Triacylglycerol synthesis during nitrogen stress involves the prokaryotic lipid synthesis pathway and acyl chain remodeling in the microalgae Coccomyxa subellipsoidea

Exploring Extremotolerant and Extremophilic Microalgae: New Frontiers in Sustainable Biotechnological Applications

Exploring extremotolerant and extremophilic microalgae opens new frontiers in sustainable biotechnological applications. These microorganisms thrive in extreme environments and exhibit specialized metabolic pathways, making them valuable for various industries. The study focuses on the ecological adaptation and biotechnological potential of these microalgae, highlighting their ability to produce bioactive compounds under stress conditions. The literature reveals that extremophilic microalgae can significantly enhance biomass production, reduce contamination risks in large-scale systems, and produce valuable biomolecules such as carotenoids, lipids, and proteins. These insights suggest that extremophilic microalgae have promising applications in food, pharmaceutical, cosmetic, and biofuel industries, offering sustainable and efficient alternatives to traditional resources. The review concludes that further exploration and utilization of these unique microorganisms can lead to innovative and environmentally friendly solutions in biotechnology.

An integration of algae-mediated wastewater treatment and resource recovery through anaerobic digestion

Eutrophication is one of the major emerging challenges in aquatic environment. Industrial facilities, including food, textile, leather, and paper, generate a significant amount of wastewater during their manufacturing process. Discharge of nutrient-rich industrial effluent into aquatic systems causes eutrophication, eventually disturbs the aquatic system. On the other hand, algae provide a sustainable approach to treat wastewater, while the resultant biomass may be used to produce biofuel and other valuable products such as biofertilizers. This review aims to provide new insight into the application of algal bloom biomass for biogas and biofertilizer production. The literature review suggests that algae can treat all types of wastewater (high strength, low strength, and industrial). However, algal growth and remediation potential mainly depend on growth media composition and operation conditions such as light intensity, wavelength, light/dark cycle, temperature, pH, and mixing. Further, the open pond raceways are cost-effective compared to closed photobioreactors, thus commercially applied for biomass generation. Additionally, converting wastewater-grown algal biomass into methane-rich biogas through anaerobic digestion seems appealing. Environmental factors such as substrate, inoculum-to-substrate ratio, pH, temperature, organic loading rate, hydraulic retention time, and carbon/nitrogen ratio significantly impact the anaerobic digestion process and biogas production. Overall, further pilot-scale studies are required to warrant the real-world applicability of the closed-loop phycoremediation coupled biofuel production technology.

Direct processing of alginate-immobilized microalgae into polyhydroxybutyrate using marine bacterium of Saccharophagus degradans

Alginate immobilized microalgae (AIM) was found efficient in algal cells separation and pollutants removal, however, its processing required alginate removal. In present study, polysaccharide-degrading bacterium of Saccharophagus degradans was used to biodegrade alginate and microalgae in AIM and produce polyhydroxybutyrate (PHB). Results showed that AIM cultivated in wastewater contained 34.0% carbohydrate and 45.7% protein. S. degradans effectively degraded and utilized polysaccharide of AIM to maintain five-day continuous growth at 7.1-8.8 log CFU/mL. Compared with glucose, S. degradans metabolism of mixed polysaccharide in AIM maintained the medium pH at 7.1-7.8. Increasing the inoculum concentration did not enhance AIM utilization by S. degradans due to the carbon catabolite repression of glucose which likely inactivated hydrolysis enzymes. PHB production in S. degradans peaked at 64.9 mg/L after 72 h cultivation but was later degraded to provide energy. Conclusively, S. degradans was effective in direct processing of AIM while showing potential in PHB production.

Characterization of fatty acid desaturases reveals stress-induced synthesis of C18 unsaturated fatty acids enriched in triacylglycerol in the oleaginous alga Chromochloris zofingiensis

BACKGROUND

The green microalga Chromochloris zofingiensis is capable of producing high levels of triacylglycerol rich in C18 unsaturated fatty acids (UFAs). FA desaturation degree is regulated by FA desaturases (FADs). Nevertheless, it remains largely unknown regarding what FADs are involved in FA desaturations and how these FADs collaborate to contribute to the high abundance of C18 UFAs in triacylglycerol in C. zofingiensis.

RESULTS

To address these issues, we firstly determined the transcription start sites of 11 putative membrane-bound FAD-coding genes (CzFADs) and updated their gene models. Functional validation of these CzFADs in yeast and cyanobacterial cells revealed that seven are bona fide FAD enzymes with distinct substrates. Combining the validated functions and predicted subcellular compartments of CzFADs and the FA profiles of C. zofingiensis, the FA desaturation pathways in this alga were reconstructed. Furthermore, a multifaceted lipidomic analysis by systematically integrating thin-layer chromatography, gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry techniques was conducted, unraveling profiles of polar membrane lipids in C. zofingiensis and major desaturation steps occurring in these lipids. By correlating transcriptional patterns of CzFAD genes and changes of lipids upon abiotic stress conditions, our results highlighted collaboration of CzFADs for C18 UFA synthesis and supported that both de novo FA synthesis and membrane lipid remodeling contributed C18 UFAs to triacylglycerol for storage.

CONCLUSIONS

Taken together, our study for the first time elucidated the pathways of C18 FA desaturations and comprehensive profiles of polar membrane lipids in C. zofingiensis and shed light on collaboration of CzFADs for the synthesis and enrichment of C18 UFAs in triacylglycerol.

Fatty Acid Production and Direct Acyl Transfer through Polar Lipids Control TAG Biosynthesis during Nitrogen Deprivation in the Halotolerant Alga Dunaliella tertiolecta

The aims of this work were to evaluate the contribution of the free fatty acid (FA) pool to triacylglyceride (TAG) biosynthesis and to try to characterize the mechanism by which FA are assimilated into TAG in the green alga Dunaliella tertiolecta. A time-resolved lipidomic analysis showed that nitrogen (N) deprivation induces a redistribution of total lipidome, particularly of free FA and major polar lipid (PL), in parallel to enhanced accumulation of polyunsaturated TAG. The steady-state concentration of the FA pool, measured by prolonged ¹⁴C-bicarbonate pre-labeling, showed that N deprivation induced a 50% decrease in total FA level within the first 24 h and up to 85% after 96 h. The abundance of oleic acid increased from 50 to 70% of total free FA while polyunsaturated FA (PUFA) disappeared under N deprivation. The FA flux, measured by the rate of incorporation of ¹⁴C-palmitic acid (PlA), suggests partial suppression of phosphatidylcholine (PC) acyl editing and an enhanced turnover of the FA pool and of total digalactosyl-diacylglycerol (DGDG) during N deprivation. Taken together, these results imply that FA biosynthesis is a major rate-controlling stage in TAG biosynthesis in D. tertiolecta and that acyl transfer through PL such as PC and DGDG is the major FA assimilation pathway into TAG in that alga and possibly in other green microalgae. Increasing the availability of FA could lead to enhanced TAG biosynthesis and to improved production of high-value products from microalgae.

Metabolomics and lipid profile analysis of Coccomyxa melkonianii SCCA 048

With an unsupervised GC-MS metabolomics approach, polar metabolite changes of the microalgae Coccomyxa melkonianii SCCA 048 grown under standard conditions for seven weeks were studied. C. melkonianii was sampled at the Rio Irvi River, in the mining site of Montevecchio-Ingurtosu (Sardinia, Italy), which is severely contaminated by heavy metals and shows high concentrations of sulfates. The partial-least-square (PLS) analysis of the GC-MS data indicated that growth of C. melkonianii was characterized by an increase of the levels of threonic acid, myo-inositol, malic acid, and fumaric acid. Furthermore, at the sixth week of exponential phase the lipid fingerprint of C. melkonianii was studied by LC-QTOF-MS. C. melkonianii lipid extract characterized through an iterative MS/MS analysis showed the following percent levels: 61.34 ± 0.60% for triacylglycerols (TAG); 11.55 ± 0.09% for diacylglyceryltrimethyl homoserines (DGTS), 11.34 ± 0.10% for sulfoquinovosyldiacylglycerols (SQDG) and, 5.29 ± 0.04% for lysodiacylglyceryltrimethyl homoserines (LDGTS). Noteworthy, we were able to annotate different fatty acid ester of hydroxyl fatty acid, such as FAHFA (18:1_20:3), FAHFA (18:2_20:4), FAHFA (18:0_20:2), and FAHFA (18:1_18:0), with relevant biological activity. These approaches can be useful to study the biochemistry of this extremophile algae in the view of its potential exploitation in the phycoremediation of polluted mining areas.

Large fluxes of fatty acids from membranes to triacylglycerol and back during N-deprivation and recovery in Chlamydomonas

Microalgae accumulate triacylglycerol (TAG) during nutrient deprivation and break it down after nutrient resupply, and these processes involve dramatic shifts in cellular carbon allocation. Due to the importance of algae in the global carbon cycle, and the potential of algal lipids as feedstock for chemical and fuel production, these processes are of both ecophysiological and biotechnological importance. However, the metabolism of TAG is not well understood, particularly the contributions of fatty acids (FAs) from different membrane lipids to TAG accumulation and the fate of TAG FAs during degradation. Here, we used isotopic labeling time course experiments on Chlamydomonas reinhardtii to track FA synthesis and transfer between lipid pools during nitrogen (N)-deprivation and resupply. When cells were labeled before N-deprivation, total levels of label in cellular FAs were unchanged during subsequent N-deprivation and later resupply, despite large fluxes into and out of TAG and membrane lipid pools. Detailed analyses of FA levels and labeling revealed that about one-third of acyl chains accumulating in TAG during N-deprivation derive from preexisting membrane lipids, and in total, at least 45% of TAG FAs passed through membrane lipids at one point. Notably, most acyl chains in membrane lipids during recovery after N-resupply come from TAG. Fluxes of polyunsaturated FAs from plastidic membranes into TAG during N-deprivation were particularly noteworthy. These findings demonstrate a high degree of integration of TAG and membrane lipid metabolism and highlight a role for TAG in storage and supply of membrane lipid components.

Evaluation of Thirty Microalgal Isolates as Biodiesel Feedstocks Based on Lipid Productivity and Triacylglycerol (TAG) Content

Microalgae are considered feedstock for biodiesel production due to their capability to accumulate triacylglycerols, which have a 99% conversion rate into biodiesel, under certain conditions. This study aims to evaluate thirty native microalgal strains as feedstock for biodiesel production based on their biomass and lipid productivities, and total lipid and triacylglycerol contents under nitrogen-sufficient and nitrogen starvation conditions. In addition, Chlamydomonas reinhardtii cw15 mutant strain was utilized as a reference strain for triacylglycerol accumulation. Among the eight potent strains, Chlorella vulgaris KP2 was considered as a most promising strain with the highest triacylglycerol content, highest total lipid content (28.56% of dry cell weight), and the highest lipid productivity (4.56 mg/L/day) under nitrogen starvation. Under nitrogen starvation, the major fatty acids in the triacylglycerol of Chlorella vulgaris KP2 were C18:1 (37.56%), C16:0 (23.16%), C18:0 (23.07), C18:2 (7.00%), and C18:3 (3.12%), and the percentages of saturated fatty acids, monounsaturated fatty acids, and polyunsaturated fatty acids represented 49.26, 38.73, and 10.12% of the total fatty acids, respectively. Furthermore, the fatty acid methyl esters of triacylglycerol displayed remarkable biodiesel properties with a lower iodine value (59.00 gI₂/100 g), higher oxidative stability (14.24 h) and higher cetane number (58.73) under nitrogen starvation. This study suggests that nitrogen-starved Chlorella vulgaris KP2 could be used as a feedstock for biodiesel production due to the considerable amounts of triacylglycerol and favorable biodiesel properties.

The potential growth and lipid accumulation in Coccomyxa subellipsoidea triggered by glucose combining with sodium acetate

Carbon sources whether types or magnitudes were fateful in terms of stimulating growth and lipids accumulation in microalgae applied for biodiesel production. The set scenario of this work was to investigate the feasibilities of glucose (G) combining with sodium acetate (SA) carbon sources in enhancing biomass and lipid accumulation in Coccomyxa subellipsoidea. The results demonstrated that C. subellipsoidea subjected to the combination feeding of G (20 g/L) and SA (12 g/L) achieved the favorable biomass (5.22 g/L) and lipid content (52.16%). The resulting lipid productivity (388.96 mg/L/day) was 1.33- to 7.60-fold more than those of sole G or SA as well as other combinations of G and SA. Even though the total fatty acids of C. subellipsoidea cells treated with the optimal combination of G and SA showed no noticeable increment in comparison with sole G or SA, the proportion of monounsaturated C18:1 (over 48.69%) and the content of C18:3 (< 12%) were commendable in high-quality algal biodiesel production. Further, such fascinating lipid accumulation in C. subellipsoidea cells treated with G combining with SA might be attributed to that G promoted glycolysis as well as SA activated glyoxylate shunt and TCA cycle to synergistically provide sufficient acetyl-CoA precursors for lipid accumulation. These findings hinted the potential of the combination of carbon sources in enhancing the overall lipid productivity to offset alga-based biodiesel production cost and would guide other alga strains cultivation.

The lipid biochemistry of eukaryotic algae

Algal lipid metabolism fascinates both scientists and entrepreneurs due to the large diversity of fatty acyl structures that algae produce. Algae have therefore long been studied as sources of genes for novel fatty acids; and, due to their superior biomass productivity, algae are also considered a potential feedstock for biofuels. However, a major issue in a commercially viable "algal oil-to-biofuel" industry is the high production cost, because most algal species only produce large amounts of oils after being exposed to stress conditions. Recent studies have therefore focused on the identification of factors involved in TAG metabolism, on the subcellular organization of lipid pathways, and on interactions between organelles. This has been accompanied by the development of genetic/genomic and synthetic biological tools not only for the reference green alga Chlamydomonas reinhardtii but also for Nannochloropsis spp. and Phaeodactylum tricornutum. Advances in our understanding of enzymes and regulatory proteins of acyl lipid biosynthesis and turnover are described herein with a focus on carbon and energetic aspects. We also summarize how changes in environmental factors can impact lipid metabolism and describe present and potential industrial uses of algal lipids.

Induction of oil accumulation by heat stress is metabolically distinct from N stress in the green microalgae Coccomyxa subellipsoidea C169

Algae are often promoted as feedstock organisms to produce a sustainable petroleum fossil fuel alternative. However, to induce lipid accumulation most often requires a severe stress that is difficult to induce in large batch cultures. The objective of this study is to analyze and mathematically model heat stress on growth, chlorophyll content, triacylglyceride, and starch synthesis in algae. We initially screened 30 algal species for the most pronounced induction of lipid droplets from heat stress using confocal microscopy and mass spectroscopy techniques. One species, Coccomyxa subellipsoidea C169, was selected and subjected to further biochemical analyses using a jacketed bioreactor amended with 1% CO2 at 25°C, 30°C, 32°C, 33°C, 34°C, 35°C, and 36°C. Lipid and starch accumulation was less extreme than N stress. Growth was reduced above 25°C, but heat stress induced lipid droplet synthesis was negatively correlated with growth only past a demonstrated threshold temperature above 32°C. The optimal temperature for lipid accumulation was 35°C, which led to 6% of dry weight triglyceride content and a 72% reduction from optimal growth after 5 days. Fatty acid influx rates into triglycerides and 15N labeling of amino acids and proteins indicate that heat stress is mechanistically distinct from N stress. Thus, this study lends support to a novel hypothesis that lipid droplet triglycerides result from a redistribution of carbon flux as fatty acids to neutral storage lipids over membrane or other lipids.

Phytotoxicity of amoxicillin to the duckweed Spirodela polyrhiza: Growth, oxidative stress, biochemical traits and antibiotic degradation

The increasing availability of antibiotics in wastewater has created a serious threat to non-target organisms in the environment. The aim of this study was to evaluate the potential toxicity of amoxicillin on duckweed Spirodela polyrhiza during a short-term exposure (7 d). The duckweed was exposed to a range of environmentally relevant (0.0001-0.01 mg L^-1) and high (0.1 and 1 mg L^-1) concentrations of amoxicillin. Subsequently, biomarkers of toxicity such as growth, pigments (Chl a, Chl b and carotenoids), antioxidative enzyme activity (catalase, CAT; superoxide dismutase, SOD; and ascorbate peroxidases, APX), and biochemical content (protein, lipid and starch) were analysed in their fronds. The high dose (1 mg L^-1) of amoxicillin caused a significant (p < 0.05) decrease in photopigments, protein, starch and lipid content and an increase in carotenoids/total Chl and Chl a/Chl b ratios in fronds of Spirodela polyrhiza. The results showed a shift in biomarkers: a decrease in frond growth and relative growth rate (RGR) (16.2-53.8%) and an increase in the activities (mmol mg protein^-1) of CAT (0.021-0.041), APX (0.84-2.49) and SOD (0.12-0.23) in fronds. The significantly (p < 0.05) greater reduction in amoxicillin content in duckweed setups (84.6-100%) than in the control (62.1-73%) suggested that phytodegradation is an important mechanism in removing antibiotics from water, apart from hydrolysis and photodegradation, which occur in control setups. Overall, the results suggested a toxic effect of amoxicillin on Spirodela polyrhiza, even at low concentrations, and nonetheless, the duckweed contributed directly to the degradation of antibiotics in the water and throughout the phytoremediation process.

The roles of starch and lipid in Chlorella sp. during cell recovery from nitrogen starvation

The influence of N-replenishment on cell growth, chlorophyll content, nitrogen uptake, and accumulation of starch and lipid was studied in Chlorella sp. N-starved algal cells were fully recovered within 2days after N-replenishment. Stored starch in N-deficient culture was degraded immediately during recovery process. However, lipid response had a period of delay when suffered from long starvation. During the recovery process, neutral lipid was reduced accompanied by an increase in membrane lipid. It was speculated that starch served as a source of carbon and energy catabolized to support quick recovery of the culture, while lipid preferred to play a structural role that specific fatty acid species were released from storage lipid and as building blocks for quick synthesis of membrane lipid. In light of rapid growth recovery and no net degradation in total fatty acids, a semi-continuous cultivation process might be a potential way to enhance lipid productivity.

Analysis of the lipid body proteome of the oleaginous alga Lobosphaera incisa

BACKGROUND

Lobosphaera incisa (L. incisa) is an oleaginous microalga that stores triacylglycerol (TAG) rich in arachidonic acid in lipid bodies (LBs). This organelle is gaining attention in algal research, since evidence is accumulating that proteins attached to its surface fulfill important functions in TAG storage and metabolism.

RESULTS

Here, the composition of the LB proteome in L incisa was investigated by comparing different cell fractions in a semiquantitative proteomics approach. After applying stringent filters to the proteomics data in order to remove contaminating proteins from the list of possible LB proteins (LBPs), heterologous expression of candidate proteins in tobacco pollen tubes, allowed us to confirm 3 true LBPs: A member of the algal Major Lipid Droplet Protein family, a small protein of unknown function and a putative lipase. In addition, a TAG lipase that belongs to the SUGAR DEPENDENT 1 family of TAG lipases known from oilseed plants was identified. Its activity was verified by functional complementation of an Arabidopsis thaliana mutant lacking the major seed TAG lipases.

CONCLUSIONS

Here we describe 3 LBPs as well as a TAG lipase from the oleaginous microalga L. incisa and discuss their possible involvement in LB metabolism. This study highlights the importance of filtering LB proteome datasets and verifying the subcellular localization one by one, so that contaminating proteins can be recognized as such. Our dataset can serve as a valuable resource in the identification of additional LBPs, shedding more light on the intriguing roles of LBs in 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.

Metabolome Analysis Reveals Betaine Lipids as Major Source for Triglyceride Formation, and the Accumulation of Sedoheptulose during Nitrogen-Starvation of Phaeodactylum tricornutum

Oleaginous microalgae are considered as a promising resource for the production of biofuels. Especially diatoms arouse interest as biofuel producers since they are most productive in carbon fixation and very flexible to environmental changes in the nature. Naturally, triacylglycerol (TAG) accumulation in algae only occurs under stress conditions like nitrogen-limitation. We focused on Phaeodactylum strain Pt4 (UTEX 646), because of its ability to grow in medium with low salinity and therefore being suited when saline water is less available or for wastewater cultivation strategies. Our data show an increase in neutral lipids during nitrogen-depletion and predominantly 16:0 and 16:1(n-7) accumulated in the TAG fraction. The molecular species composition of TAG suggests a remodeling primarily from the betaine lipid diacylglyceroltrimethylhomoserine (DGTS), but a contribution of the chloroplast galactolipid monogalactosyldiacylglycerol (MGDG) cannot be excluded. Interestingly, the acyl-CoA pool is rich in 20:5(n-3) and 22:6(n-3) in all analyzed conditions, but these fatty acids are almost excluded from TAG. Other metabolites most obviously depleted under nitrogen-starvation were amino acids, lyso-phospholipids and tricarboxylic acid (TCA) cycle intermediates, whereas sulfur-containing metabolites as dimethylsulfoniopropionate, dimethylsulfoniobutyrate and methylsulfate as well as short acyl chain carnitines, propanoyl-carnitine and butanoyl-carnitine increased upon nitrogen-starvation. Moreover, the Calvin cycle may be de-regulated since sedoheptulose accumulated after nitrogen-depletion. Together the data provide now the basis for new strategies to improve lipid production and storage in Phaeodactylum strain Pt4.

Microalgae Recovery from Water for Biofuel Production Using CO2-Switchable Crystalline Nanocellulose

There is a pressing need to develop efficient and sustainable approaches to harvesting microalgae for biofuel production and water treatment. CO2-switchable crystalline nanocellulose (CNC) modified with 1-(3-aminopropyl)imidazole (APIm) is proposed as a reversible coagulant for harvesting microalgae. Compared to native CNC, the positively charged APIm-modified CNC, which dispersed well in carbonated water, showed appreciable electrostatic interaction with negatively charged Chlorella vulgaris upon CO2-treatment. The gelation between the modified CNC, triggered by subsequent air sparging, can also enmesh adjacent microalgae and/or microalgae-modified CNC aggregates, thereby further enhancing harvesting efficiencies. Moreover, the surface charges and dispersion/gelation of APIm-modified CNC could be reversibly adjusted by alternatively sparging CO2/air. This CO2-switchability would make the reusability of redispersed CNC for further harvesting possible. After harvesting, the supernatant following sedimentation can be reused for microalgal cultivation without detrimental effects on cell growth. The use of this approach for harvesting microalgae presents an advantage to other current methods available because all materials involved, including the cellulose, CO2, and air, are natural and biocompatible without adverse effects on the downstream processing for biofuel production.

Transcriptome analysis reveals global regulation in response to CO2 supplementation in oleaginous microalga Coccomyxa subellipsoidea C-169

BACKGROUND

Microalgae are emerging as suitable feedstock for renewable biofuel production and providing a promising way to alleviate green house gas CO2. Characterizing the metabolic pathways involved in the biosynthesis of energy-rich compounds and their global regulation upon elevated CO2 is necessary to explore the mechanism underlying rapid growth and lipid accumulation, so as to realize the full potential of these organisms as energy resources.

RESULTS

In the present study, 2 and 5 % CO2 increased growth rate and lipid accumulation in autotrophically cultured green alga Coccomyxa subellipsoidea C-169. Overall biomass productivity as 222 mg L(-1) day(-1) and fatty acid content as 48.5 % dry cell weight were attained in 2 % CO2, suggesting C-169 as a great candidate for lipid production via CO2 supplementation. Transcriptomic analysis of 2 % against 0.04 % CO2-cultured C-169 unveiled the global regulation of important metabolic processes. Other than enhancing gene expression in the Calvin cycle, C-169 upregulated the expression of phosphoenolpyruvate carboxylase, pyruvate carboxylase and carbamoyl-phosphate synthetase II to enhance the anaplerotic carbon assimilation reactions upon elevated CO2. Upregulation of ferredoxin and ferredoxin-NADP(+) reductase implied that plentiful energy captured through photosynthesis was transferred through ferredoxin to sustain rapid growth and lipid accumulation. Genes involved in the glycolysis, TCA cycle and oxidative phosphorylation were predominantly upregulated presumably to provide abundant intermediates and metabolic energy for anabolism. Coordinated upregulation of nitrogen acquisition and assimilation genes, together with activation of specific carbamoyl-phosphate synthetase and ornithine pathway genes, might help C-169 to maintain carbon/nitrogen balance upon elevated CO2. Significant downregulation of fatty acid degradation genes, as well as the upregulation of fatty acid synthesis genes at the later stage might contribute to the tremendous lipid accumulation.

CONCLUSION

Global and collaborative regulation was employed by C-169 to assimilate more carbon and maintain carbon/nitrogen balance upon elevated CO2, which provide abundant carbon skeleton and affluent metabolic energy to sustain rapid growth and lipid accumulation. Data here for the first time bring significant insights into the regulatory profile of metabolism and acclimation to elevated CO2 in C-169, which provide important information for future metabolic engineering in the development of sustainable microalgae-based biofuels.