Proteomic and biophysical analyses reveal a metabolic shift in nitrogen deprived Nannochloropsis oculata

Reconstruction of Long-Chain Polyunsaturated Acid Synthesis Pathways in Marine Red Microalga Porphyridium cruentum Using Lipidomics and Transcriptomics

The marine red microalga Porphyridium can simultaneously synthesize long-chain polyunsaturated fatty acids, including eicosapentaenoic acid (C20:5, EPA) and arachidonic acid (C20:4, ARA). However, the distribution and synthesis pathways of EPA and ARA in Porphyridium are not clearly understood. In this study, Porphyridium cruentum CCALA 415 was cultured in nitrogen-replete and nitrogen-limited conditions. Fatty acid content determination, transcriptomic, and lipidomic analyses were used to investigate the synthesis of ARA and EPA. The results show that membrane lipids were the main components of lipids, while storage lipids were present in a small proportion in CCALA 415. Nitrogen limitation enhanced the synthesis of storage lipids and ω6 fatty acids while inhibiting the synthesis of membrane lipids and ω3 fatty acids. A total of 217 glycerolipid molecular species were identified, and the most abundant species included monogalactosyldiglyceride (C16:0/C20:5) (MGDG) and phosphatidylcholine (C16:0/C20:4) (PC). ARA was mainly distributed in PC, and EPA was mainly distributed in MGDG. Among all the fatty acid desaturases (FADs), the expressions of Δ5FAD, Δ6FAD, Δ9FAD, and Δ12FAD were up-regulated, whereas those of Δ15FAD and Δ17FAD were down-regulated. Based on these results, only a small proportion of EPA was synthesized through the ω3 pathway, while the majority of EPA was synthesized through the ω6 pathway. ARA synthesized in the ER was likely shuttled into the chloroplast by DAG and was converted into EPA by Δ17FAD.

Insight into the mechanism of nitrogen sufficiency conversion strategy for microalgae-based ammonium-rich wastewater treatment

The strategy of nitrogen sufficiency conversion can improve ammonium nitrogen (NH4+-N) removal with microalgal cells from ammonium-rich wastewater. We selected and identified one promising isolated algal strain, NCU-7, Chlorella sorokiniana, which showed a high algal yield and tolerance to ammonium in wastewater, as well as strong adaptability to N deprivation. The transition from N deprivation through mixotrophy (DN, M) to N sufficiency through autotrophy (SN, P) achieved the highest algal yields (optical density = 1.18 and 1.59) and NH4+-N removal rates (2.5 and 4.2 mg L-1 d-1) from synthetic wastewaters at two NH4+-N concentrations (160 and 320 mg L-1, respectively). Algal cells in DN, M culture obtained the lowest protein content (20.6%) but the highest lipid content (34.0%) among all cultures at the end of the stage 2. After transferring to stage 3, the lowest protein content gradually recovered to almost the same level as SN, P culture on the final day. Transmission electron microscopy and proteomics analysis demonstrated that algal cells had reduced intracellular protein content but accumulated lipids under N deprivation by regulating the reduction in synthesis of protein, carbohydrate, and chloroplast, while enhancing lipid synthesis. After transferring to N sufficiency, algal cells accelerated their growth by recovering protein synthesis, leading to excessive uptake of NH4+-N from wastewater. This study provides specific insights into a nitrogen sufficiency conversion strategy to enhance algal growth and NH4+-N removal/uptake during microalgae-based ammonium-rich wastewater treatment.

Microalgae biofuels: illuminating the path to a sustainable future amidst challenges and opportunities

The development of microalgal biofuels is of significant importance in advancing the energy transition, alleviating food pressure, preserving the natural environment, and addressing climate change. Numerous countries and regions across the globe have conducted extensive research and strategic planning on microalgal bioenergy, investing significant funds and manpower into this field. However, the microalgae biofuel industry has faced a downturn due to the constraints of high costs. In the past decade, with the development of new strains, technologies, and equipment, the feasibility of large-scale production of microalgae biofuel should be re-evaluated. Here, we have gathered research results from the past decade regarding microalgae biofuel production, providing insights into the opportunities and challenges faced by this industry from the perspectives of microalgae selection, modification, and cultivation. In this review, we suggest that highly adaptable microalgae are the preferred choice for large-scale biofuel production, especially strains that can utilize high concentrations of inorganic carbon sources and possess stress resistance. The use of omics technologies and genetic editing has greatly enhanced lipid accumulation in microalgae. However, the associated risks have constrained the feasibility of large-scale outdoor cultivation. Therefore, the relatively controllable cultivation method of photobioreactors (PBRs) has made it the mainstream approach for microalgae biofuel production. Moreover, adjusting the performance and parameters of PBRs can also enhance lipid accumulation in microalgae. In the future, given the relentless escalation in demand for sustainable energy sources, microalgae biofuels should be deemed a pivotal constituent of national energy planning, particularly in the case of China. The advancement of synthetic biology helps reduce the risks associated with genetically modified (GM) microalgae and enhances the economic viability of their biofuel production.

Multiomics and optobiotechnological approaches for the development of microalgal strain for production of aviation biofuel and biorefinery

Demand and consumption of fossil fuels is increasing daily, and oil reserves are depleting. Technological developments are required towards developing sustainable renewable energy sources and microalgae are emerging as a potential candidate for various application-driven research. Molecular understanding attained through omics and system biology approach empowering researchers to modify various metabolic pathways of microalgal system for efficient extraction of biofuel and important biomolecules. This review furnish insight into different "advanced approaches" like optogenetics, systems biology and multi-omics for enhanced production of FAS (Fatty Acid Synthesis) and lipids in microalgae and their associated challenges. These new approaches would be helpful in the path of developing microalgae inspired technological platforms for optobiorefinery, which could be explored as source material to produce biofuels and other valuable bio-compounds on a large scale.

Life-cycle assessment of flue gas CO2 fixation from coal-fired power plant and coal chemical plant by microalgae

The technology of flue gas CO₂ fixation by microalgae is highly attractive in the era of CO₂ neutrality. However, CO₂ emission along the whole process has yet to be sufficiently evaluated. Here, a life-cycle assessment was performed to evaluate the energy conversion characteristics and environmental impacts of flue gas CO₂ fixation from coal-fired power plant (Case 1) and coal chemical plant (Case 2) by microalgae. The results show that total energy consumption and CO₂ gas emissions for Case 1 are 27.5-38.0 MJ/kg microalgae power (MP) and 5.7-7.7 kg CO₂ equiv/kg MP, respectively, which are lower than that for Case 2 (122.5-181.3 MJ/kg MP and 32.7-48.6 kg CO₂ equiv/kg MP). The CO₂ gas aeration rate and microalgae growth rate are the two most sensitive parameters for the energy conversion and net CO₂ emission. Therefore, increasing the CO₂ aeration efficiency and microalgae growth rate are key to advance the technology of flue gas CO₂ fixation by microalgae which will contribute to carbon naturality.

Transcriptome analysis of lipid metabolism in response to cerium stress in the oleaginous microalga Nannochloropsis oculata

Nannochloropsis oculata can accumulate large amounts of lipids under rare earth element (REE) conditions. However, the lipid accumulation mechanism responsible for REE stress has not been elucidated. In this study, the effects of cerium (the most abundant REE) on the growth and lipid accumulation of N. oculata were investigated. The de novo transcriptome data of N. oculata under cerium conditions were subsequently collected and analyzed. The results showed that N. oculata exhibited good cerium-resistance ability, showed slightly decrease in biomass but significantly increase in lipid content (55.8 % dry cell weight) under 6.0 mg/L cerium condition. Meanwhile, about 83.4 % cerium was biological fixated. Through transcriptome analysis, we found that the inhibited photosynthesis and carbon fixation pathways coupled with the stress-sensitive expression of ribosome biogenesis genes acclimatized the cells to REE stress. The active glycolysis pathway accelerated carbon flux to pyruvate and acetyl-CoA, and the upregulation of glycerol kinase and phosphatidate cytidylyltransferase genes further induced lipid accumulation. In addition, cerium downregulated the acyl-CoA oxidase and triacylglycerol lipase genes, which inhibited the degradation of lipids. Therefore, different responses to cerium demonstrate how N. oculata cells adapt to REE stress, and this knowledge may be used to extend our understanding of triacylglycerol (TAG) and the synthesis of other important metabolites.

Comparative Proteomics Reveals Evidence of Enhanced EPA Trafficking in a Mutant Strain of Nannochloropsis oculata

The marine microalga Nannochloropsis oculata is a bioproducer of eicosapentaenoic acid (EPA), a fatty acid. EPA is incorporated into monogalactosyldiacylglycerol within N. oculata thylakoid membranes, and there is a biotechnological need to remodel EPA synthesis to maximize production and simplify downstream processing. In this study, random mutagenesis and chemical inhibitor-based selection method were devised to increase EPA production and accessibility for improved extraction. Ethyl methanesulfonate was used as the mutagen with selective pressure achieved by using two enzyme inhibitors of lipid metabolism: cerulenin and galvestine-1. Fatty acid methyl ester analysis of a selected fast-growing mutant strain had a higher percentage of EPA (37.5% of total fatty acids) than the wild-type strain (22.2% total fatty acids), with the highest EPA quantity recorded at 68.5 mg/g dry cell weight, while wild-type cells had 48.6 mg/g dry cell weight. Label-free quantitative proteomics for differential protein expression analysis revealed that the wild-type and mutant strains might have alternative channeling pathways for EPA synthesis. The mutant strain showed potentially improved photosynthetic efficiency, thus synthesizing a higher quantity of membrane lipids and EPA. The EPA synthesis pathways could also have deviated in the mutant, where fatty acid desaturase type 2 (13.7-fold upregulated) and lipid droplet surface protein (LDSP) (34.8-fold upregulated) were expressed significantly higher than in the wild-type strain. This study increases the understanding of EPA trafficking in N. oculata, leading to further strategies that can be implemented to enhance EPA synthesis in marine microalgae.

Insights into the genetic and metabolic engineering approaches to enhance the competence of microalgae as biofuel resource: A review

Conventional fuel resources are overburden with speedy global energy demand which ensued the urgent need of alternate energy resources. Biofuel generation efficiency of microalgae is notable due to their comparatively rapid biomass production rate and high oil content. But, the employment of microalgae as biofuel resource is in infancy due to low productivity and high production cost. The issues can be addressed by employing engineered microalgal strains that would be able to efficiently generate enhanced levels of biomass with augmented lipid and/or carbohydrate content for proficient biofuel production. Genetic alterations and metabolic engineering of microalgal species might be helpful in developing high stress-tolerant strains with improved properties for biofuel generation. Various omics approaches appeared significant to upgrade the microalgal lipid production. Intervention of genetic and metabolic engineering approaches would facilitate the development of microalgae as a competent biofuel resource and inflate the economic commercialization of biofuels.

In situ monitoring of chlorophyll a fluorescence in Nannochloropsis oceanica cultures to assess photochemical changes and the onset of lipid accumulation during nitrogen deprivation

In situ chlorophyll a fluorescence measurements were applied to monitor changes in the photochemical variables of Nannochloropsis oceanica cultures under nitrogen-deplete and nitrogen-replete (control) conditions. In addition, growth, lipid, fatty acid, and pigment contents were also followed. In the control culture, growth was promoted along with pigment content, electron transport rate (ETR), and polyunsaturated fatty acids, while total lipid content and fatty acid saturation level diminished. Under nitrogen-deplete conditions, the culture showed a higher de-epoxidation state of the xanthophyll cycle pigments. Fast transients revealed a poor processing efficiency for electron transfer beyond Q_A , which was in line with the low ETR due to nitrogen depletion. Lipid content and the de-epoxidation state were the first biochemical variables triggered by the change in nutrient status, which coincided with a 20% drop in the in situ effective quantum yield of PSII (ΔF'/F_m '), and a raise in the V_j measurements. A good correlation was found between the changes in ΔF'/F_m ' and lipid content (r = -0.96, p < 0.01). The results confirm the reliability and applicability of in situ fluorescence measurements to monitor lipid induction in N. oceanica.

Isolation of Industrial Important Bioactive Compounds from Microalgae

Microalgae are known as a rich source of bioactive compounds which exhibit different biological activities. Increased demand for sustainable biomass for production of important bioactive components with various potential especially therapeutic applications has resulted in noticeable interest in algae. Utilisation of microalgae in multiple scopes has been growing in various industries ranging from harnessing renewable energy to exploitation of high-value products. The focuses of this review are on production and the use of value-added components obtained from microalgae with current and potential application in the pharmaceutical, nutraceutical, cosmeceutical, energy and agri-food industries, as well as for bioremediation. Moreover, this work discusses the advantage, potential new beneficial strains, applications, limitations, research gaps and future prospect of microalgae in industry.

Development of New Antiproliferative Compound against Human Tumor Cells from the Marine Microalgae Nannochloropsis gaditana by Applied Proteomics

Proteomics is a crucial tool for unravelling the molecular dynamics of essential biological processes, becoming a pivotal technique for basic and applied research. Diverse bioinformatic tools are required to manage and explore the huge amount of information obtained from a single proteomics experiment. Thus, functional annotation and protein-protein interactions are evaluated in depth leading to the biological conclusions that best fit the proteomic response in the system under study. To gain insight into potential applications of the identified proteins, a novel approach named "Applied Proteomics" has been developed by comparing the obtained protein information with the existing patents database. The development of massive sequencing technology and mass spectrometry (MS/MS) improvements has allowed the application of proteomics nonmodel microorganisms, which have been deeply described as a novel source of metabolites. Between them, Nannochloropsis gaditana has been pointed out as an alternative source of biomolecules. Recently, our research group has reported the first complete proteome analysis of this microalga, which was analysed using the applied proteomics concept with the identification of 488 proteins with potential industrial applications. To validate our approach, we selected the UCA01 protein from the prohibitin family. The recombinant version of this protein showed antiproliferative activity against two tumor cell lines, Caco2 (colon adenocarcinoma) and HepG-2 (hepatocellular carcinoma), proving that proteome data have been transformed into relevant biotechnological information. From Nannochloropsis gaditana has been developed a new tool against cancer-the protein named UCA01. This protein has selective effects inhibiting the growth of tumor cells, but does not show any effect on control cells. This approach describes the first practical approach to transform proteome information in a potential industrial application, named "applied proteomics". It is based on a novel bioalgorithm, which is able to identify proteins with potential industrial applications. From hundreds of proteins described in the proteome of N. gaditana, the bioalgorithm identified over 400 proteins with potential uses; one of them was selected as UCA01, "in vitro" and its potential was demonstrated against cancer. This approach has great potential, but the applications are potentially numerous and undefined.

Bioengineering of Microalgae: Recent Advances, Perspectives, and Regulatory Challenges for Industrial Application

Microalgae, due to their complex metabolic capacity, are being continuously explored for nutraceuticals, pharmaceuticals, and other industrially important bioactives. However, suboptimal yield and productivity of the bioactive of interest in local and robust wild-type strains are of perennial concerns for their industrial applications. To overcome such limitations, strain improvement through genetic engineering could play a decisive role. Though the advanced tools for genetic engineering have emerged at a greater pace, they still remain underused for microalgae as compared to other microorganisms. Pertaining to this, we reviewed the progress made so far in the development of molecular tools and techniques, and their deployment for microalgae strain improvement through genetic engineering. The recent availability of genome sequences and other omics datasets form diverse microalgae species have remarkable potential to guide strategic momentum in microalgae strain improvement program. This review focuses on the recent and significant improvements in the omics resources, mutant libraries, and high throughput screening methodologies helpful to augment research in the model and non-model microalgae. Authors have also summarized the case studies on genetically engineered microalgae and highlight the opportunities and challenges that are emerging from the current progress in the application of genome-editing to facilitate microalgal strain improvement. Toward the end, the regulatory and biosafety issues in the use of genetically engineered microalgae in commercial applications are described.

Biphasic optimization approach for maximization of lipid production by the microalga Chlorella pyrenoidosa

The aim of the study was to identify the optimum cultivation conditions for the microalgal growth and lipid production of the oleaginous microalga Chlorella pyrenoidosa Chick (IPPAS C2). Moreover, an appropriate NO₃^- concentration in the cultivation medium for maximized lipid accumulation was determined. The experimental design involved a biphasic cultivation strategy with an initial biomass accumulating phase under optimized light (400 μmol/m² per s), temperature (25 °C), and elevated CO₂ concentration in the air mixture (3%), followed by a mid-elevated CO₂ concentration (0.5%) for lipid induction. The highest lipid yields of 172.47 ± 18.1 and 179.65 ± 25.4 mg/L per day were detected for NO₃^- concentrations of 100 and 150 mg/L. The optimization approach presented here led not only to the maximization of lipid yield but also to the development of a biphasic cultivation strategy easily applicable to the cultivation process without the necessity for algal cell harvesting between the first and second cultivation phases.

Transcriptomic survey on the microcystins production and growth of Microcystis aeruginosa under nitrogen starvation

Cyanobacteria are a vital component of freshwater phytoplankton, and many species are recognized for their ability to produce toxins and harmful algal blooms (HABs). Nitrogen is an essential element of all the complex macromolecules in algal cells. However, the underlying molecular mechanism of the changes in transcriptomic patterns and physiological responses in response to N starvation is poorly understood. The transcriptomes were generated via RNA-sequencing (RNA-Seq) technology to study the major metabolic pathway under N starvation. The results shed light on the mechanism of toxin production and physiological adaptations in Microcystis aeruginosa (M. aeruginosa). The cell density gradually increased during the first two days then declined over time and was finally stable at (15.50 ± 0.5) × 10⁵ cell mL^-1 after 6 days. The chlorophyll-a content and phycocyanin content of M. aeruginosa increased during the first two days and subsequently decreased markedly over time under N starvation. The variable to maximum chlorophyll fluorescence ratio (F_v/F_m ratio) decreased with time under N starvation. Most photosynthesis genes have similarity decreasing trends with growth physiological changes. The microcystins (MCs) levels generally increased first, reaching a peak value with 1.35 pg cell^-1 on the fifth day, and then remained roughly constant. The genes involved in N metabolism-related gene expression were upregulated to maintain normal biological activity, while the genes involved in photosynthesis-related gene expression were downregulated to save energy. All genes encoding algae toxin synthesis were upregulated under N starvation. The observed expression patterns demonstrate that all MCs genes respond similarly to MCs production within the cell. Our results indicate the response mechanism of M. aeruginosa under N starvation and provide a comprehensive understanding of N-controlling cyanobacteria and MCs synthesis.

Integration of proteome and transcriptome refines key molecular processes underlying oil production in Nannochloropsis oceanica

BACKGROUND

Under nitrogen deficiency situation, Nannochloropsis spp. accumulate large amounts of lipids in the form of triacylglycerides (TAG). Mechanisms of this process from the perspective of transcriptome and metabolome have been obtained previously, yet proteome analysis is still sparse which hinders the analysis of dynamic adaption to nitrogen deficiency. Here, proteomes for 3 h, 6 h, 12 h, 24 h, 48 h and 10th day of nitrogen deplete (N-) and replete (N+) conditions were obtained and integrated with previous transcriptome data for N. oceanica.

RESULTS

Physiological adaptations to N- not apparent from transcriptome data were unveiled: (a) abundance of proteins related to photosynthesis only slightly decreased in the first 48 h, indicating that photosynthesis is still working efficiently, and protein amounts adjust gradually with reduction in chloroplast size. (b) Most proteins related to the TCA cycle were strongly upregulated after 48 h under N-, suggesting that respiration is enhanced after 48 h and that TCA cycle efflux supports the carbon required for lipid synthesis. (c) Proteins related to lipid accumulation via the Kennedy pathway increased their abundance at 48 h, synchronous with the previously reported diversification of fatty acids after 48 h.

CONCLUSIONS

This study adds a proteome perspective on the major pathways for TAG accumulation in Nannochloropsis spp. Temporal changes of proteome exhibited distinct adaptation phases that are usually delayed relative to transcriptomic responses. Notably, proteome data revealed that photosynthesis and carbon fixation are still ongoing even after 48 h of N-. Moreover, sometimes completely opposite trends in proteome and transcriptome demonstrate the relevance of underexplored post-transcriptional regulation for N- adaptation.

Use of magnetic fields and nitrate concentration to optimize the growth and lipid yield of Nannochloropsis oculata

Microalgae produce increased lipid content accompanied by a significant decrease in cell density with decreasing nitrate concentration. Magnetic fields (MF) have been reported as a factor that could accelerate metabolism and growth in microalgae culture. Thus, this study aimed to optimize the influence of MF and nitrate concentration (sodium nitrate, N) on the growth and lipid productivity of Nannochloropsis oculata. A single-factor experiment integrated with response surface methodology (RSM) via central composite design (CCD) was performed. The results showed that the maximum specific growth rate (0.24 d^-1) and maximum lipid productivity (38 mg L^-1 d^-1) obtained in this study were higher than those of the control culture (by 166% and 103%, respectively). This study also found that the two-way interaction term MF × N had a significant effect on cell growth but not on lipid production. It was concluded that to design appropriate MF for enhanced lipid productivity due to cell growth, further research must focus on developing an understanding of the relationship between the bioeffects of the magnetic field and the proteomic changes involved in lipid accumulation strategies. This approach would enable the design of conditions to obtain inexpensive high-value products from N. oculata.

Omics approaches for microalgal applications: Prospects and challenges

In recent impetus of phycological research, microalgae have emerged as a potential candidate for various arena of application-driven research. Omics-based tactics are used for disentangling the regulation and network integration for biosynthesis/degradation of metabolic precursors, intermediates, end products, and identifying the networks that regulate the metabolic flux. Multi-omics coupled with data analytics have facilitated understanding of biological processes and allow ample access to diverse metabolic pathways utilized for genetic manipulations making microalgal factories more efficient. The present review discusses state-of-art "Algomics" and the prospect of microalgae and their role in symbiotic association by using omics approaches including genomics, transcriptomics, proteomics and metabolomics. Microalgal based uni- and multi-omics approaches are critically analyzed in wastewater treatment, metal toxicity and remediation, biofuel production, and therapeutics to provide an imminent outlook for an array of environmentally sustainable and economically viable microalgal applications.

Can Omics Approaches Improve Microalgal Biofuels under Abiotic Stress?

Microalgae hold the promise of an inexpensive and sustainable source of biofuels. The existing microalgal cultivation technologies need significant improvement to outcompete other biofuel sources such as terrestrial plants. Application of 'algomics' approaches under different abiotic stress conditions could be an effective strategy for optimization of microalgal growth and production of high-quality biofuels. In this review, we discuss the roles of omics in understanding genome structure and biocomponents metabolism in various microalgal species to optimize sustainable biofuel production. Application of individual and integrated omics revealed that genes and metabolic pathways of microalgae have been altered under multiple stress conditions, resulting in an increase in biocomponents, providing a research platform for expansion of genetic engineering studies in microalgal strains.

Evaluation of changes in Microcystis aeruginosa growth and microcystin production by urea via transcriptomic surveys

The freshwater cyanobacteria, Microcystis aeruginosa (M. aeruginosa), is well known to produce microcystins (MCs) and induce the formation of harmful algal blooms (HABs) in aquatic environments, but the effects that urea fertilizer has on cyanobacterial growth and toxin production from a molecular biology perspective remain poorly understood. We evaluated changes in the growth and toxicity of M. aeruginosa cultured under different conditions of nitrogen (N) starvation (NN), low nitrogen (LN), and high nitrogen (HN). Cell density and chlorophyll-a concentrations decreased in cyanobacteria exposed to N starvation and increased following the addition of urea, whereas MCs content increased to a peak and then decreased after urea addition. Transcriptomic analysis confirmed that most genes encoding MCs and genes involved in N metabolic pathways were upregulated under N starvation and LN conditions, whereas these genes were downregulated under HN conditions. Our results offer important insights into the exploring N in controlling the formation of HABs and toxin production based on both physiological and molecular response.

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.

Microalgae Chlorella vulgaris biomass harvesting by natural flocculant: effects on biomass sedimentation, spent medium recycling and lipid extraction

BACKGROUND

Microalgal biomass harvesting using traditional chemicals is costly for the production of biofuels, hindering the scale-up process of the technology. Thus, the search for a cost-effective microalgal harvesting method is extremely important. Using chitosan as a natural flocculant to harvest microalgal biomass seems to be an efficient and convenient solution. Although microalgal biomass flocculation by chitosan has been reported in some previous studies, literature on the harvesting of microalgae C. vulgaris biomass using such polymer is scanty. In addition, there is limited information available on whether the usage of chitosan during the harvesting will affect downstream lipid extraction. Still, whether microalgae can be re-grown with the spent medium after chitosan flocculation is still unknown.

RESULTS

In this study, microalgal biomass harvesting using chitosan as a natural flocculant and aluminum sulfate as a traditional flocculant was compared and evaluated. Optimal doses and effects on biomass sedimentation, spent medium recycling and lipid extraction were investigated. The results showed that the optimal doses for chitosan and aluminum sulfate to achieve more than 90% biomass recovery were 0.25 and 2.5 g/L, respectively. The sedimentation time of 10 min was found to be the most appropriate to remove over 90% biomass from culture. The spent medium after chitosan flocculation could be potentially recycled for the re-cultivation of microalgae, which demonstrated robust growth in comparison with those grown in the recycled medium from aluminum sulfate flocculation. The lipid content of microalgae harvested by chitosan reached 32.9, 4.6% higher than that of those harvested by aluminum sulfate, indicating that the application of the natural flocculant would not impact the downstream extraction of microalgal lipids.

CONCLUSION

The results herein presented, demonstrated that chitosan is applicable for microalgal harvesting during the upscaling process. Flocculation method developed by using chitosan as a natural flocculant is a worthy microalgal harvesting method for microalgae-based biofuel production. There is hope that chitosan can be reasonably and technically realistically applied in a full-scale process for the harvesting of microalgal biomass.

A Mathematical Model of Neutral Lipid Content in terms of Initial Nitrogen Concentration and Validation in Coelastrum sp. HA-1 and Application in Chlorella sorokiniana

Microalgae are considered to be a potential major biomass feedstock for biofuel due to their high lipid content. However, no correlation equations as a function of initial nitrogen concentration for lipid accumulation have been developed for simplicity to predict lipid production and optimize the lipid production process. In this study, a lipid accumulation model was developed with simple parameters based on the assumption protein synthesis shift to lipid synthesis by a linear function of nitrogen quota. The model predictions fitted well for the growth, lipid content, and nitrogen consumption of Coelastrum sp. HA-1 under various initial nitrogen concentrations. Then the model was applied successfully in Chlorella sorokiniana to predict the lipid content with different light intensities. The quantitative relationship between initial nitrogen concentrations and the final lipid content with sensitivity analysis of the model were also discussed. Based on the model results, the conversion efficiency from protein synthesis to lipid synthesis is higher and higher in microalgae metabolism process as nitrogen decreases; however, the carbohydrate composition content remains basically unchanged neither in HA-1 nor in C. sorokiniana.