₂O supplementation were analyzed by GC-MS and/or LC-MS over time courses during nitrogen starvation to address the roles of catabolic carbon recycling, acyl chain redistribution, and de novo fatty acid (FA) synthesis during the expansion of the lipid bodies. The metabolic origin of stress-induced triglyceride was found to be a continuous 8:2 ratio between de novo synthesized FA and acyl chain transfer from pre-stressed membrane lipids with little input from lipid remodeling. Membrane lipids were continually synthesized with associated acyl chain editing during nitrogen stress, in contrast to an overall decrease in total membrane lipid. The incorporation rates of de novo synthesized FA into lipid classes were measured over a time course of nitrogen starvation. The synthesis of triglycerides, phospholipids, and galactolipids followed a two-stage pattern where nitrogen starvation resulted in a 2.5-fold increase followed by a gradual decline. Acyl chain flux into membrane lipids was dominant in the first stage followed by triglycerides. These data indicate that the level of metabolic control that determines acyl chain flux between membrane lipids and triglycerides during nitrogen stress relies primarily on the Kennedy pathway and de novo FA synthesis with limited, defined input from acyl editing reactions.

Nitrogen-induced metabolic changes and molecular determinants of carbon allocation in Dunaliella tertiolecta

Certain species of microalgae are natural accumulators of lipids, while others are more inclined to store starch. However, what governs the preference to store lipids or starch is not well understood. In this study, the microalga Dunaliella tertiolecta was used as a model to study the global gene expression profile regulating starch accumulation in microalgae. D. tertiolecta, when depleted of nitrogen, produced only 1% of dry cell weight (DCW) in neutral lipids, while starch was rapidly accumulated up to 46% DCW. The increased in starch content was accompanied by a coordinated overexpression of genes shunting carbon towards starch synthesis, a response not seen in the oleaginous microalgae Nannochloropsis oceanica, Chlamydomonas reinhardtii or Chlorella vulgaris. Genes in the central carbon metabolism pathways, particularly those of the tricarboxylic acid cycle, were also simultaneously upregulated, indicating a robust interchange of carbon skeletons for anabolic and catabolic processes. In contrast, fatty acid and triacylglycerol synthesis genes were downregulated or unchanged, suggesting that lipids are not a preferred form of storage in these cells. This study reveals the transcriptomic influence behind storage reserve allocation in D. tertiolecta and provides valuable insights into the possible manipulation of genes for engineering microorganisms to synthesize products of interest.

Concurrent production of carotenoids and lipid by a filamentous microalga Trentepohlia arborum

During the study of Trentepohlia arborum it became clear that its cells are rich in lipids and carotenoids. Thus, lipid content, composition and fatty acids profiles in individual lipid classes, as well as pigment profiles, responding to different culture conditions, were further investigated. The results showed that the predominant carotenoids and lipid fraction in total lipid in this study was β-carotene and TAG, respectively. The lipid content increased significantly under high light while nitrogen-replete conditions induced the highest carotenoids content. However, only with a double stress of high light and nitrogen-deficiency it was possible to maximize the productivities of both carotenoids and lipids. Carotenoids (mainly β-carotene) accounted for ca. 5% of the microalgal lipid under the double stress. Data herein show the potential of T. arborum for the production of both lipids and carotenoids, and hence provide an appropriate way to produce different products from T. arborum.