Triacylglycerol is produced from starch and polar lipids in the green alga Dunaliella tertiolecta

Polyunsaturated triacylglycerol accumulation mainly attributes to turnover of de novo-synthesized membrane lipids in stress-induced starchless Chlamydomonas

KEY MESSAGE

Assembly of PUFA-attached TAGs is intimately correlated to turnover of newly formed membrane lipids in starch-deficient Chlamydomonas exposed to high light and nitrogen stress under air-aerated mixotrophic conditions. Triacylglycerols (TAGs) rich in polyunsaturated fatty acids (PUFAs) in microalgae have attracted extensive attention due to its promising application in nutraceuticals and other high-value compounds. Previous studies revealed that PUFAs accumulated in TAG primarily derived from the dominant membrane lipids, monogalactosyldiacylglycerolipid, digalactosyldiacylglycerol and diacylglycerol-N,N,N-trimethylhomoserine (DGTS), in the model alga Chlamydomonas reinhardtii. However, their respective contribution to PUFA-attached TAG integration has not been clearly deciphered, particularly in starchless Chlamydomonas that hyper-accumulates TAG. In this study, the starchless C. reinhardtii BAFJ5 was mixotrophically cultivated in photobioreactors aerated with air (0.04% CO2), and we monitored the dynamic changes in growth, cellular carbon and nitrogen content, photosynthetic activity, biochemical compositions, and glycerolipid remodeling under high light and nitrogen starvation conditions. The results indicated that multiple PUFAs continually accumulated in total lipids and TAG, and the primary distributors of these PUFAs gradually shifted from membrane lipids to TAG in stress-induced BAFJ5. The stoichiometry analyses showed that the PUFA-attached TAG assembly attributed to turnover of not only the major glycerolipids, but also the phospholipids, phosphatidylethanolamine (PE) and phosphatidylglycerol. Specifically, the augmented C16:3n3 and C18:3n3 in TAG mainly originated from de novo-synthesized galactolipids, while the cumulative C18:3n6 and C18:4n3 in TAG were intimately correlated with conversion of the newly formed DGTS and PE. These findings emphasized significance of PUFA-attached TAG formation dependent on turnover of de novo assembled membrane lipids in starch-deficient Chlamydomonas, beneficial for enhanced production of value-added lipids in microalgae.

Effects of nitrogen starvation on growth and biochemical composition of some microalgae species

Nitrogen is one of the most important nutrient sources for the growth of microalgae. We studied the effects of nitrogen starvation on the growth responses, biochemical composition, and fatty acid profile of Dunaliella tertiolecta, Phaeodactylum tricornutum, and Nannochloropsis oculata. The lack of nitrogen caused changes in carbohydrate, protein, lipid, and fatty acid composition in all examined microalgae. The carbohydrate content increased 59% in D. tertiolecta, while the lipid level increased 139% in P. tricornutum under nitrogen stress conditions compared to the control groups. Nitrogen starvation increased the oligosaccharide and polysaccharide contents of D. tertiolecta 4.1-fold and 3.6-fold, respectively. Furthermore, triacylglycerol (TAG) levels in N. oculata and P. tricornutum increased 2.3-fold and 7.4-fold, respectively. The dramatic increase in the amount of TAG is important for the use of these microalgae as raw materials in biodiesel. Nitrogen starvation increased the amounts of oligosaccharides and polysaccharides of D. tertiolecta, while increased eicosapentaenoic acid (EPA) in N. oculata and docosahexaenoic acid (DHA) content in P. tricornutum. The amount of polyunsaturated fatty acids (PUFAs), EPA, DHA, oligosaccharides, and polysaccharides in microalgal species can be increased without using the too costly nitrogen source in the culture conditions, which can reduce the most costly of living feeding.

Deciphering the β-carotene hyperaccumulation in Dunaliella by the comprehensive analysis of Dunaliella salina and Dunaliella tertiolecta under high light conditions

The green microalga Dunaliella salina hyperaccumulates β-carotene in the chloroplast, which turns its cells orange. This does not occur in the sister species Dunaliella tertiolecta. However, the molecular mechanisms of β-carotene hyperaccumulation were still unclear. Here, we discovered the reasons for β-carotene hyperaccumulation by comparing the morphology, physiology, genome, and transcriptome between the carotenogenic D. salina and the noncarotenogenic D. tertiolecta after transfer to high light. The differences in photosynthetic capacity, cell growth, and the concentration of stored carbon suggest that these species regulate the supply and utilization of carbon differently. The number of β-carotene-containing plastid lipid globules increased in both species, but much faster and to a greater extent in D. salina than in D. tertiolecta. Consistent with the accumulation of plastid lipid globules, the expression of the methyl-erythritol-phosphate and carotenoid biosynthetic pathways increased only in D. salina, which explains the de novo synthesis of β-carotene. In D. salina, the concomitantly upregulated expression of the carotene globule proteins suggests that hyperaccumulation of β-carotene also requires a simultaneous increase in its sink capacity. Based on genomic analysis, we propose that D. salina has genetic advantages for routing carbon from growth to carotenoid metabolism.

Elevated inorganic carbon and salinity enhances photosynthesis and ATP synthesis in picoalga Picocystis salinarum as revealed by label free quantitative proteomics

Saline soda lakes are of immense ecological value as they niche some of the most exclusive haloalkaliphilic communities dominated by bacterial and archaeal domains, with few eukaryotic algal representatives. A handful reports describe Picocystis as a key primary producer with great production rates in extremely saline alkaline habitats. An extremely haloalkaliphilic picoalgal strain, Picocystis salinarum SLJS6 isolated from hypersaline soda lake Sambhar, Rajasthan, India, grew robustly in an enriched soda lake medium containing mainly Na₂CO₃, 50 g/l; NaHCO_3, 50 g/l, NaCl, 50 g/l (salinity ≈150‰) at pH 10. To elucidate the molecular basis of such adaptation to high inorganic carbon and NaCl concentrations, a high-throughput label-free quantitation based quantitative proteomics approach was applied. Out of the total 383 proteins identified in treated samples, 225 were differentially abundant proteins (DAPs), of which 150 were statistically significant (p < 0.05) including 70 upregulated and 64 downregulated proteins after 3 days of growth in highly saline-alkaline medium. Most DAPs were involved in photosynthesis, oxidative phosphorylation, glucose metabolism and ribosomal structural components envisaging that photosynthesis and ATP synthesis were central to the salinity-alkalinity response. Key components of photosynthetic machinery like photosystem reaction centres, adenosine triphosphate (ATP) synthase ATP, Rubisco, Fructose-1,6-bisphosphatase, Fructose-bisphosphate aldolase were highly upregulated. Enzymes peptidylprolyl isomerases (PPIase), important for correct protein folding showed remarkable marked-up regulation along with other chaperon proteins indicating their role in osmotic adaptation. Enhanced photosynthetic activity exhibited by P. salinarum in highly saline-alkaline condition is noteworthy as photosynthesis is suppressed under hyperosmotic conditions in most photosynthetic organisms. The study provided the first insights into the proteome of extremophilic alga P. salinarum exhibiting extraordinary osmotic adaptation and proliferation in polyextreme conditions prevailing in saline sodic ecosystems, potentially unraveling the basis of resilience in this not so known organism and paves the way for a promising future candidate for biotechnological applications and model organism for deciphering the molecular mechanisms of osmotic adaptation. The mass spectrometry proteomics data is available at the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD037170.

Effects of heat shock on photosynthesis-related characteristics and lipid profile of Cycas multipinnata and C. panzhihuaensis

BACKGROUND

Cycas multipinnata and C. panzhihuaensis are two attractive ornamental tree species. With the global climate change, the temperature in the natural habitats of both the species shows a marked rising trend. However, how the two species respond to extreme high temperatures are not clear. Chlorophyll fluorescence parameters, chlorophyll content, chloroplast ultrastructure and lipid metabolism in the two species were determined following plant exposure to heat stress.

RESULTS

The results demonstrated that the photosynthetic efficiency decreased significantly in both the species following heat shock and recovery, but to a greater extent in C. panzhihuaensis. Compared to the control, chlorophyll content of C. multipinnata did not change significantly following heat stress and recovery. However, chlorophyll content of C. panzhihuaensis increased significantly after 1 d of recovery in comparison with the control. Chloroplast ultrastructures of C. panzhihuaensis were more severely affected by heat shock than C. multipinnata. C. multipinnata and C. panzhihuaensis followed a similar change trend in the amounts of most of the lipid categories after heat stress. However, only the amounts of lysophospholipids and fatty acyls differed significantly between the two species following heat treatment. Additionally, the unsaturation levels of the major lipid classes in C. multipinnata were significantly lower than or equal to those in C. panzhihuaensis.

CONCLUSIONS

C. multipinnata was less affected by extremely high temperatures than C. panzhihuaensis. The differential stability of chlorophyll and chloroplast ultrastructure and the differential adjustment of lipid metabolism might contribute to the different responses to heat shock between the two species.

Increased Lipids in Chlamydomonas reinhardtii by Multiple Regulations of DOF, LACS2, and CIS1

Microalgal lipids are essential for biofuel and dietary supplement production. Lipid engineering for higher production has been studied for years. However, due to the complexity of lipid metabolism, single-gene engineering gradually encounters bottlenecks. Multiple gene regulation is more beneficial to boosting lipid accumulation and further clarifying the complex regulatory mechanism of lipid biosynthesis in the homeostasis of lipids, carbohydrates, and protein metabolism. Here, three lipid-related genes, DOF, LACS2, and CIS, were co-regulated in Chlamydomonas reinhartii by two circles of transformation to overexpress DOF and knock down LACS2 and CIS simultaneously. With the multiple regulations of these genes, the intracellular lipids and FA content increased by 142% and 52%, respectively, compared with CC849, whereas the starch and protein contents decreased by 45% and 24%. Transcriptomic analysis showed that genes in TAG and FA biosynthesis were up-regulated, and genes in starch and protein metabolism were down-regulated. This revealed that more carbon precursor fluxes from starch and protein metabolism were redirected towards lipid synthesis pathways. These results showed that regulating genes in various metabolisms contributed to carbon flux redirection and significantly improved intracellular lipids, demonstrating the potential of multiple gene regulation strategies and providing possible candidates for lipid overproduction in microalgae.

Differences in Glycerolipid Response of Chlamydomonas reinhardtii Starchless Mutant to High Light and Nitrogen Deprivation Stress Under Three Carbon Supply Regimes

Carbon source serves as a crucial factor for microalgal lipid biosynthesis. The supplied exogenous inorganic or organic carbon affects lipid accumulation in microalgae under stress conditions. However, the impacts of different carbon availability on glycerolipid metabolism, triacylglycerol (TAG) metabolism in particular, still remain elusive in microalgae. Chlamydomonas starchless mutant BAFJ5 has emerged as a model system to study TAG metabolism, due to its property of hyper-accumulating TAG. In this study, the glycerolipidomic response of the starchless BAFJ5 to high light and nitrogen-deprived (HL-N) stress was deciphered in detail to distinguish glycerolipid metabolism under three carbon supply regimes. The results revealed that the autotrophically and mixotrophically grown BAFJ5 cells aerated with air containing 2% CO2 presented similar changes in growth, photosynthetic activity, biochemical components, and glycerolipid metabolism under HL-N conditions. But the mixotrophically grown BAFJ5 aerated with air containing 0.04% CO2 exhibited more superior accumulation in TAG, which was esterified with a significantly higher proportion of C18:1n9 and prominently the lower proportions of polyunsaturated fatty acids. In addition, these cells increased the relative levels of C18:2n6 in the membrane lipids, i.e., monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), in priority, and decreased that of C18:3n3 and C18:4n3 in the betaine lipid, N,N,N-trimethylhomoserine diacylglycerol (DGTS), subsequently, to adapt to the HL-N stress conditions, compared to the cells under the other two conditions. Thus, it was suggested that C. reinhardtii starchless mutant appeared to present distinct metabolism for TAG biosynthesis involving membrane lipid remodeling under distinct carbon supply regimes. This study provides insights into how the different carbon supply regimes affect lipid metabolism in Chlamydomonas starchless cells, which will benefit the optimized production of storage lipids in microalgae.

Microalgae Bioactive Carbohydrates as a Novel Sustainable and Eco-Friendly Source of Prebiotics: Emerging Health Functionality and Recent Technologies for Extraction and Detection

There is a global interest in the novel consumption, nutritional trends, and the market of new prebiotic sources and their potential functional impacts. Commercially available nutritional supplements based on microalgae that are approved to be edible by FDA, like Arthrospira platensis (Cyanobacteria) and Chlorella vulgaris (Chlorophyta) become widely attractive. Microalgae are rich in carbohydrates, proteins, and polyunsaturated fatty acids that have high bioactivity. Recently, scientists are studying the microalgae polysaccharides (PS) or their derivatives (as dietary fibers) for their potential action as a novel prebiotic source for functional foods. Besides, the microalgae prebiotic polysaccharides are used for medication due to their antioxidant, anticancer, and antihypertensive bioactivities. This review provides an overview of microalgae prebiotics and other macromolecules' health benefits. The phytochemistry of various species as alternative future sources of novel polysaccharides were mentioned. The application as well as the production constraints and multidisciplinary approaches for evaluating microalgae phytochemistry were discussed. Additionally, the association between this potential of combining techniques like spectroscopic, chromatographic, and electrochemical analyses for microalgae sensation and analysis novelty compared to the chemical methods was emphasized.

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.

To Divide or Not to Divide? How Deuterium Affects Growth and Division of Chlamydomonas reinhardtii

Extensive in vivo replacement of hydrogen by deuterium, a stable isotope of hydrogen, induces a distinct stress response, reduces cell growth and impairs cell division in various organisms. Microalgae, including Chlamydomonas reinhardtii, a well-established model organism in cell cycle studies, are no exception. Chlamydomonas reinhardtii, a green unicellular alga of the Chlorophyceae class, divides by multiple fission, grows autotrophically and can be synchronized by alternating light/dark regimes; this makes it a model of first choice to discriminate the effect of deuterium on growth and/or division. Here, we investigate the effects of high doses of deuterium on cell cycle progression in C. reinhardtii. Synchronous cultures of C. reinhardtii were cultivated in growth medium containing 70 or 90% D2O. We characterize specific deuterium-induced shifts in attainment of commitment points during growth and/or division of C. reinhardtii, contradicting the role of the "sizer" in regulating the cell cycle. Consequently, impaired cell cycle progression in deuterated cultures causes (over)accumulation of starch and lipids, suggesting a promising potential for microalgae to produce deuterated organic compounds.

Quantitative proteomic comparison of salt stress in Chlamydomonas reinhardtii and the snow alga Chlamydomonas nivalis reveals mechanisms for salt-triggered fatty acid accumulation via reallocation of carbon resources

BACKGROUND

Chlamydomonas reinhardtii is a model green alga strain for molecular studies; its fully sequenced genome has enabled omic-based analyses that have been applied to better understand its metabolic responses to stress. Here, we characterised physiological and proteomic changes between a low-starch C. reinhardtii strain and the snow alga Chlamydomonas nivalis, to reveal insights into their contrasting responses to salinity stress.

RESULTS

Each strain was grown in conditions tailored to their growth requirements to encourage maximal fatty acid (as a proxy measure of lipid) production, with internal controls to allow comparison points. In 0.2 M NaCl, C. nivalis accumulates carbohydrates up to 10.4% DCW at 80 h, and fatty acids up to 52.0% dry cell weight (DCW) over 12 days, however, C. reinhardtii does not show fatty acid accumulation over time, and shows limited carbohydrate accumulation up to 5.5% DCW. Analysis of the C. nivalis fatty acid profiles showed that salt stress improved the biofuel qualities over time. Photosynthesis and respiration rates are reduced in C. reinhardtii relative to C. nivalis in response to 0.2 M NaCl. De novo sequencing and homology matching was used in conjunction with iTRAQ-based quantitative analysis to identify and relatively quantify proteomic alterations in cells exposed to salt stress. There were abundance differences in proteins associated with stress, photosynthesis, carbohydrate and lipid metabolism proteins. In terms of lipid synthesis, salt stress induced an increase in dihydrolipoyl dehydrogenase in C. nivalis (1.1-fold change), whilst levels in C. reinhardtii remained unaffected; this enzyme is involved in acetyl CoA production and has been linked to TAG accumulation in microalgae. In salt-stressed C. nivalis there were decreases in the abundance of UDP-sulfoquinovose (- 1.77-fold change), which is involved in sulfoquinovosyl diacylglycerol metabolism, and in citrate synthase (- 2.7-fold change), also involved in the TCA cycle. Decreases in these enzymes have been shown to lead to increased TAG production as fatty acid biosynthesis is favoured. Data are available via ProteomeXchange with identifier PXD018148.

CONCLUSIONS

These differences in protein abundance have given greater understanding of the mechanism by which salt stress promotes fatty acid accumulation in the un-sequenced microalga C. nivalis as it switches to a non-growth state, whereas C. reinhardtii does not have this response.

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

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

Ecological and Industrial Implications of Dynamic Seaweed-Associated Microbiota Interactions

Seaweeds are broadly distributed and represent an important source of secondary metabolites (e.g., halogenated compounds, polyphenols) eliciting various pharmacological activities and playing a relevant ecological role in the anti-epibiosis. Importantly, host (as known as basibiont such as algae)–microbe (as known as epibiont such as bacteria) interaction (as known as halobiont) is a driving force for coevolution in the marine environment. Nevertheless, halobionts may be fundamental (harmless) or detrimental (harmful) to the functioning of the host. In addition to biotic factors, abiotic factors (e.g., pH, salinity, temperature, nutrients) regulate halobionts. Spatiotemporal and functional exploration of such dynamic interactions appear crucial. Indeed, environmental stress in a constantly changing ocean may disturb complex mutualistic relations, through mechanisms involving host chemical defense strategies (e.g., secretion of secondary metabolites and antifouling chemicals by quorum sensing). It is worth mentioning that many of bioactive compounds, such as terpenoids, previously attributed to macroalgae are in fact produced or metabolized by their associated microorganisms (e.g., bacteria, fungi, viruses, parasites). Eventually, recent metagenomics analyses suggest that microbes may have acquired seaweed associated genes because of increased seaweed in diets. This article retrospectively reviews pertinent studies on the spatiotemporal and functional seaweed-associated microbiota interactions which can lead to the production of bioactive compounds with high antifouling, theranostic, and biotechnological potential.

Effects of the water-soluble fraction of the mixture fuel oil/diesel on the microalgae Dunaliella tertiolecta through growth

There is evidence that water-soluble fraction (WSF) from fuel oil/diesel mixture affects marine microbiota. In order to establish a sequence of WSF effects during microalgal growth, this work aimed to monitor Dunaliella tertiolecta exposed to WSF during 15 days. Three different pigments (chlorophyll a, lutein, and β-carotene) and four metabolites (protein, lipids, fatty acids, and phenols) were studied, and FTIR spectroscopy was used to determine the biomolecular transitions of lipids and their accumulation. The results show that D. tertiolecta triggered a physiological and biochemical response with changes in growth rate, pigments, phenols, lipids, and proteins of the microalga, although fatty acid profile was unaltered. For all the biochemical parameters altered, there were significant differences with the controls. At the end of the assay, exposed D. tertiolecta showed similar values with the control on all the compounds analyzed, except lipids. FTIR absorbance showed an increase in unsaturated acyl chains within the exposed microalgae, giving support for a possible uptake of hydrocarbons from WSF. Variation in pigments and phenol contents is presented as an integrated antioxidant response to the stress imposed by WSF. Overall, this research provides information about the effects of WSF on D. tertiolecta, and the ability of this microalga to recover after long-term exposure to the water-soluble fraction of fuel oil/diesel.

The cadmium-induced changes in the polar and neutral lipid compositions suggest the involvement of triacylglycerol in the defense response in maize

Cadmium (Cd) is a heavy metal ion leading to morphological and physiological disorders in plants; a specific toxicity target is the membrane lipids. The total lipids were separated by thin-layer chromatography, and the fatty acid composition of the total (TLs), polar lipids (PLs) and triacylglycerol (TAG)-a neutral lipid-was analyzed in maize seedlings in hydroponics and treated by various Cd concentrations (0-200 µM Cd). The TLs and PLs significantly decreased in roots after Cd treatment, suggesting the onset of lipid peroxidation mediated by oxygen free radicals, that induce alterations of the membrane structure and function. There were also increases in the TAG from 28.2 to 36.9% of TLs, and the TAG/PLs ratio varied from 0.59 to 0.84, in control and after exposure to 200 µM Cd, respectively. The TAG plays potent roles in membrane turnover serving as energy and carbon resources for the biosynthesis of membrane lipids, to preserve membrane structure and function, and therefore cell homeostasis in response to Cd. In shoots, a significant increase in the levels of C16:0, C18:1, and C18:2, while a decrease in that of C18:3 was observed, suggesting inhibition of desaturases enzymes. These lead to impairment of the chloroplast membrane. The total lipid content did not change under Cd stress. The PLs, however, decreased from 22.4 to 13.6 mg g^-1 DW; their percent to TLs varied from 86.6 to 52.5%, in control, and after Cd treatment, respectively. In conclusion, the accumulation of TAG may represent a defense strategy by which maize seedlings can withstand the effects of Cd toxicity, leading to reduced oxidative stress.

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.

Dynamic Modeling of Microalgae Growth and Lipid Production under Transient Light and Nitrogen Conditions

We developed a new dynamic model to characterize how light and nitrogen regulate the cellular processes of photosynthetic microalgae leading to transient changes in the production of neutral lipids, carbohydrates, and biomass. Our model recapitulated the versatile neutral lipid synthesis pathways via (i) carbon reuse from carbohydrate metabolism under nitrogen sufficiency and (ii) fixed carbon redirection under nitrogen depletion. We also characterized the effects of light adaptation, light inhibition hysteresis, and nitrogen limitation on photosynthetic carbon fixation. The formulated model was calibrated and validated with experimental data of Dunaliella viridis cultivated in a lab-scale photobioreactor (PBR) under various light (low/moderate/high) and nitrogen (sufficient/limited) conditions. We conducted the identifiability, uncertainty, and sensitivity analyses to verify the model reliability using the profile likelihood method, the Markov chain Monte Carlo (MCMC) technique, and the extended Fourier Amplitude Sensitivity Test (eFAST). Our model predictions agreed well with experimental observations and suggested potential model improvement by incorporating a lipid degradation mechanism. The insights from our model-driven analysis helped improve the mechanistic understanding of transient algae growth and bioproducts formation under environmental variations and could be applied to optimize biofuel and biomass production.

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.

Xylan in the Middle: Understanding Xylan Biosynthesis and Its Metabolic Dependencies Toward Improving Wood Fiber for Industrial Processing

Lignocellulosic biomass, encompassing cellulose, lignin and hemicellulose in plant secondary cell walls (SCWs), is the most abundant source of renewable materials on earth. Currently, fast-growing woody dicots such as Eucalyptus and Populus trees are major lignocellulosic (wood fiber) feedstocks for bioproducts such as pulp, paper, cellulose, textiles, bioplastics and other biomaterials. Processing wood for these products entails separating the biomass into its three main components as efficiently as possible without compromising yield. Glucuronoxylan (xylan), the main hemicellulose present in the SCWs of hardwood trees carries chemical modifications that are associated with SCW composition and ultrastructure, and affect the recalcitrance of woody biomass to industrial processing. In this review we highlight the importance of xylan properties for industrial wood fiber processing and how gaining a greater understanding of xylan biosynthesis, specifically xylan modification, could yield novel biotechnology approaches to reduce recalcitrance or introduce novel processing traits. Altering xylan modification patterns has recently become a focus of plant SCW studies due to early findings that altered modification patterns can yield beneficial biomass processing traits. Additionally, it has been noted that plants with altered xylan composition display metabolic differences linked to changes in precursor usage. We explore the possibility of using systems biology and systems genetics approaches to gain insight into the coordination of SCW formation with other interdependent biological processes. Acetyl-CoA, s-adenosylmethionine and nucleotide sugars are precursors needed for xylan modification, however, the pathways which produce metabolic pools during different stages of fiber cell wall formation still have to be identified and their co-regulation during SCW formation elucidated. The crucial dependence on precursor metabolism provides an opportunity to alter xylan modification patterns through metabolic engineering of one or more of these interdependent pathways. The complexity of xylan biosynthesis and modification is currently a stumbling point, but it may provide new avenues for woody biomass engineering that are not possible for other biopolymers.

Microalgae and cyanobacteria modeling in water resource recovery facilities: A critical review

Microalgal and cyanobacterial resource recovery systems could significantly advance nutrient recovery from wastewater by achieving effluent nitrogen (N) and phosphorus (P) levels below the current limit of technology. The successful implementation of phytoplankton, however, requires the formulation of process models that balance fidelity and simplicity to accurately simulate dynamic performance in response to environmental conditions. This work synthesizes the range of model structures that have been leveraged for algae and cyanobacteria modeling and core model features that are required to enable reliable process modeling in the context of water resource recovery facilities. Results from an extensive literature review of over 300 published phytoplankton models are presented, with particular attention to similarities with and differences from existing strategies to model chemotrophic wastewater treatment processes (e.g., via the Activated Sludge Models, ASMs). Building on published process models, the core requirements of a model structure for algal and cyanobacterial processes are presented, including detailed recommendations for the prediction of growth (under phototrophic, heterotrophic, and mixotrophic conditions), nutrient uptake, carbon uptake and storage, and respiration.

Microalgae for biofuel production

Microalgae have been used commercially since the 1950s and 1960s, particularly in the Far East for human health foods and in the United States for wastewater treatment. Initial attempts to produce bulk chemicals such as biofuels from microalgae were not successful, despite commercially favorable conditions during the 1970s oil crisis. However, research initiatives at this time, many using extremophilic microalgae and cyanobacteria (e.g., Dunaliella and Spirulina), did solve many problems and clearly identified biomass productivity and harvesting as the two main constraints stopping microalgae producing bulk chemicals, such as biofuels, on a large scale. In response to the growing unease around global warming, induced by anthropogenic CO₂ emissions, microalgae were again suggested as a carbon neutral process to produce biofuels. This recent phase of microalgae biofuels research can be thought to have started around 2007, when a very highly cited review by Chisti was published. Since 2007, a large body of scientific publications have appeared on all aspects of microalgae biotechnology, but with a clear emphasis on neutral lipid (triacylglycerol) synthesis and the use of neutral lipids as precursors for biodiesel production. In this review, the key research on microalgal biotechnology that took place prior to 2007 will be summarized and then the research trends post 2007 will be examined emphasizing the research into producing biodiesel from microalgae.

Sustainable production of biomass and biodiesel by acclimation of non-acidophilic microalgae to acidic conditions

The overwhelming response towards algal biodiesel production has been well-recognized recently as a sustainable alternative to conventional fuels. Most microalgae cannot grow well at acidic pH. The present study, therefore, investigated whether non-acidophilic microalgae Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3 can be acclimated to extreme-acidic pH for sustainable production of biomass and biodiesel. Growth analysis indicated that both the microalgal strains possessed a passive uptake of CO₂ at pH 3.0 with biomass production of 0.25 g dry wt. L^-1 in Desmodemus sp. and 0.45 g dry wt. L^-1 in Heterochlorella sp.. Flow-cytometry analysis for reactive oxygen species, membrane permeability and neutral-lipids revealed the capabilities of both strains to adapt to the stress imposed by acidic pH. Lipid production was doubled in both the strains when grown at pH 3.0. In-situ transesterification of biomass resulted in 13-15% FAME yield in the selected microalgae, indicating their great potential in biofuel production.

A hypothesis about the origin of carotenoid lipid droplets in the green algae Dunaliella and Haematococcus

Hypercarotenogenesis in green algae evolved by mutation of PSY that increased its transcription at high light, disintegration of the eyespot in Dunaliella and acquisition of the capacity to export carotenoids from chloroplasts in Haematococcus. Carotenoids (Car) are lipid-soluble pigments synthesized in plants, algae, bacteria and fungi. Car have strong antioxidative properties and as such are utilized to reduce the danger of different diseases in humans. Two green microalgae are utilized as rich natural sources for Car: Dunaliella salina/bardawil accumulates 10% (w/w) β-carotene (βC), which is also pro-vitamin A, and Haematococcus pluvialis accumulates 4% (w/w) astaxanthin (Ast), the strongest antioxidant among Car. D. bardawil accumulates βC in plastoglobules within the chloroplast, whereas H. pluvialis deposits Ast in cytoplasmic lipid droplets (CLD). In this review we compare the hypercarotenogenic responses (HCR) in Dunaliella and in Haematococcus and try to outline hypothetical evolutionary pathways for its origin. We propose that a mutation in phytoene synthetase that increased its transcription level in response to high light stress had a pivotal role in the evolution of the HCR. Proteomic analyses indicated that in D. bardawil/salina the HCR evolved from dissociation and amplification of eyespot lipid globules. The more robust HCR in algae that accumulate carotenoids in CLD, such as H. pluvialis, required also acquisition of the capacity to export βC out of the chloroplast and its enzymatic conversion into Ast.

Optimization of triacylglycerol and starch production in Chlamydomonas debaryana NIES-2212 with regard to light intensity and CO2 concentration

Triacylglycerol (TAG) and starch produced by micro-algae are potential sources of biofuel. Our previous studies showed that the unicellular green alga, Chlamydomonas debaryana NIES-2212, which is a rare species of Chlamydomonas that possesses phosphatidylcholine (PC), is a seed organism for the development of biofuel producers. This alga accumulates large amounts of TAG and starch under completely photo-autotrophic conditions during stationary phase without nutrient deprivation. The present study was performed to optimize the growth conditions of this alga with regard to light intensity and CO2 concentration to improve the efficiency of TAG and starch production. The growth rate of C. debaryana was greater at higher light intensity, although there was no significant difference in the final cell density of the culture. The highest contents of TAG and starch, approximately 200 fmol cell^-1 and 600 pg cell^-1, respectively, were achieved with a light intensity of 200 µmol m^-2 s^-1 bubbled with air containing 5.0 % CO2. These results suggest that optimization of light intensity and CO2 concentration can enhance the productivity of TAG and starch by C. debaryana NIES-2212.

High-value biomass from microalgae production platforms: strategies and progress based on carbon metabolism and energy conversion

Microalgae are capable of producing sustainable bioproducts and biofuels by using carbon dioxide or other carbon substances in various cultivation modes. It is of great significance to exploit microalgae for the economical viability of biofuels and the revenues from high-value bioproducts. However, the industrial performance of microalgae is still challenged with potential conflict between cost of microalgae cultivation and revenues from them, which is mainly ascribed to the lack of comprehensive understanding of carbon metabolism and energy conversion. In this review, we provide an overview of the recent advances in carbon and energy fluxes of light-dependent reaction, Calvin-Benson-Bassham cycle, tricarboxylic acid cycle, glycolysis pathway and processes of product biosynthesis in microalgae, with focus on the increased photosynthetic and carbon efficiencies. Recent strategies for the enhanced production of bioproducts and biofuels from microalgae are discussed in detail. Approaches to alter microbial physiology by controlling light, nutrient and other environmental conditions have the advantages of increasing biomass concentration and product yield through the efficient carbon conversion. Engineering strategies by regulating carbon partitioning and energy route are capable of improving the efficiencies of photosynthesis and carbon conversion, which consequently realize high-value biomass. The coordination of carbon and energy fluxes is emerging as the potential strategy to increase efficiency of carbon fixation and product biosynthesis. To achieve more desirable high-value products, coordination of multi-stage cultivation with engineering and stress-based strategies occupies significant positions in a long term.