Time-resolved transcriptome analysis and lipid pathway reconstruction of the oleaginous green microalga Monoraphidium neglectum reveal a model for triacylglycerol and lipid hyperaccumulation

Enhanced growth and metabolite production from a novel strain of Porphyridium sp

Microalgae are capable of generating numerous metabolites that possess notable biological activities and hold substantial promise for various industrial applications. Nevertheless, the taxonomic diversity of these photosynthetic microorganisms has not received thorough investigation. Using the 18S rRNA encoding gene, a recently discovered strain originating from the Tunisian coast (the governorate of Mahdia) was identified as a member of the Porphyridium genus. The growth response as well as the metabolite accumulation of Porphyridium sp. to different culture media (Pm, F/2, and Hemerick) was investigated over a period of 52 days. The highest biomass production was recorded with Pm medium (2 × 107 cell/mL). The apparent growth rates (µ) and the doubling time (Dt) were about 0.081 day-1 and 12.34 days, respectively. The highest chlorophyll a (0.678 ± 0.005 pg/cell), total carotenoids (0.18 ± 0.003 pg/cell), phycoerythrin (3.88 ± 0.003 pg/cell), and proteins (14.58 ± 0.35 pg/cell) contents were observed with F/2 medium. Cultivating Porphyridium sp. in both F/2 and Hemerick media yielded similar levels of starch accumulation. The Hemerick medium has proven to be the most suitable for the production of lipids (2.23% DW) and exopolysaccharides (5.41 ± 0.56 pg/cell).

Integration of Epigenome and Lactylome Reveals the Regulation of Lipid Production in Nannochloropsis oceanica

Lysine lactylation (Kla) is a kind of novel post-translational modification (PTM) that participates in gene expression and various metabolic processes. Nannochloropsis has a remarkable capacity for triacylglycerol (TAG) production under nitrogen stress. To elucidate the involvement of lactylation in lipid synthesis, we conducted chromatin immunoprecipitation sequencing (ChIP-seq) and mRNA-seq analyses to monitor lactylation modifications and transcriptome alterations in Nannochloropsis oceanica. In all, 2057 genes showed considerable variation between nitrogen deprivation (ND) and nitrogen repletion (NR) conditions. Moreover, a total of 5375 differential Kla peaks were identified, including 5331 gain peaks and 44 loss peaks under ND vs NR. The differential Kla peaks were primarily distributed in the promoter (≤1 kb) (71.07%), 5'UTR (22.64%), and exon (4.25%). Integrative analysis of ChIP-seq, transcriptome, and previous proteome and lactylome data elucidates the potential mechanism by which lactylation promotes lipid accumulation under ND. Lactylation facilitates autophagy and protein degradation, leading to the recycling of carbon into the tricarboxylic acid (TCA) cycle, thereby providing carbon precursors for lipid synthesis. Additionally, lactylation induces the redirection of carbon from membrane lipids to TAG by upregulating lipases and enhancing the TCA cycle and β-oxidation pathways. This research offers a new perspective for the investigation of lipid biosynthesis in Nannochloropsis.

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.

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.

Data-Independent-Acquisition-Based Proteomic Approach towards Understanding the Acclimation Strategy of Oleaginous Microalga Microchloropsis gaditana CCMP526 in Hypersaline Conditions

Salinity is one of the significant factors that affect growth and cellular metabolism, including photosynthesis and lipid accumulation, in microalgae and higher plants. Microchloropsis gaditana CCMP526 can acclimatize to different salinity levels by accumulating compatible solutes, carbohydrates, and lipids as energy storage molecules. We used proteomics to understand the molecular basis for acclimation of M. gaditana to increased salinity levels [55 and 100 PSU (practical salinity unit)]. Correspondence analysis was used for the identification of salinity-responsive proteins (SRPs). The highest number of salinity-induced proteins was observed in 100 PSU. Gene ontology enrichment analysis revealed a separate path of acclimation for cells exposed to 55 and 100 PSU. Osmolyte and lipid biosynthesis were upregulated in hypersaline conditions. Concomitantly, lipid oxidation pathways were also upregulated in hypersaline conditions, providing acetyl-CoA for energy metabolism through the tricarboxylic acid cycle. Carbon fixation and photosynthesis were tightly regulated, while chlorophyll biosynthesis was affected in hypersaline conditions. Importantly, temporal proteome analysis of salinity-induced M. gaditana revealed vital SRPs which could be used for engineering salinity resilient microalgal strains for improved productivity in hypersaline culture conditions.

Physiological and transcriptome analysis elucidates the metabolic mechanism of versatile Porphyridium purpureum under nitrogen deprivation for exopolysaccharides accumulation

Porphyridium purpureum is a mesophilic, unicellular red alga rich in phycoerythrin, sulfate polysaccharides, and polyunsaturated fatty acids. Nitrogen deficiency inhibited the growth of P. purpureum and resulted in yellowing of the cells and thickening of the extracellular viscousness sheath. Under nitrogen stress, the contents of total lipids and exopolysaccharides in P. purpureum were increased by 65.2% and 188.0%, respectively. We demonstrate that the immediate response of P. purpureum to nitrogen deficiency is mediated by carbon flow to polysaccharide synthesis, while the synthesis of lipids is enhanced as a permanent energy storage substance at the later stage. Based on transcriptome annotation information, we elucidate the synthesis pathway of polysaccharides from P. purpureum from the perspective of glycosyl-donor interconversion, and demonstrate that the n-6 pathway is the main synthesis pathway of polyunsaturated fatty acids. This study not only provides a production strategy for polysaccharides and fatty acids by single-celled marine red algae P. purpureum, but also provides targets for further genetic modification.

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.

The oleaginous astaxanthin-producing alga Chromochloris zofingiensis: potential from production to an emerging model for studying lipid metabolism and carotenogenesis

The algal lipids-based biodiesel, albeit having advantages over plant oils, still remains high in the production cost. Co-production of value-added products with lipids has the potential to add benefits and is thus believed to be a promising strategy to improve the production economics of algal biodiesel. Chromochloris zofingiensis, a unicellular green alga, has been considered as a promising feedstock for biodiesel production because of its robust growth and ability of accumulating high levels of triacylglycerol under multiple trophic conditions. This alga is also able to synthesize high-value keto-carotenoids and has been cited as a candidate producer of astaxanthin, the strongest antioxidant found in nature. The concurrent accumulation of triacylglycerol and astaxanthin enables C. zofingiensis an ideal cell factory for integrated production of the two compounds and has potential to improve algae-based production economics. Furthermore, with the advent of chromosome-level whole genome sequence and genetic tools, C. zofingiensis becomes an emerging model for studying lipid metabolism and carotenogenesis. In this review, we summarize recent progress on the production of triacylglycerol and astaxanthin by C. zofingiensis. We also update our understanding in the distinctive molecular mechanisms underlying lipid metabolism and carotenogenesis, with an emphasis on triacylglycerol and astaxanthin biosynthesis and crosstalk between the two pathways. Furthermore, strategies for trait improvements are discussed regarding triacylglycerol and astaxanthin synthesis in C. zofingiensis.

Recent advances in the application of metabolomics for food safety control and food quality analyses

As one of the omics fields, metabolomics has unique advantages in facilitating the understanding of physiological and pathological activities in biology, physiology, pathology, and food science. In this review, based on developments in analytical chemistry tools, cheminformatics, and bioinformatics methods, we highlight the current applications of metabolomics in food safety, food authenticity and quality, and food traceability. Additionally, the combined use of metabolomics with other omics techniques for "foodomics" is comprehensively described. Finally, the latest developments and advances, practical challenges and limitations, and requirements related to the application of metabolomics are critically discussed, providing new insight into the application of metabolomics in food analysis.

The Microalga Nannochloropsis during Transition from Quiescence to Autotrophy in Response to Nitrogen Availability

The marine microalgae Nannochloropsis oceanica (CCMP1779) is a prolific producer of oil and is considered a viable and sustainable resource for biofuel feedstocks. Nitrogen (N) availability has a strong impact on the physiological status and metabolism of microalgal cells, but the exact nature of this response is poorly understood. To fill this gap we performed transcriptomic profiling combined with cellular and molecular analyses of N. oceanica CCMP1779 during the transition from quiescence to autotrophy. N deprivation-induced quiescence was accompanied by a strong reorganization of the photosynthetic apparatus and changes in the lipid homeostasis, leading to accumulation of triacylglycerol. Cell cycle activation and re-establishment of photosynthetic activity observed in response to resupply of the growth medium with N were accompanied by a rapid degradation of triacylglycerol stored in lipid droplets (LDs). Besides observing LD translocation into vacuoles, we also provide evidence for direct interaction between the LD surface protein (NoLDSP) and AUTOPHAGY-RELATED8 (NoATG8) protein and show a role of microlipophagy in LD turnover in N. oceanica CCMP1779. This knowledge is crucial not only for understanding the fundamental mechanisms controlling the cellular energy homeostasis in microalgal cells but also for development of efficient strategies to achieve higher algal biomass and better microalgal lipid productivity.

Degradation of Lipid Droplets in Plants and Algae-Right Time, Many Paths, One Goal

In eukaryotic cells, lipids in the form of triacylglycerols (TAGs) are the major reservoir of cellular carbon and energy. These TAGs are packed into specialized organelles called lipid droplets (LDs). They can be found in most, if not all, types of cells, from bacteria to human. Recent data suggest that rather than being simple storage organelles, LDs are very dynamic structures at the center of cellular metabolism. This is also true in plants and algae, where LDs have been implicated in many processes including energy supply; membrane structure, function, trafficking; and signal transduction. Plant and algal LDs also play a vital role in human life, providing multiple sources of food and fuel. Thus, a lot of attention has been paid to metabolism and function of these organelles in recent years. This review summarizes the most recent advances on LDs degradation as a key process for TAGs release. While the initial knowledge on this process came from studies in oilseeds, the findings of the last decade revealed high complexity and specific mechanisms of LDs degradation in plants and algae. This includes identification of numerous novel proteins associated with LDs as well as a prominent role for autophagy in this process. This review outlines, systemizes, and discusses the most current data on LDs catabolism in plants and algae.

Time‑resolved transcriptome analysis during transitions of sulfur nutritional status provides insight into triacylglycerol (TAG) and astaxanthin accumulation in the green alga Chromochloris zofingiensis

BACKGROUND

Chromochloris zofingiensis, an oleaginous microalga, is a promising feedstock for the co-production of triacylglycerol (TAG)-based biodiesel and the high-value product astaxanthin. To reveal the molecular mechanism of TAG and astaxanthin biosynthesis during transitions of sulfur nutritional status, namely sulfur-starvation (SS) and sulfur-replenishment (SR), the physiological responses and the transcriptomic dynamics of C. zofingiensis were examined.

RESULTS

The results revealed a reversible TAG and astaxanthin accumulation under SS, which is correlated with the reduction of cell growth and protein content, indicating the reallocation of carbon. By correlating the data on the physiological and transcriptional responses to different sulfur nutritional status, a model for the underlying mechanism of TAG and astaxanthin accumulation in C. zofingiensis was postulated, which involved up-regulation of key genes including diacylglycerol acyltransferase (DGTT5) and beta-carotene ketolase (BKT1), increased energy and NADPH supply by elevating the tricarboxylic acid (TCA) cycle and the oxidative pentose phosphate (OPP) pathway, and the increased carbon precursors (pyruvate and acetyl-CoA) through central carbon metabolism. In addition, the net enhancement of the de novo biosynthesis of fatty acids and the re-direction of the terpenoid precursors toward the branch catalyzed by lycopene beta cyclase (LCYb) and BKT1 escalated the substrate availability for the biosynthesis of TAG and astaxanthin, respectively.

CONCLUSIONS

In this study, the time-resolved transcriptional analysis of C. zofingiensis under SS and SR conditions was reported for the first time to elucidate the regulatory roles of key enzymes, including DGTT5, BKT1 and LCYb, in the underlying mechanisms of TAG and astaxanthin accumulation.

Novel insights into salinity-induced lipogenesis and carotenogenesis in the oleaginous astaxanthin-producing alga Chromochloris zofingiensis: a multi-omics study

BACKGROUND

Chromochloris zofingiensis, a freshwater alga capable of synthesizing both triacylglycerol (TAG) and astaxanthin, has been receiving increasing attention as a leading candidate producer. While the mechanism of oleaginousness and/or carotenogenesis has been studied under such induction conditions as nitrogen deprivation, high light and glucose feeding, it remains to be elucidated in response to salt stress, a condition critical for reducing freshwater footprint during algal production processes.

RESULTS

Firstly, the effect of salt concentrations on growth, lipids and carotenoids was examined for C. zofingiensis, and 0.2 M NaCl demonstrated to be the optimal salt concentration for maximizing both TAG and astaxanthin production. Then, the time-resolved lipid and carotenoid profiles and comparative transcriptomes and metabolomes were generated in response to the optimized salt concentration for congruent analysis. A global response was triggered in C. zofingiensis allowing acclimation to salt stress, including photosynthesis impairment, ROS build-up, protein turnover, starch degradation, and TAG and astaxanthin accumulation. The lipid metabolism involved a set of stimulated biological pathways that contributed to carbon precursors, energy and reductant molecules, pushing and pulling power, and storage sink for TAG accumulation. On the other hand, salt stress suppressed lutein biosynthesis, stimulated astaxanthin biosynthesis (mainly via ketolation), yet had little effect on total carotenoid flux, leading to astaxanthin accumulation at the expense of lutein. Astaxanthin was predominantly esterified and accumulated in a well-coordinated manner with TAG, pointing to the presence of common regulators and potential communication for the two compounds. Furthermore, the comparison between salt stress and nitrogen deprivation conditions revealed distinctions in TAG and astaxanthin biosynthesis as well as critical genes with engineering potential.

CONCLUSIONS

Our multi-omics data and integrated analysis shed light on the salt acclimation of C. zofingiensis and underlying mechanisms of TAG and astaxanthin biosynthesis, provide engineering implications into future trait improvements, and will benefit the development of this alga for production uses under saline environment, thus reducing the footprint of freshwater.

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.

Applications of metabolomics in assessing ecological effects of emerging contaminants and pollutants on plants

Metabolomics, the global profiling of metabolite composition, is a powerful technique that can be applied to answer a diverse set of research questions concerning effects of toxicants on organisms. It has recently emerged as a tool to understand complex environmental perturbations in biological systems, especially at sub-lethal concentrations. Organisms can be affected by different stressors such as xenobiotics or increase in concentration of natural compounds such as nitrogen, phosphorous, and sulfur. Metabolomics has facilitated a better understanding of the effects of these perturbations on organisms such as plants, animals, and humans providing phenotypic and biological information in a high throughput manner. In this review, we will discuss recent applications of metabolomics to study the ecological effects of different environmental perturbations, including nanoparticles, pharmaceuticals and personal care products, pesticides, as well as the changes in natural compounds found in the environment with a focus on plant systems.

Multiomics analysis reveals a distinct mechanism of oleaginousness in the emerging model alga Chromochloris zofingiensis

Chromochloris zofingiensis, featured due to its capability to simultaneously synthesize triacylglycerol (TAG) and astaxanthin, is emerging as a leading candidate alga for production uses. To better understand the oleaginous mechanism of this alga, we conducted a multiomics analysis by systematically integrating time-resolved transcriptomes, lipidomes and metabolomes in response to nitrogen deprivation. The data analysis unraveled the distinct mechanism of TAG accumulation, which involved coordinated stimulation of multiple biological processes including supply of energy and reductants, carbon reallocation from protein and starch, and 'pushing' and 'pulling' carbon to TAG synthesis. Unlike the model alga Chlamydomonas, de novo fatty acid synthesis in C. zofingiensis was promoted, together with enhanced turnover of both glycolipids and phospholipids, supporting the drastic need of acyls for TAG assembly. Moreover, genomewide analysis identified many key functional enzymes and transcription factors that had engineering potential for TAG modulation. Two genes encoding glycerol-3-phosphate acyltransferase (GPAT), the first committed enzyme for TAG assembly, were found in the C. zofingiensis genome; in vivo functional characterization revealed that extrachloroplastic GPAT instead of chloroplastic GPAT played a central role in TAG synthesis. These findings illuminate distinct oleaginousness mechanisms in C. zofingiensis and pave the way towards rational manipulation of this alga to becone an emerging model for trait improvements.

Comparative transcriptome analysis of a long-time span two-step culture process reveals a potential mechanism for astaxanthin and biomass hyper-accumulation in Haematococcus pluvialis JNU35

BACKGROUND

Among all organisms tested, Haematococcus pluvialis can accumulate the highest levels of natural astaxanthin. Nitrogen starvation and high irradiance promote the accumulation of starch, lipid, and astaxanthin in H. pluvialis, yet their cell division is significantly retarded. Accordingly, adaptive regulatory mechanisms are very important and necessary to optimize the cultivation conditions enabling an increase in biomass; as well as promoting astaxanthin accumulation by H. pluvialis. To clarify the intrinsic mechanism of high-level astaxanthin and biomass accumulation in the newly isolated strain, H. pluvialis JNU35, nitrogen-sufficiency and nitrogen-depletion conditions were employed. Time-resolved comparative transcriptome analysis was also conducted by crossing the two-step culture process.

RESULTS

In the present study, we report the overall growth and physiological, biochemical, and transcriptomic characteristics of H. pluvialis JNU35 in response to nitrogen variation. From eight sampling time-points (2 days, 4 days, 8 days, 10 days, 12 days, 14 days, 16 days, and 20 days), 25,480 differentially expressed genes were found. These genes included the significantly responsive unigenes associated with photosynthesis, astaxanthin biosynthesis, and nitrogen metabolic pathways. The expressions of all key and rate-limiting genes involved in astaxanthin synthesis were significantly upregulated. The photosynthetic pathway was found to be attenuated, whereas the ferredoxin gene was upregulated, which might activate the cyclic electron-transport chain as compensation. Moreover, the expressions of genes related to nitrogen transport and assimilation were upregulated. The expressions of genes in the proteasome pathway were also upregulated. In contrast, the chloroplasts and nonessential proteins were gradually degraded, activating the specific ornithine-urea cycle pathway. These changes may promote the sustained accumulation of astaxanthin and biomass.

CONCLUSIONS

To the best of our knowledge, this paper is the first to investigate the long-term differences of gene expression from two-step culture process in the astaxanthin producer, H. pluvialis JNU35. According to our results, β-carotene ketolase (bkt1 and bkt2) serves as the key enzyme regulating astaxanthin accumulation in H. pluvialis JNU35. The cyclic electron-transport chain and novel nitrogen metabolic process were used adaptively as the regulatory mechanism compensating for different levels of stress. The in-depth study of these metabolic pathways and related key genes can reveal the underlying relationship between cell growth and astaxanthin accumulation in H. pluvialis JNU35.

Time-resolved carotenoid profiling and transcriptomic analysis reveal mechanism of carotenogenesis for astaxanthin synthesis in the oleaginous green alga Chromochloris zofingiensis

BACKGROUND

Chromochloris zofingiensis is emerging as an industrially relevant alga given its robust growth for the production of lipids and astaxanthin, a value-added carotenoid with broad applications. Nevertheless, poor understanding of astaxanthin synthesis has limited engineering of this alga for rational improvements.

RESULTS

To reveal the molecular mechanism underlying astaxanthin accumulation in C. zofingiensis, here we conducted an integrated analysis by combining the time-resolved transcriptomes and carotenoid profiling in response to nitrogen deprivation (ND). A global response was triggered for C. zofingiensis to cope with the ND stress. Albeit the little variation in total carotenoid content, individual carotenoids responded differentially to ND: the primary carotenoids particularly lutein and β-carotene decreased, while the secondary carotenoids increased considerably, with astaxanthin and canthaxanthin being the most increased ones. The carotenogenesis pathways were reconstructed: ND had little effect on the carbon flux to carotenoid precursors, but stimulated astaxanthin biosynthesis while repressing lutein biosynthesis, thereby diverting the carotenoid flux from primary carotenoids to secondary carotenoids particularly astaxanthin. Comparison between C. zofingiensis and Haematococcus pluvialis revealed the distinctive mechanism of astaxanthin synthesis in C. zofingiensis. Furthermore, potential bottlenecks in astaxanthin synthesis were identified and possible engineering strategies were proposed for the alga.

CONCLUSIONS

Collectively, these findings shed light on distinctive mechanism of carotenogenesis for astaxanthin biosynthesis in C. zofingiensis, identify key functional enzymes and regulators with engineering potential and will benefit rational manipulation of this alga for improving nutritional traits.

Photosynthetic physiological performance and proteomic profiling of the oleaginous algae Scenedesmus acuminatus reveal the mechanism of lipid accumulation under low and high nitrogen supplies

In this study, we presented cellular morphological changes, time-resolved biochemical composition, photosynthetic performance and proteomic profiling to capture the photosynthetic physiological response of Scenedesmus acuminatus under low nitrogen (3.6 mM NaNO₃, N^-) and high nitrogen supplies (18.0 mM NaNO₃, N⁺). S. acuminatus cells showed extensive lipid accumulation (53.7% of dry weight) and were enriched in long-chain fatty acids (C16 & C18) under low nitrogen supply. The activity of PSII and photosynthetic rate decreases, whereas non-photochemical quenching and dark respiration rates were increased in the N^- group. In addition, the results indicated a redistribution of light excitation energy between PSII and PSI in S. acuminatus exists before lipid accumulation. The iTRAQ results showed that, under high nitrogen supply, protein abundance of the chlorophyll biosynthesis, the Calvin cycle and ribosomal proteins decreased in S. acuminatus. In contrast, proteins associated with the photosynthetic machinery, except for F-type ATPase, were increased in the N⁺ group (N⁺, 3 vs. 9 days and 3 days, N⁺ vs. N^-). Under low nitrogen supply, proteins involved in central carbon metabolism, fatty acid synthesis and branched-chain amino acid metabolism were increased, whereas the abundance of proteins of the photosynthetic machinery had decreased, with exception of PSI (N^-, 3 vs. 9 days and 9 days, N⁺ vs. N^-). Collectively, the current study has provided a basis for the metabolic engineering of S. acuminatus for biofuel production.

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.

Exploring stress tolerance mechanism of evolved freshwater strain Chlorella sp. S30 under 30 g/L salt

Enhancement of stress tolerance to high concentration of salt and CO₂ is beneficial for CO₂ capture by microalgae. Adaptive evolution was performed for improving the tolerance of a freshwater strain, Chlorella sp. AE10, to 30 g/L salt. A resulting strain denoted as Chlorella sp. S30 was obtained after 46 cycles (138 days). The stress tolerance mechanism was analyzed by comparative transcriptomic analysis. Although the evolved strain could tolerate 30 g/L salt, high salinity caused loss to photosynthesis, oxidative phosphorylation, fatty acid biosynthesis and tyrosine metabolism. The related genes of antioxidant enzymes, CO₂ fixation, amino acid biosynthesis, central carbon metabolism and ABC transporter proteins were up-regulated. Besides the up-regulation of several genes in Calvin-Benson cycle, they were also identified in C4 photosynthetic pathway and crassulacean acid metabolism pathway. They were essential for the survival and CO₂ fixation of Chlorella sp. S30 under 30 g/L salt and 10% CO₂.

Time-series lipidomic analysis of the oleaginous green microalga species Ettlia oleoabundans under nutrient stress

BACKGROUND

Microalgae are uniquely advantageous organisms cultured and harvested for several value-added biochemicals. A majority of these compounds are lipid-based, such as triacylglycerols (TAGs), which can be used for biofuel production, and their accumulation is most affected under nutrient stress conditions. As such, the balance between cellular homeostasis and lipid metabolism becomes more intricate to achieve efficiency in bioproduct synthesis. Lipidomics studies in microalgae are of great importance as biochemical diversity also plays a major role in lipid regulation among oleaginous species.

METHODS

The aim of this study was to analyze time-series changes in lipid families produced by microalga under different nutrient conditions and growth phases to gain comprehensive information at the cellular level. For this purpose, we worked with a highly adaptable, oleaginous, non-model green microalga species, Ettlia oleoabundans (a.k.a. Neochloris oleoabundans). Using a mass spectrometry-based untargeted and targeted metabolomics' approach, we analyzed the changes in major lipid families under both replete and deplete nitrogen and phosphorus conditions at four different time points covering exponential and stationary growth phases.

RESULTS

Comprehensive analysis of the lipid metabolism highlighted the accumulation of TAGs, which can be utilized for the production of biodiesel via transesterification, and depletion of chlorophylls and certain structural lipids required for photosynthesis, under nutrient deprived conditions. We also found a correlation between the depletion of digalactosyldiacylglycerols (DGDGs) and sulfoquinovosyldiacylglycerols (SQDGs) under nutrient deprivation.

CONCLUSIONS

High accumulation of TAGs under nutrient limitation as well as a depletion of other lipids of interest such as phosphatidylglycerols (PGs), DGDGs, SQDGs, and chlorophylls seem to be interconnected and related to the microalgal photosynthetic efficiency. Overall, our results provided key biochemical information on the lipid regulation and physiology of a non-model green microalga, along with optimization potential for biodiesel and other value-added product synthesis.