Systems-level analysis of nitrogen starvation-induced modifications of carbon metabolism in a Chlamydomonas reinhardtii starchless mutant

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.

A mechanistic insight on how Compromised Hydrolysis of Triacylglycerol 7 (CHT7) restrains the involvement of it's CXC domain from quiescence repression

CHT7 is a regulator of quiescence repression in Chlamydomonas reinhardtii. Initially, CHT7's repression activity was thought to be managed by its DNA-binding CXC domain. Later, it was found that the CHT7-CXC domain is dispensable for CHT7's activities. Rather, CHT7's predicted protein domains were proposed to be involved in regulation activities by binding to other repressors in the cell. Yet, it remains unclear why and how CHT7 refrains its CXC domain from participating in any transcriptional activities. The question becomes more intriguing, since CXC binding regions are available in promoter regions of some of the misregulated genes in CHT7 mutant (cht7). Through biophysical experiments and molecular dynamics approaches, we studied the DNA recognition behavior of CHT7-CXC. The results indicate that this domain possesses sequence selectivity due to the differential binding abilities of its subdomains. Further, to understand if the case is that CXC loses its DNA binding capabilities in the vicinity of other repressors, we examined CHT7-CXC's DNA binding stability under the spatial constraint conditions created through fusing CHT7-CXC with AsLOV2. The results show limited ability of CHT7-CXC to withstand steric forces and provide insights to why and how algal cells may hold back CHT7-CXC's indulgence in quiescence repression. CLASSIFICATIONS: Biological Sciences, Biophysics and Computational Biology.

Lipid Droplets in Endosymbiotic Symbiodiniaceae spp. Associated with Corals

Symbiodiniaceae species is a dinoflagellate that plays a crucial role in maintaining the symbiotic mutualism of reef-building corals in the ocean. Reef-building corals, as hosts, provide the nutrition and habitat to endosymbiotic Symbiodiniaceae species and Symbiodiniaceae species transfer the fixed carbon to the corals for growth. Environmental stress is one of the factors impacting the physiology and metabolism of the corals-dinoflagellate association. The environmental stress triggers the metabolic changes in Symbiodiniaceae species resulting in an increase in the production of survival organelles related to storage components such as lipid droplets (LD). LDs are found as unique organelles, mainly composed of triacylglycerols surrounded by phospholipids embedded with some proteins. To date, it has been reported that investigation of lipid droplets significantly present in animals and plants led to the understanding that lipid droplets play a key role in lipid storage and transport. The major challenge of investigating endosymbiotic Symbiodiniaceae species lies in overcoming the strategies in isolating lesser lipid droplets present in its intercellular cells. Here, we review the most recent highlights of LD research in endosymbiotic Symbiodiniaceae species particularly focusing on LD biogenesis, mechanism, and major lipid droplet proteins. Moreover, to comprehend potential novel ways of energy storage in the symbiotic interaction between endosymbiotic Symbiodiniaceae species and its host, we also emphasize recent emerging environmental factors such as temperature, ocean acidification, and nutrient impacting the accumulation of lipid droplets in endosymbiotic Symbiodiniaceae species.

Restricting electron flow at cytochrome b6f when downstream electron acceptors are severely limited

Photosynthetic organisms frequently experience abiotic stress that restricts their growth and development. Under such circumstances, most absorbed solar energy cannot be used for CO2 fixation and can cause the photoproduction of reactive oxygen species (ROS) that can damage the photosynthetic reaction centers of PSI and PSII, resulting in a decline in primary productivity. This work describes a biological "switch" in the green alga Chlamydomonas reinhardtii that reversibly restricts photosynthetic electron transport (PET) at the cytochrome b6f (Cyt b6f) complex when the capacity for accepting electrons downstream of PSI is severely limited. We specifically show this restriction in STARCHLESS6 (sta6) mutant cells, which cannot synthesize starch when they are limited for nitrogen (growth inhibition) and subjected to a dark-to-light transition. This restriction represents a form of photosynthetic control that causes diminished electron flow to PSI and thereby prevents PSI photodamage but does not appear to rely on a ΔpH. Furthermore, when electron flow is restricted, the plastid alternative oxidase (PTOX) becomes active, functioning as an electron valve that dissipates some excitation energy absorbed by PSII and allows the formation of a proton motive force (PMF) that would drive some ATP production (potentially sustaining PSII repair and nonphotochemical quenching [NPQ]). The restriction at the Cyt b6f complex can be gradually relieved with continued illumination. This study provides insights into how PET responds to a marked reduction in availability of downstream electron acceptors and the protective mechanisms involved.

Exploiting AGPase genes and encoded proteins to prioritize development of optimum engineered strains in microalgae towards sustainable biofuel production

Although ADP glucose pyrophosphorylase (AGPase), with two large subunits (ls) and two small subunits (ss), is a promising knockout target for increasing the neutral lipid content, the details regarding the sequence-structure features and their distribution within metabolic system in microalgae is rather limited. Against this backdrop, a comprehensive genome-wide comparative analysis on 14 sequenced microalgal genomes was performed. For the first time the heterotetrameric structure of the enzyme and the interaction of the catalytic unit with the substrate was also studied. Novel findings of the present study includes: (i) at the DNA level, the genes controlling the ss are more conserved than those controlling the ls; the variation in both the gene groups is mainly due to exon number, exon length and exon phase distribution; (ii) at protein level, the ss genes are more conserved relative to those for ls; (III) three putative key consensus sequences 'LGGGAGTRLYPLTKNRAKPAV', 'WFQGTADAV' and 'ASMGIYVFRKD' were ubiquitously conserved in all the AGPases; (iv) molecular dynamics investigations revealed that the modeled AGPase heterotetrameric structure, from oleaginous algae Chlamydomonas reinharditii, was completely stable in real time environment; (v) The binding interfaces of catalytic unit, ssAGPase, from C. reinharditii with α-D-glucose 1-phosphate (αGP) was also analyzed. The results of the present study have provided system-based insights into the structure-function of the genes and encoded proteins, which provided clues for exploitation of variability in these genes that, could be further utilized to design site-specific mutagenic experiments for engineering of microalgal strains towards sustainable development of biofuel.

From Feasting to Fasting: The Arginine Pathway as a Metabolic Switch in Nitrogen-Deprived Chlamydomonas reinhardtii

The metabolism of the model microalgae Chlamydomonas reinhardtii under nitrogen deprivation is of special interest due to its resulting increment of triacylglycerols (TAGs), that can be applied in biotechnological applications. However, this same condition impairs cell growth, which may limit the microalgae's large applications. Several studies have identified significant physiological and molecular changes that occur during the transition from an abundant to a low or absent nitrogen supply, explaining in detail the differences in the proteome, metabolome and transcriptome of the cells that may be responsible for and responsive to this condition. However, there are still some intriguing questions that reside in the core of the regulation of these cellular responses that make this process even more interesting and complex. In this scenario, we reviewed the main metabolic pathways that are involved in the response, mining and exploring, through a reanalysis of omics data from previously published datasets, the commonalities among the responses and unraveling unexplained or non-explored mechanisms of the possible regulatory aspects of the response. Proteomics, metabolomics and transcriptomics data were reanalysed using a common strategy, and an in silico gene promoter motif analysis was performed. Together, these results identified and suggested a strong association between the metabolism of amino acids, especially arginine, glutamate and ornithine pathways to the production of TAGs, via the de novo synthesis of lipids. Furthermore, our analysis and data mining indicate that signalling cascades orchestrated with the indirect participation of phosphorylation, nitrosylation and peroxidation events may be essential to the process. The amino acid pathways and the amount of arginine and ornithine available in the cells, at least transiently during nitrogen deprivation, may be in the core of the post-transcriptional, metabolic regulation of this complex phenomenon. Their further exploration is important to the discovery of novel advances in the understanding of microalgae lipids' production.

Palmelloid Formation and Cell Aggregation Are Essential Mechanisms for High Light Tolerance in a Natural Strain of Chlamydomonas reinhardtii

Photosynthetic organisms, such as higher plants and algae, require light to survive. However, an excessive amount of light can be harmful due to the production of reactive oxygen species (ROS), which cause cell damage and, if it is not effectively regulated, cell death. The study of plants' responses to light can aid in the development of methods to improve plants' growth and productivity. Due to the multicellular nature of plants, there may be variations in the results based on plant age and tissue type. Chlamydomonas reinhardtii, a unicellular green alga, has also been used as a model organism to study photosynthesis and photoprotection. Nonetheless, the majority of the research has been conducted with strains that have been consistently utilized in laboratories and originated from the same source. Despite the availability of many field isolates of this species, very few studies have compared the light responses of field isolates. This study examined the responses of two field isolates of Chlamydomonas to high light stress. The light-tolerant strain, CC-4414, managed reactive oxygen species (ROS) slightly better than the sensitive strain, CC-2344, did. The proteomic data of cells subjected to high light revealed cellular modifications of the light-tolerant strain toward membrane proteins. The morphology of cells under light stress revealed that this strain utilized the formation of palmelloid structures and cell aggregation to shield cells from excessive light. As indicated by proteome data, morphological modifications occur simultaneously with the increase in protein degradation and autophagy. By protecting cells from stress, cells are able to continue to upregulate ROS management mechanisms and prevent cell death. This is the first report of palmelloid formation in Chlamydomonas under high light stress.

A conserved MYB transcription factor is involved in regulating lipid metabolic pathways for oil biosynthesis in green algae

Green algae can accumulate high levels of triacylglycerol (TAG), yet knowledge remains fragmented on the regulation of lipid metabolic pathways by transcription factors (TFs). Here, via bioinformatics and in vitro and in vivo analyses, we revealed the roles of a myeloblastosis (MYB) TF in regulating TAG accumulation in green algae. CzMYB1, an R2R3-MYB from Chromochloris zofingiensis, was transcriptionally upregulated upon TAG-inducing conditions and correlated well with many genes involved in the de novo fatty acid synthesis, fatty acid activation and desaturation, membrane lipid turnover, and TAG assembly. Most promoters of these genes were transactivated by CzMYB1 in the yeast one-hybrid assay and contained the binding elements CNGTTA that were recognized by CzMYB1 through the electrophoretic mobility shift assay. CrMYB1, a close homologue of CzMYB1 from Chlamydomonas reinhardtii that recognized similar elements for binding, also transcriptionally correlated with many lipid metabolic genes. Insertional disruption of CrMYB1 severely suppressed the transcriptional expression of CrMYB1, as well as of key lipogenic genes, and impaired TAG level considerably under stress conditions. Our results reveal that this MYB, conserved in green algae, is involved in regulating global lipid metabolic pathways for TAG biosynthesis and accumulation.

The Chlamydomonas transcription factor MYB1 mediates lipid accumulation under nitrogen depletion

Microalgae accumulate high levels of oil under stress, but the underlying biosynthetic pathways are not fully understood. We sought to identify key regulators of lipid metabolism under stress conditions. We found that the Chlamydomonas reinhardtii gene encoding the MYB-type transcription factor MYB1 is highly induced under stress conditions. Two myb1 mutants accumulated less total fatty acids and storage lipids than their parental strain upon nitrogen (N) depletion. Transcriptome analysis revealed that genes involved in lipid metabolism are highly enriched in the wild-type but not in the myb1-1 mutant after 4 h of N depletion. Among these genes were several involved in the transport of fatty acids from the chloroplast to the endoplasmic reticulum (ER): acyl-ACP thioesterase (FAT1), Fatty Acid EXporters (FAX1, FAX2), and long-chain acyl-CoA synthetase1 (LACS1). Furthermore, overexpression of FAT1 in the chloroplast increased lipid production. These results suggest that, upon N depletion, MYB1 promotes lipid accumulation by facilitating fatty acid transport from the chloroplast to the ER. This study identifies MYB1 as an important positive regulator of lipid accumulation in C. reinhardtii upon N depletion, adding another player to the established regulators of this process, including NITROGEN RESPONSE REGULATOR 1 (NRR1) and TRIACYLGLYCEROL ACCUMULATION REGULATOR 1 (TAR1).

Rhythm of the Night (and Day): Predictive Metabolic Modeling of Diurnal Growth in Chlamydomonas

Economical production of photosynthetic organisms requires the use of natural day/night cycles. These induce strong circadian rhythms that lead to transient changes in the cells, requiring complex modeling to capture. In this study, we coupled times series transcriptomic data from the model green alga Chlamydomonas reinhardtii to a metabolic model of the same organism in order to develop the first transient metabolic model for diurnal growth of algae capable of predicting phenotype from genotype. We first transformed a set of discrete transcriptomic measurements (D. Strenkert, S. Schmollinger, S. D. Gallaher, P. A. Salomé, et al., Proc Natl Acad Sci U S A 116:2374–2383, 2019, https://doi.org/10.1073/pnas.1815238116) into continuous curves, producing a complete database of the cell’s transcriptome that can be interrogated at any time point. We also decoupled the standard biomass formation equation to allow different components of biomass to be synthesized at different times of the day. The resulting model was able to predict qualitative phenotypical outcomes of a starchless mutant. We also extended this approach to simulate all single-knockout mutants and identified potential targets for rational engineering efforts to increase productivity. This model enables us to evaluate the impact of genetic and environmental changes on the growth, biomass composition, and intracellular fluxes for diurnal growth.

IMPORTANCE We have developed the first transient metabolic model for diurnal growth of algae based on experimental data and capable of predicting phenotype from genotype. This model enables us to evaluate the impact of genetic and environmental changes on the growth, biomass composition and intracellular fluxes of the model green alga, Chlamydomonas reinhardtii. The availability of this model will enable faster and more efficient design of cells for production of fuels, chemicals, and pharmaceuticals.

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.

Expression of Chlamydomonas reinhardtii chloroplast diacylglycerol acyltransferase 3 is induced by light in concert with triacylglycerol accumulation

Considerable progress has been made towards the understanding of triacylglycerol (TAG) accumulation in algae. One key aspect is finding conditions that trigger TAG production without reducing cell division. Previously, we identified a soluble diacylglycerol acyltransferase (DGAT), related to plant DGAT3, with heterologous DGAT activity. In this work, we demonstrate that Chlamydomonas reinhardtii DGAT3 localizes to the chloroplast and that its expression is induced by light, in correspondence with TAG accumulation. Dgat3 mRNAs and TAGs increase in both wild-type and starch-deficient cells grown with acetate upon transferring them from dark or low light to higher light levels, albeit affected by the particularities of each strain. The response of dgat3 mRNAs and TAGs to light depends on the pre-existing levels of TAGs, suggesting the existence of a negative regulatory loop in the synthesis pathway, although an effect of TAG turnover cannot be ruled out. Altogether, these results hint towards a possible role of DGAT3 in light-dependent TAG accumulation in C. reinhardtii.

Photosynthetic assimilation of CO2 regulates TOR activity

The target of rapamycin (TOR) kinase is a master regulator that integrates nutrient signals to promote cell growth in all eukaryotes. It is well established that amino acids and glucose are major regulators of TOR signaling in yeast and metazoan, but whether and how TOR responds to carbon availability in photosynthetic organisms is less understood. In this study, we showed that photosynthetic assimilation of CO₂ by the Calvin-Benson-Bassham (CBB) cycle regulates TOR activity in the model single-celled microalga Chlamydomonas reinhardtii Stimulation of CO₂ fixation boosted TOR activity, whereas inhibition of the CBB cycle and photosynthesis down-regulated TOR. We uncovered a tight link between TOR activity and the endogenous level of a set of amino acids including Ala, Glu, Gln, Leu, and Val through the modulation of CO₂ fixation and the use of amino acid synthesis inhibitors. Moreover, the finding that the Chlamydomonas starch-deficient mutant sta6 displayed disproportionate TOR activity and high levels of most amino acids, particularly Gln, further connected carbon assimilation and amino acids to TOR signaling. Thus, our results showed that CO₂ fixation regulates TOR signaling, likely through the synthesis of key amino acids.

Long-chain acyl-CoA synthetases activate fatty acids for lipid synthesis, remodeling and energy production in Chlamydomonas

Long-chain acyl-CoA synthetases (LACSs) play many roles in mammals, yeasts and plants, but knowledge on their functions in microalgae remains fragmented. Here via genetic, biochemical and physiological analyses, we unraveled the function and roles of LACSs in the model microalga Chlamydomonas reinhardtii. In vitro assays on purified recombinant proteins revealed that CrLACS1, CrLACS2 and CrLACS3 all exhibited bona fide LACS activities toward a broad range of free fatty acids. The Chlamydomonas mutants compromised in CrLACS1, CrLACS2 or CrLACS3 did not show any obvious phenotypes in lipid content or growth under nitrogen (N)-replete condition. But under N-deprivation, CrLACS1 or CrLACS2 suppression resulted in c. 50% less oil, yet with a higher amount of chloroplast lipids. By contrast, CrLACS3 suppression impaired oil remobilization and cell growth severely during N-recovery, supporting its role in fatty acid β-oxidation to provide energy and carbon sources for regrowth. Transcriptomics analysis suggested that the observed lipid phenotypes are likely not due to transcriptional reprogramming but rather a shift in metabolic adjustment. Taken together, this study provided solid experimental evidence for essential roles of the three Chlamydomonas LACS enzymes in lipid synthesis, remodeling and catabolism, and highlighted the importance of lipid homeostasis in cell growth under nutrient fluctuations.

MicroRNA Expression Profile Analysis of Chlamydomonas reinhardtii during Lipid Accumulation Process under Nitrogen Deprivation Stresses

Lipid accumulation in various microalgae has been found induced by nitrogen deprivation, and it controls many different genes expression. Yet, the underlying molecular mechanisms still remain largely unknown. MicroRNA (miRNAs) play a critical role in post-transcriptional gene regulation. In this study, miRNAs were hypothesized involved in lipid accumulation by nitrogen deprivation. A deep-sequencing platform was used to explore miRNAs-mediated responses induced by nitrogen deprivation in Chlamydomonas reinhardtii. The eukaryotic orthologous groups of proteins (KOG) function in the predicted target genes of miRNA with response to nitrogen deprivation were mainly involved in signal transduction mechanisms, including transcription, lipid transport, and metabolism. A total of 109 miRNA were predicted, including 79 known miRNA and 30 novel miRNA. A total of 29 miRNAs showed significantly differential expressions after nitrogen deprivation, and most of them were upregulated. A total of 10 miRNAs and their targeting genes might involve in lipid transport and metabolism biological process. This study first investigates nitrogen deprivation-regulated miRNAs in microalgae and broadens perspectives on miRNAs importance in microalgae lipid accumulation via nitrogen deprivation. This study provides theoretical guidance for the application of microalgae in bio-oil engineering production.

Transcriptomic survey reveals multiple adaptation mechanisms in response to nitrogen deprivation in marine Porphyridium cruentum

Single-cell red microalga Porphyridium cruentum is potentially considered to be the bioresource for biofuel and pharmaceutical production. Nitrogen is a kind of nutrient component for photosynthetic P. cruentum. Meanwhile, nitrogen stress could induce to accumulate some substances such as lipid and phycoerythrin and affect its growth and physiology. However, how marine microalga Porphyridium cruentum respond and adapt to nitrogen starvation remains elusive. Here, acclimation of the metabolic reprogramming to changes in the nutrient environment was studied by high-throughput mRNA sequencing in the unicellular red alga P. cruentum. Firstly, to reveal transcriptional regulation, de novo transcriptome was assembled and 8,244 unigenes were annotated based on different database. Secondly, under nitrogen deprivation, 2100 unigenes displayed differential expression (1134 upregulation and 966 downregulation, respectively) and some pathways including carbon/nitrogen metabolism, photosynthesis, and lipid metabolism would be reprogrammed in P. cruentum. The result demonstrated that nitrate assimilation (with related unigenes of 8–493 fold upregulation) would be strengthen and photosynthesis (with related unigenes of 6–35 fold downregulation) be impaired under nitrogen deprivation. Importantly, compared to other green algae, red microalga P. cruentum presented a different expression pattern of lipid metabolism in response to nitrogen stress. These observations will also provide novel insight for understanding adaption mechanisms and potential targets for metabolic engineering and synthetic biology in P. cruentum.

The Hypoxia-Associated Localization of Chemotaxis Protein CheZ in Azorhizorbium caulinodans

Spatial organization of chemotactic proteins is important for cooperative response to external stimuli. However, factors affecting the localization dynamics of chemotaxis proteins are less studied. According to some reports, the polar localization of chemotaxis system I is induced by hypoxia and starvation in Vibrio cholerae. However, in V. cholerae, the chemotaxis system I is not involved in flagellum-mediated chemotaxis, and it may play other alternative cellular functions. In this study, we found that the polar localization of CheZ, a phosphatase regulating chemotactic movement in Azorhizobium caulinodans ORS571, can also be affected by hypoxia and cellular energy-status. The conserved phosphatase active site D165 and the C-terminus of CheZ are essential for the energy-related localization, indicating a cross link between hypoxia-related localization changes and phosphatase activity of CheZ. Furthermore, three of five Aer-like chemoreceptors containing PAS domains participate in the cellular localization of CheZ. In contrast to carbon starvation, free-living nitrogen fixation can alleviate the role of nitrogen limitation and hypoxia on polar localization of CheZ. These results showed that the localization changes induced by hypoxia might be a strategy for bacteria to adapt to complex environment.

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.

2-D DIGE Combined with Pro-Q Diamond Staining for the Identification of Protein Phosphorylation for Chlamydomonas reinhardtii : A Successful Approach

The green alga Chlamydomonas reinhardtii is an extremely useful model organism, and protein phosphorylation is an extremely important posttranslational modification. We have established the protocol 2-D difference gel electrophoresis (DIGE), combined with the fluorescence staining with Pro-Q Diamond, that successfully detects subtle changes of spot mobility caused by protein phosphorylation between Chlamydomonas samples.

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.

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.

Single-cell RNA sequencing of batch Chlamydomonas cultures reveals heterogeneity in their diurnal cycle phase

The photosynthetic unicellular alga Chlamydomonas (Chlamydomonas reinhardtii) is a versatile reference for algal biology because of its ease of culture in the laboratory. Genomic and systems biology approaches have previously described transcriptome responses to environmental changes using bulk data, thus representing the average behavior from pools of cells. Here, we apply single-cell RNA sequencing (scRNA-seq) to probe the heterogeneity of Chlamydomonas cell populations under three environments and in two genotypes differing by the presence of a cell wall. First, we determined that RNA can be extracted from single algal cells with or without a cell wall, offering the possibility to sample natural algal communities. Second, scRNA-seq successfully separated single cells into nonoverlapping cell clusters according to their growth conditions. Cells exposed to iron or nitrogen deficiency were easily distinguished despite a shared tendency to arrest photosynthesis and cell division to economize resources. Notably, these groups of cells not only recapitulated known patterns observed with bulk RNA-seq but also revealed their inherent heterogeneity. A substantial source of variation between cells originated from their endogenous diurnal phase, although cultures were grown in constant light. We exploited this result to show that circadian iron responses may be conserved from algae to land plants. We document experimentally that bulk RNA-seq data represent an average of typically hidden heterogeneity in the population.

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.

Co-expression networks in Chlamydomonas reveal significant rhythmicity in batch cultures and empower gene function discovery

The unicellular green alga Chlamydomonas reinhardtii is a choice reference system for the study of photosynthesis and chloroplast metabolism, cilium assembly and function, lipid and starch metabolism, and metal homeostasis. Despite decades of research, the functions of thousands of genes remain largely unknown, and new approaches are needed to categorically assign genes to cellular pathways. Growing collections of transcriptome and proteome data now allow a systematic approach based on integrative co-expression analysis. We used a dataset comprising 518 deep transcriptome samples derived from 58 independent experiments to identify potential co-expression relationships between genes. We visualized co-expression potential with the R package corrplot, to easily assess co-expression and anti-correlation between genes. We extracted several hundred high-confidence genes at the intersection of multiple curated lists involved in cilia, cell division, and photosynthesis, illustrating the power of our method. Surprisingly, Chlamydomonas experiments retained a significant rhythmic component across the transcriptome, suggesting an underappreciated variable during sample collection, even in samples collected in constant light. Our results therefore document substantial residual synchronization in batch cultures, contrary to assumptions of asynchrony. We provide step-by-step protocols for the analysis of co-expression across transcriptome data sets from Chlamydomonas and other species to help foster gene function discovery.

Trophic Transition Enhanced Biomass and Lipid Production of the Unicellular Green Alga Scenedesmus acuminatus

Microalgal heterotrophic cultivation is an emerging technology that can enable producing high cell-density algal cell cultures, which can be coupled with photoautotrophic cultivation for valuable chemicals such as lipids manufacturing. However, how the heterotrophically grown algal cells respond to the lipid-inducing conditions has not been fully elucidated so far. In this study, when the heterotrophically grown Scenedesmus acuminatus cells were subjected to the high light (HL) and nitrogen-limited (NL) conditions, both the biomass and lipid productivity were enhanced as compared to that of the photoautotrophically grown counterparts. The chlorophyll a fluorometry analysis showed that the Fv/Fm and Y(II) of the heterotrophically grown cells subjected to the HL and NL conditions was recovered to the maximum value of 0.75 and 0.43, respectively, much higher than those of the photoautotrophically grown cells under the same stress conditions. Transcriptomic analysis revealed that heterotrophically grown cells fully expressed the genes coding for the photosystems proteins, including the key photoprotective proteins D1, PsbS, light-harvesting-complex (LHC) I and LHC II. Meanwhile, downregulation of the carotenoid biosynthesis and upregulation of the glycolysis/gluconeogenesis, tricarboxylic acid (TCA) cycle and oxidative phosphorylation pathways were observed when the heterotrophically grown cells were subjected to the HL and N-limited conditions for lipid production. It was deduced that regulation of these pathways not only enhanced the light utilization but also provided the reducing power and ATP by which the biomass accumulation was significantly elevated. Besides, upregulation of the acetyl-CoA carboxylase/biotin carboxylase, digalactosyl diacylglycerol synthase and diacylglycerol acyltransferase 2 encoding genes may be attributable to the enhanced lipid production. Understanding the cellular responses during the trophic transition process could guide improvement of the strength of trophic transition enhancing microalgal biomass and lipid production.

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.

Identification and Biotechnical Potential of a Gcn5-Related N-Acetyltransferase Gene in Enhancing Microalgal Biomass and Starch Production

Microalgae are promising feedstocks for starch production, which are precursors for bioenergy and chemicals manufacturing. Though starch biosynthesis has been intensively studied in the green alga Chlamydomonas reinhardtii, regulatory mechanisms governing starch metabolism in this model species have remained largely unknown to date. We proposed that altering triacylglycerol (TAG) biosynthesis may trigger intrinsic regulatory pathways governing starch metabolism. In accordance with the hypothesis, it was observed in this study that overexpression of the plastidial lysophosphatidic acid acyltransferase gene (i.e. LPAAT1) in C. reinhardtii significantly enhanced TAG biosynthesis under nitrogen (N)-replete conditions, whereas the starch biosynthesis was enhanced in turn under N depletion. By the exploitation of transcriptomics analysis, a putative regulatory gene coding Gcn5-related N-acetyltransferase (GNAT19) was identified, which was up-regulated by 11-12 times in the CrLPAAT1 OE lines. Overexpression of the cloned full-length CrGNAT19 cDNA led to significant increase in the starch content of C. reinhardtii cells grown under both N-replete and N-depleted conditions, which was up to 4 times and 26.7% higher than that of the empty vector control, respectively. Moreover, the biomass yield of the CrGNAT19 OE lines reached 1.5 g L^-1 after 2 days under N-depleted conditions, 72% higher than that of the empty vector control (0.87 g L^-1). Overall, the yield of starch increased by 118.5% in CrGNAT19 OE lines compared to that of the control. This study revealed the great biotechnical potentials of an unprecedented GNAT19 gene in enhancing microalgal starch and biomass production.

Multi-omics reveals mechanisms of total resistance to extreme illumination of a desert alga

The unparalleled performance of Chlorella ohadii under irradiances of twice full sunlight underlines the gaps in our understanding of how the photosynthetic machinery operates, and what sets its upper functional limit. Rather than succumbing to photodamage under extreme irradiance, unique features of photosystem II function allow C. ohadii to maintain high rates of photosynthesis and growth, accompanied by major changes in composition and cellular structure. This remarkable resilience allowed us to investigate the systems response of photosynthesis and growth to extreme illumination in a metabolically active cell. Using redox proteomics, transcriptomics, metabolomics and lipidomics, we explored the cellular mechanisms that promote dissipation of excess redox energy, protein S-glutathionylation, inorganic carbon concentration, lipid and starch accumulation, and thylakoid stacking. C. ohadii possesses a readily available capacity to utilize a sudden excess of reducing power and carbon for growth and reserve formation, and post-translational redox regulation plays a pivotal role in this rapid response. Frequently the response in C. ohadii deviated from that of model species, reflecting its life history in desert sand crusts. Comparative global and case-specific analyses provided insights into the potential evolutionary role of effective reductant utilization in this extreme resistance of C. ohadii to extreme irradiation.

Potassium channel KCN11 is required for maintaining cellular osmolarity during nitrogen starvation to control proper cell physiology and TAG accumulation in Chlamydomonas reinhardtii

BACKGROUND

Nitrogen (N) starvation in algae induces a variety of structural and metabolic changes including accumulation of triacylglycerol (TAG). Given the promising prospect of using algae as feedstock for biofuel production, accumulation of TAG upon N starvation becomes an ideal system to study TAG biosynthesis. Under nitrogen-depleted conditions, algae also accumulate compatible solutes such as sugar and certain amino acids, which is expected to elevate osmolarity in the cytoplasm. However, how osmoregulation is maintained and how it impacts on carbon metabolism, especially TAG accumulation under N starvation, are not well understood.

RESULTS

We show here that potassium channel KCN11 localized in the contractile vacuole (CV) mediates osmoregulation during N starvation and loss of KCN11 profoundly affects cell physiology and TAG biosynthesis. KCN11 level is increased and the CV pulsation is accelerated. Loss of KCN11 induces aberrant CV cycle, inhibition of cell growth, increase of cell size, inhibition of chlorophyll loss and TAG accumulation. These effects are rescued by addition of sucrose to raise osmolarity in the culture medium, indicating that osmoregulation is required for cell adaptation to N starvation. Metabolomic analysis shows reduction of acetyl-CoA and accumulation of glyceraldehyde-3-phosphate in kcn11 mutant relative to the control under N starvation, indicating that defects in acetyl-CoA biosynthesis and some metabolic steps from glyceraldehyde-3-phosphate to TAG contribute to the decreased TAG accumulation due to loss of osmoregulation.

CONCLUSIONS

This work provides novel insight of osmoregulation during N starvation in the control of cell physiology and metabolism especially TAG accumulation. According to these findings, we propose that osmolarity should be carefully monitored during the industrial production of biodiesel.

Nitrate deficiency decreased photosynthesis and oxidation-reduction processes, but increased cellular transport, lignin biosynthesis and flavonoid metabolism revealed by RNA-Seq in Oryza sativa leaves

Rice cultivar "Weiyou916" (Oryza sativa L. ssp. Indica) were cultured with control (10 mM NO3-) and nitrate deficient solution (0 mM NO3-) for four weeks. Nitrogen (N) deficiency significantly decreased the content of N and P, dry weight (DW) of the shoots and roots, but increased the ratio of root to shoot in O. sativa. N deficiency decreased the photosynthesis rate and the maximum quantum yield of primary photochemistry (Fv/Fm), however, increased the intercellular CO2 concentration and primary fluorescence (Fo). N deficiency significantly increased the production of H2O2 and membrane lipid peroxidation revealed as increased MDA content in O. sativa leaves. N deficiency significantly increased the contents of starch, sucrose, fructose, and malate, but did not change that of glucose and total soluble protein in O. sativa leaves. The accumulated carbohydrates and H2O2 might further accelerate biosynthesis of lignin in O. sativa leaves under N limitation. A total of 1635 genes showed differential expression in response to N deficiency revealed by Illumina sequencing. Gene Ontology (GO) analysis showed that 195 DEGs were found to highly enrich in nine GO terms. Most of DEGs involved in photosynthesis, biosynthesis of ethylene and gibberellins were downregulated, whereas most of DEGs involved in cellular transport, lignin biosynthesis and flavonoid metabolism were upregulated by N deficiency in O. sativa leaves. Results of real-time quantitative PCR (RT-qPCR) further verified the RNA-Seq data. For the first time, DEGs involved oxygen-evolving complex, phosphorus response and lignin biosynthesis were identified in rice leaves. Our RNA-Seq data provided a global view of transcriptomic profile of principal processes implicated in the adaptation of N deficiency in O. sativa and shed light on the candidate direction in rice breeding for green and sustainable agriculture.

Fixing the Broken Phosphorus Cycle: Wastewater Remediation by Microalgal Polyphosphates

Phosphorus (P), in the form of phosphate derived from either inorganic (Pi) or organic (Po) forms is an essential macronutrient for all life. P undergoes a biogeochemical cycle within the environment, but anthropogenic redistribution through inefficient agricultural practice and inadequate nutrient recovery at wastewater treatment works have resulted in a sustained transfer of P from rock deposits to land and aquatic environments. Our present and near future supply of P is primarily mined from rock P reserves in a limited number of geographical regions. To help ensure that this resource is adequate for humanity's food security, an energy-efficient means of recovering P from waste and recycling it for agriculture is required. This will also help to address excess discharge to water bodies and the resulting eutrophication. Microalgae possess the advantage of polymeric inorganic polyphosphate (PolyP) storage which can potentially operate simultaneously with remediation of waste nitrogen and phosphorus streams and flue gases (CO2, SOx, and NOx). Having high productivity in photoautotrophic, mixotrophic or heterotrophic growth modes, they can be harnessed in wastewater remediation strategies for biofuel production either directly (biodiesel) or in conjunction with anaerobic digestion (biogas) or dark fermentation (biohydrogen). Regulation of algal P uptake, storage, and mobilization is intertwined with the cellular status of other macronutrients (e.g., nitrogen and sulphur) in addition to the manufacture of other storage products (e.g., carbohydrate and lipids) or macromolecules (e.g., cell wall). A greater understanding of controlling factors in this complex interaction is required to facilitate and improve P control, recovery, and reuse from waste streams. The best understood algal genetic model is Chlamydomonas reinhardtii in terms of utility and shared resources. It also displays mixotrophic growth and advantageously, species of this genus are often found growing in wastewater treatment plants. In this review, we focus primarily on the molecular and genetic aspects of PolyP production or turnover and place this knowledge in the context of wastewater remediation and highlight developments and challenges in this field.

Transcriptomic and Physiological Responses to Oxidative Stress in a Chlamydomonas reinhardtii Glutathione Peroxidase Mutant

Aerobic photosynthetic organisms such as algae produce reactive oxygen species (ROS) as by-products of metabolism. ROS damage biomolecules such as proteins and lipids in cells, but also act as signaling molecules. The mechanisms that maintain the metabolic balance in aerobic photosynthetic organisms and how the cells specifically respond to different levels of ROS are unclear. Glutathione peroxidase (GPX) enzymes detoxify hydrogen peroxide or organic hydroperoxides, and thus are important components of the antioxidant system. In this study, we employed a Chlamydomonas reinhardtii glutathione peroxidase knockout (gpx5) mutant to identify the genetic response to singlet oxygen (¹O₂) generated by the photosensitizer rose bengal (RB). To this end, we compared the transcriptomes of the parental strain CC4348 and the gpx5 mutant sampled before, and 1 h after, the addition of RB. Functional annotation of differentially expressed genes showed that genes encoding proteins related to ROS detoxification, stress-response-related molecular chaperones, and ubiquitin–proteasome pathway genes were upregulated in CC4338. When GPX5 was mutated, higher oxidative stress specifically induced the TCA cycle and enhanced mitochondrial electron transport. Transcription of selenoproteins and flagellar-associated proteins was depressed in CC4348 and the gpx5 mutant. In addition, we found iron homeostasis played an important role in maintaining redox homeostasis, and we uncovered the relationship between ¹O₂ stress and iron assimilation, as well as selenoproteins. Based on the observed expression profiles in response to different levels of oxidative stress, we propose a model for dose-dependent responses to different ROS levels in Chlamydomonas.

Chlamydomonas CHT7 Is Required for an Effective Quiescent State by Regulating Nutrient-Responsive Cell Cycle Gene Expression

COMPROMISED HYDROLYSIS OF TRIACYLGLYCEROLS7 (CHT7) in Chlamydomonas (Chlamydomonas reinhardtii) was previously shown to affect the transcription of a subset of genes during nitrogen (N)-replete growth and following N refeeding. Here, we show that an extensive derepression of genes involved in DNA metabolism and cell cycle-related processes, as well as downregulation of genes encoding oxidoreductases and nutrient transporters, occurs in the cht7 mutant during N deprivation. Cellular mutant phenotypes are consistent with the observed transcriptome misregulation, as cht7 cells fail to properly arrest growth, nuclear replication, and cell division following N deprivation. Reduction in cht7 colony formation following N refeeding is explained by its compromised viability during N deprivation and by the occurrence of abortive divisions during N refeeding. Surprisingly, the largely unstructured C-terminal half of CHT7 with predicted protein binding domains, but not the canonical CXC DNA binding domain, is essential for the ability of CHT7 to form stable complexes and reverse the cellular phenotypes and transcription levels in the cht7 mutant. Hence, although lacking the presumed DNA binding domain, CHT7 modulates the expression of cell cycle genes in response to N availability, which is essential for establishing an effective quiescent state and the coordinated resumption of growth following N refeeding.

Lipid droplets in plants and algae: Distribution, formation, turnover and function

Plant oils represent an energy-rich and carbon-dense group of hydrophobic compounds. These oils are not only of economic interest, but also play important, fundamental roles in plant and algal growth and development. The subcellular storage compartments of plant lipids, referred to as lipid droplets (LDs), have long been considered relatively inert oil vessels. However, research in the last decade has revealed that LDs play far more dynamic roles in plant biology than previously appreciated, including transient neutral lipid storage, membrane remodeling, lipid signaling, and stress responses. Here we discuss recent developments in the understanding of LD formation, turnover and function in land plants and algae.

CrABCA2 Facilitates Triacylglycerol Accumulation in Chlamydomonas reinhardtii under Nitrogen Starvation

The microalga Chlamydomonas reinhardtii accumulates triacylglycerols (TAGs) in lipid droplets under stress conditions, such as nitrogen starvation. TAG biosynthesis occurs mainly at the endoplasmic reticulum (ER) and requires fatty acid (FA) substrates supplied from chloroplasts. How FAs are transferred from chloroplast to ER in microalgae was unknown. We previously reported that an Arabidopsis thaliana ATP-binding cassette (ABC) transporter, AtABCA9, facilitates FA transport at the ER during seed development. Here we identified a gene homologous to AtABCA9 in the C. reinhardtii genome, which we named CrABCA2. Under nitrogen deprivation conditions, CrABCA2 expression was upregulated, and the CrABCA2 protein level also increased. CrABCA2 knockdown lines accumulated less TAGs and CrABCA2 overexpression lines accumulated more TAGs than their untransformed parental lines. Transmission electron microscopy showed that CrABCA2 was localized in swollen ER. These results suggest that CrABCA2 transports substrates for TAG biosynthesis to the ER during nitrogen starvation . Our study provides a potential tool for increasing lipid production in microalgae.

Symbiont population control by host-symbiont metabolic interaction in Symbiodiniaceae-cnidarian associations

In cnidarian-Symbiodiniaceae symbioses, algal endosymbiont population control within the host is needed to sustain a symbiotic relationship. However, the molecular mechanisms that underlie such population control are unclear. Here we show that a cnidarian host uses nitrogen limitation as a primary mechanism to control endosymbiont populations. Nitrogen acquisition and assimilation transcripts become elevated in symbiotic Breviolum minutum algae as they reach high-densities within the sea anemone host Exaiptasia pallida. These same transcripts increase in free-living algae deprived of nitrogen. Symbiotic algae also have an elevated carbon-to-nitrogen ratio and shift metabolism towards scavenging nitrogen from purines relative to free-living algae. Exaiptasia glutamine synthetase and glutamate synthase transcripts concomitantly increase with the algal endosymbiont population, suggesting an increased ability of the host to assimilate ammonium. These results suggest algal growth and replication in hospite is controlled by access to nitrogen, which becomes limiting for the algae as their population within the host increases.

The relationship between the coral animal and symbiotic algae is essential to coral health, and researchers are turning to Exaiptasia, a model cnidarian system, to study this relationship mechanistically. Here the authors find that endosymbiotic algae become limited by nitrogen at high population densities and provide the host with high levels of fixed carbon.

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.

Multiscale Multiobjective Systems Analysis (MiMoSA): an advanced metabolic modeling framework for complex systems

In natural environments, cells live in complex communities and experience a high degree of heterogeneity internally and in the environment. Even in 'ideal' laboratory environments, cells can experience a high degree of heterogeneity in their environments. Unfortunately, most of the metabolic modeling approaches that are currently used assume ideal conditions and that each cell is identical, limiting their application to pure cultures in well-mixed vessels. Here we describe our development of Multiscale Multiobjective Systems Analysis (MiMoSA), a metabolic modeling approach that can track individual cells in both space and time, track the diffusion of nutrients and light and the interaction of cells with each other and the environment. As a proof-of concept study, we used MiMoSA to model the growth of Trichodesmium erythraeum, a filamentous diazotrophic cyanobacterium which has cells with two distinct metabolic modes. The use of MiMoSA significantly improves our ability to predictively model metabolic changes and phenotype in more complex cell cultures.

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.

Enhancement of β-carotene production by regulating the autophagy-carotenoid biosynthesis seesaw in Chlamydomonas reinhardtii

This work aimed to demonstrate a new strategy for enhancing the production of carotenoids through the regulation of seesaw cross-talk between autophagy and carotenoid biosynthesis pathways in Chlamydomonas reinhardtii. Autophagy-related ATG1 and ATG8 genes were first silenced using artificial microRNA, which in turn reduced the mRNA expression of ATG1 and ATG8 by 84.4% and 74.3%, respectively. While ATG1 kinase controls early step in autophagy induction and ATG8 is an essential factor for the downstream formation of autophagosome membranes, the decreased expression of these genes led to a 2.34-fold increase in the amount of β-carotene content (i.e., 23.75 mg/g DCW). Furthermore, all mutants seemed to exhibit greater biodiesel properties than that of wild-type due to increased accumulation of saturated and monounsaturated fatty acids. These results support the role of autophagy in regulating the production of valuable metabolites, which could contribute to uplifting the economic outlook of nascent algal biorefinery.

Remodeling of Chlamydomonas Metabolism Using Synthetic Inducers Results in Lipid Storage during Growth

Microalgae accumulate lipids during stress such as that of nutrient deprivation, concomitant with cessation of growth and depletion of chloroplasts. By contrast, certain small chemical compounds selected by high-throughput screening in Chlamydomonas reinhardtii can induce lipid accumulation during growth, maintaining biomass. Comprehensive pathway analyses using proteomics, transcriptomics, and metabolomics data were acquired from Chlamydomonas cells grown in the presence of one of two structurally distinct lipid activators. WD10784 stimulates both starch and lipid accumulation, whereas WD30030-treated cells accumulate only lipids. The differences in starch accumulation are largely due to differential effects of the two compounds on substrate levels that feed into starch synthesis and on genes encoding starch metabolic enzymes. The compounds had differential effects on photosynthesis, respiration, and oxidative stress pathways. Cells treated with WD10784 showed slowed growth over time and reduced abundance of photosynthetic proteins, decreased respiration, and increased oxidative stress proteins, glutathione, and reactive oxygen species specific to this compound. Both compounds maintained central carbon and nitrogen metabolism, including the tricarboxylic acid cycle, glycolysis, respiration, and the Calvin-Benson-Bassham cycle. There were few changes in proteins and transcripts related to fatty acid biosynthesis, whereas proteins and transcripts for triglyceride production were elevated, suggesting that lipid synthesis is largely driven by substrate availability. This study reports that the compound WD30030 and, to a lesser extent WD10784, increases lipid and lipid droplet synthesis and storage without restricting growth or biomass accumulation by mechanisms that are substantially different from nutrient deprivation.

Chlamydomonas cell cycle mutant crcdc5 over-accumulates starch and oil

The use of algal biomass for biofuel production requires improvements in both biomass productivity and its energy density. Green microalgae store starch and oil as two major forms of carbon reserves. Current strategies to increase the amount of carbon reserves often compromise algal growth. To better understand the cellular mechanisms connecting cell division to carbon storage, we examined starch and oil accumulation in two Chlamydomonas mutants deficient in a gene encoding a homolog of the Arabidopsis Cell Division Cycle 5 (CDC5), a MYB DNA binding protein known to be involved in cell cycle in higher plants. The two crcdc5 mutants (crcdc5-1 and crcdc5-2) were found to accumulate significantly higher amount of starch and oil than their corresponding parental lines. Flow cytometry analysis on synchronized cultures cultivated in a diurnal light/dark cycle revealed an abnormal division of the two mutants, characterized by a prolonged S/M phase, therefore demonstrating its implication in cell cycle in Chlamydomonas. Taken together, these results suggest that the energy saved by a slowdown in cell division is used for the synthesis of reserve compounds. This work highlights the importance in understanding the interplay between cell cycle and starch/oil homeostasis, which should have a critical impact on improving lipid/starch productivity.

Metabolomic foundation for differential responses of lipid metabolism to nitrogen and phosphorus deprivation in an arachidonic acid-producing green microalga

The green oleaginous microalga Lobosphaera incisa accumulates storage lipids triacylglycerols (TAG) enriched in the long-chain polyunsaturated fatty acid arachidonic acid under nitrogen (N) deprivation. In contrast, under phosphorous (P) deprivation, the production of the monounsaturated oleic acid prevails. We compared physiological responses, ultrastructural, and metabolic consequences of L. incisa acclimation to N and P deficiency to provide novel insights into the key determinants of ARA accumulation. Differential responses to nutrient deprivation on growth performance, carbon-to-nitrogen stoichiometry, membrane lipid composition and TAG accumulation were demonstrated. Ultrastructural analyses suggested a dynamic role for vacuoles in sustaining cell homeostasis under conditions of different nutrient availability and their involvement in autophagy in L. incisa. Paralleling ARA-rich TAG accumulation in lipid droplets, N deprivation triggered intensive chloroplast dismantling and promoted catabolic processes. Metabolome analysis revealed depletion of amino acids and pyrimidines, and repression of numerous biosynthetic hubs to favour TAG biosynthesis under N deprivation. Under P deprivation, despite the relatively low growth penalties, the presence of the endogenous P reserves and the characteristic lipid remodelling, metabolic signatures of energy deficiency were revealed. Metabolome adjustments to P deprivation included depletion in ATP and phosphorylated nucleotides, increased levels of TCA-cycle intermediates and osmoprotectants. We conclude that characteristic cellular and metabolome adjustments tailor the adaptive responses of L. incisa to N and P deprivation modulating its LC-PUFA production.