Biosynthesis and regulation of carotenoids in Dunaliella: progresses and prospects

High-value bioproducts from microalgae: Strategies and progress

Microalgae have been considered as alternative sustainable resources for high-value bioproducts such as lipids (especially triacylglycerides [TAGs]), polyunsaturated fatty acids (PUFAs), and carotenoids, due to their relatively high photosynthetic efficiency, no arable land requirement, and ease of scale-up. It is of great significance to exploit microalgae for the production of high-value bioproducts. How to improve the content or productivity of specific bioproducts has become one of the most urgent challenges. In this review, we will describe high-value bioproducts from microalgae and their biosynthetic pathways (mainly for lipids, PUFAs, and carotenoids). Recent progress and strategies for the enhanced production of bioproducts from microalgae are also described in detail, and these strategies take advantages of optimized cultivation conditions with abiotic stress, chemical stress (addition of metabolic precursors, phytohormones, chemical inhibitors, and chemicals inducing oxidative stress response), and molecular approaches such as metabolic engineering, transcriptional engineering, and gene disruption strategies (mainly RNAi, antisense RNA, miRNA-based knockdown, and CRISPR/Cas9).

Loss of Function in Zeaxanthin Epoxidase of Dunaliella tertiolecta Caused by a Single Amino Acid Mutation within the Substrate-Binding Site

The zea1 mutant of marine microalga Dunaliella tertiolecta accumulates zeaxanthin under normal growth conditions, and its phenotype has been speculated to be related to zeaxanthin epoxidase (ZEP). In this study, we isolated the ZEP gene from both wild-type D. tertiolecta and the mutant. We found that the zea1 mutant has a point mutation of the 1337th nucleotide of the ZEP sequence (a change from guanine to adenine), resulting in a change of glycine to aspartate in a highly conserved region in the catalytic domain. Similar expression levels of ZEP mRNA and protein in both wild-type and zea1 were confirmed by using qRT-PCR and western blot analysis, respectively. Additionally, the enzyme activity analysis of ZEPs in the presence of cofactors showed that the inactivation of ZEP in zea1 was not caused by deficiency in the levels of cofactors. From the predicted three-dimensional ZEP structure of zea1, we observed a conformational change on the substrate-binding site in the ZEP. A comparative analysis of the ZEP structures suggested that the conformational change induced by a single amino acid mutation might impact the interaction between the substrate and substrate-binding site, resulting in loss of zeaxanthin epoxidase function.

A systematic correlation analysis of carotenoids, chlorophyll, non-pigmented cell mass, and cell number for the blueprint of Dunaliella salina culture in a photobioreactor

Microalgal carotenoids are attractive health ingredients, but their production should be optimized to improve cost-effectiveness. Understanding cellular physiology centered on carotenoid synthesis is the prerequisite for this work. Therefore, systematic correlation analyses were conducted among chlorophyll, carotenoids, non-pigmented cell mass, and cell number of Dunaliella salina in a specified condition over a relatively long culture time. First, an integrated correlation was performed: a temporal profile of the carotenoids was correlated with those of other factors, including chlorophyll, non-pigmented cell mass, and cell number. Pearson and Spearman correlation analyses were performed to identify linearity and monotonicity of the correlation, respectively, and then cross-correlation was executed to determine if the correlation had a time lag. Second, to understand the cellular potential of metabolism, the procedure was repeated to provide a data set composed of the specific synthesis rates of the factors or growth rate, which additionally provided kinetic correlations among the constituting components of the cell, excluding the effect of cell number. This systematic approach could generate a blueprint model that is composed of only what it needs, which could make it possible to efficiently control and optimize the process.

Plasma proteins associated with circulating carotenoids in Nepalese school-aged children

Carotenoids are naturally occurring pigments that function as vitamin A precursors, antioxidants, anti-inflammatory agents or biomarkers of recent vegetable and fruit intake, and are thus important for population health and nutritional assessment. An assay approach that measures proteins could be more technologically feasible than chromatography, thus enabling more frequent carotenoid status assessment. We explored associations between proteomic biomarkers and concentrations of 6 common dietary carotenoids (α-carotene, β-carotene, lutein/zeaxanthin, β-cryptoxanthin, and lycopene) in plasma from 500 6-8 year old Nepalese children. Samples were depleted of 6 high-abundance proteins. Plasma proteins were quantified using tandem mass spectrometry and expressed as relative abundance. Linear mixed effects models were used to determine the carotenoid:protein associations, accepting a false discovery rate of q < 0.10. We quantified 982 plasma proteins in >10% of all child samples. Among these, relative abundance of 4 were associated with β-carotene, 11 with lutein/zeaxanthin and 51 with β-cryptoxanthin. Carotenoid-associated proteins are notably involved in lipid and vitamin A transport, antioxidant function and anti-inflammatory processes. No protein biomarkers met criteria for association with α-carotene or lycopene. Plasma proteomics may offer an approach to assess functional biomarkers of carotenoid status, intake and biological function for public health application. Original maternal micronutrient trial from which data were derived as a follow-up activity was registered at ClinicalTrials.gov: NCT00115271.

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.

Carotenoid Production by a Novel Isolate of Microbacterium paraoxydans

This study reports extraction and characterization of carotenoid pigments from Microbacterium paraoxydans, a non-photosynthetic bacterium, cultivated in Luria-Bertani (LB) medium. The isolate was identified to be moderately halo- and osmo-tolerant capable of withstanding high (~ 6%) salt and sugar (30% w/v sucrose, 20% w/v glucose) concentrations after a brief period of adaptation. The pigments were characterized using a combination of UV-Vis spectral analysis with the λ_max at 407, 436 and 466 nm and ESI-MS with an m/z value at 536.44. The absorption profile of the pigments and their nature was influenced by carbon, nitrogen source and presence of salt in the growth medium. Highest level of pigment (~ 16 g kg dry wt cells^-1) was produced in NH₄Cl supplemented LB medium. The pigment displayed free radical scavenging, anticancer activity, characteristic of the plant carotenoids. Based on the accumulation of pigments under different conditions, a biochemical pathway for synthesis of neurosporene was proposed.

Carotenogenic response in photosynthetic organisms: a colorful story

Carotenoids are a diverse group of terpenoid pigments ubiquitous in and essential for functioning of phototrophs. Most of the researchers in the field are focused on the primary carotenoids serving light harvesting, photoprotection, and supporting the structural integrity of the photosynthetic apparatus (PSA) within the thylakoid membranes. A distinct group of the pigments functionally and structurally uncoupled from the PSA and accumulating outside of the thylakoids is called secondary carotenoids. Induction of the biosynthesis and massive accumulation of the latter termed as secondary carotenogenesis and carotenogenic response (CR), respectively, is a major though insufficiently studied stress response discovered in many phototrophic organisms ranging from single-celled algae to terrestrial higher plants. The CR protects cell by means of optical shielding of cell structures vulnerable photodamage, consumption of potentially harmful dioxygen, augmenting sink capacity of photoassimilates, and exerting an antioxidant effect. The secondary carotenoids exhibit a remarkable photostability in situ. Therefore, the CR-based photoprotective mechanism, unlike, e.g., antioxidant enzyme-based protection in the chloroplast, does not require continuous investment of energy and metabolites making it highly suitable for long-term stress acclimation in phototrophs. Capability of the CR determines the strategy of acclimation of photosynthetic organisms to different stresses such as excessive irradiance, drought, extreme temperatures, and salinities. Build-up of the CR might be accompanied by gradual disengagement of 'classical' active (energy-dependent) photoprotective mechanisms such as non-photochemical quenching. In addition to that, the CR has great ecological significance. Illustrious examples of this are extremely stress-tolerant 'snow' algae and conifer species developing red coloration during winter. The CR has also considerable practical implications since the secondary carotenoids exert a plethora of beneficial effects on human and animal health. The carotenogenic microalgae are the richest biotechnological sources of natural value-added carotenoids such as astaxanthin and β-carotene. In the present review, we summarize current functional, mechanistic, and ecological insights into the CR in a broad range of organisms suggesting that it is obviously more widespread and important stress response than it is currently thought to be.

Bioenergy application of Dunaliella salina SA 134 grown at various salinity levels for lipid production

The biofuels are receiving considerable attention as a substitute for petro diesel. For microalgae, the cell density or biomass and lipid contents are key components for biodiesel production. This study was conducted to develop favorable culture conditions for Dunaliella salina to maximize its biomass and lipid accumulation. The effect of salinity (0.5 to 2.5 M NaCl) on the cell population, biochemical composition, and lipid output of Dunaliella salina was examined under a controlled environment for 21 days. Maximum growth (6.57 × 107 to 7.17 × 107cells mL-1) potentials were observed at 1.5 to 2 M NaCl. The photosynthetic pigments and carbohydrates also showed trends similar to growth. The maximum carotenoid level (5.16 mg L-1) was recorded at 2 M NaCl. Almost all physicochemical parameters increased with increases in salinity, biomass (1231.66 ± 1.26 mg L-1) and lipid content (248.33 mg L-1), as recorded at 2 M NaCl. Based on fluorescence intensity, the highest values (11.84 × 107cells/ml) of neutral lipids and total lipids (22.28%) were recorded at optimum salinity levels. The present study suggests that a high biomass and lipid accumulation of Dunaliella salina SA 134 could be obtained at the 2 M NaCl level.

Nanosalina: A Tale of Saline-Loving Algae from the Lake's Agony to Cancer Therapy

The nanoparticles (NPs) that contain the therapeutic agent within themselves without further modifications can be coined as "self-therapeutic" NPs. The development of these agents especially when derived from natural resources can lead to a paradigm shift in the field of cancer nanotechnology as they can immensely facilitate the complex chemistry procedures and the follow up biological complications. Herein, we demonstrate that inherently therapeutic NPs "integrating" β-carotene can be synthesized from Dunaliella salina microalgae in a single step without complicated chemistry. The facile synthesis involved microwave irradiation of aqueous suspension of algae which resulted in water dispersible NPs with hydrodynamic diameter of ∼80 nm. Subsequently, extensive physiochemical characterizations were performed to confirm the integrity of the particles. The pro-oxidant activities of the integrated β-carotene were triggered by photoexcitation under UV lamp (362 nm). It was demonstrated that after UV exposure, the C32 human melanoma cells incubated with NPs experienced extensive cell death as opposed to nonilluminated samples. Further cellular analysis revealed that the significant reactive oxygen species (ROS) and in particular singlet oxygen were responsible for the cells' damage while the mode of cell death was dominated by apoptosis. Moreover, detailed endocytic inhibition studies specified that UV exposure affected NPs' cellular uptake mechanism. These inherently therapeutic NPs can open new avenues for melanoma cancer treatment via ROS generation in vitro.

Analysis of the Physiological and Molecular Responses of Dunaliella salina to Macronutrient Deprivation

The halotolerant chlorophyte Dunaliella salina can accumulate up to 10% of its dry weight as β-carotene in chloroplasts when subjected to adverse conditions, including nutrient deprivation. However, the mechanisms of carotenoid biosynthesis are poorly understood. Here, the physiological and molecular responses to the deprivation of nitrogen (-N), sulfur (-S), phosphorus (-P) and different combinations of those nutrients (-N-P, -N-S, -P-S and -N-P-S) were compared to gain insights into the underlying regulatory mechanisms of carotenoid biosynthesis. The results showed that both the growth and photosynthetic rates of cells were decreased during nutrient deprivation, accompanied by lipid globule accumulation and reduced chlorophyll levels. The SOD and CAT activities of the cells were altered during nutrient deprivation, but their responses were different. The total carotenoid contents of cells subjected to multiple nutrient deprivation were higher than those of cells subjected to single nutrient deprivation and non-stressed cells. The β-carotene contents of cells subjected to -N-P, -N-S and -N-P-S were higher than those of cells subjected to single nutrient deprivation. Cells subjected to sulfur deprivation accumulated more lutein than cells subjected to nitrogen and phosphorous deprivation. In contrast, no cumulative effects of nutrient deprivation on the transcription of genes in the carotenogenic pathway were observed because MEP and carotenogenic pathway genes were up-regulated during single nutrient deprivation but were downregulated during multiple nutrient deprivation. Therefore, we proposed that the carotenoid biosynthesis pathway of D. salina is regulated at both the transcriptional and posttranscriptional levels and that a complex crosstalk occurs at the physiological and molecular levels in response to the deprivation of different nutrients.

The Effects of Light, Temperature, and Nutrition on Growth and Pigment Accumulation of Three Dunaliella salina Strains Isolated from Saline Soil

Background:

Developing algal industries in saline-alkali areas is necessary. However, suitable strains and optimal production conditions must be studied before widespread commercial use.

Objectives:

The effects of light, temperature, KNO₃, and CO(NH₂)₂ on beta-carotene and biomass accumulation were compared and evaluated in order to provide scientific guidance for commercial algal production in northeastern Thailand.

Materials and Methods:

An orthogonal design was used for evaluating optimal conditions for the algal production of three candidate Dunaliella salina strains (KU XI, KU 10 and KU 31) which were isolated from saline soils and cultured in the column photobioreactor.

Results:

The optimal light and temperature for algae growth were 135.3 μmol m^-2 s^-1 and 22°C, while the conditions of 245.6 μmol m^-2 s^-1 and 22°C induced the highest level of beta-carotene production (117.99 mg L^-1). The optimal concentrations of KNO₃, CO(NH₂)₂, and NaHCO₃ for algae growth were 0.5 g L^-1, 0.36 g L^-1, and 1.5 g L^-1, respectively, while 0, 0.12 g L^-1 and 1.5 g L^-1 were best suited for beta-carotene accumulation. The highest beta-carotene rate per cell appeared with the highest light intensity (12.21 pg) and lowest temperature (12.47 pg), and the lowest total beta-carotene content appeared at the lowest temperature (15°C). There was not a significant difference in biomass accumulation among the three Dunaliella strains; however, the beta-carotene accumulation of KU XI was higher than that of the other two strains.

Conclusions:

Light and temperature were both relevant factors that contributed to the growth and beta-carotene accumulation of the three D. salina strains, and NaHCO₃ had significantly positive effects on growth. The degree of impact of the different factors on cell growth was temperature > NaHCO₃ > light intensity > KNO₃ > CO (NH₂)₂ > strains; the impact on beta-carotene accumulation was temperature > light intensity > KNO₃ > CO (NH₂)₂ > strains > NaHCO₃

Carotenoids in Microalgae

Carotenoids are a class of isoprenoids synthesized by all photosynthetic organisms as well as by some non-photosynthetic bacteria and fungi with broad applications in food, feed and cosmetics, and also in the nutraceutical and pharmaceutical industries. Microalgae represent an important source of high-value products, which include carotenoids, among others. Carotenoids play key roles in light harvesting and energy transfer during photosynthesis and in the protection of the photosynthetic apparatus against photooxidative damage. Carotenoids are generally divided into carotenes and xanthophyls, but accumulation in microalgae can also be classified as primary (essential for survival) and secondary (by exposure to specific stimuli).In this chapter, we outline the high value carotenoids produced by commercially important microalgae, their production pathways, the improved production rates that can be achieved by genetic engineering as well as their biotechnological applications.

In Metabolic Engineering of Eukaryotic Microalgae: Potential and Challenges Come with Great Diversity

The great phylogenetic diversity of microalgae is corresponded by a wide arrange of interesting and useful metabolites. Nonetheless metabolic engineering in microalgae has been limited, since specific transformation tools must be developed for each species for either the nuclear or chloroplast genomes. Microalgae as production platforms for metabolites offer several advantages over plants and other microorganisms, like the ability of GMO containment and reduced costs in culture media, respectively. Currently, microalgae have proved particularly well suited for the commercial production of omega-3 fatty acids and carotenoids. Therefore most metabolic engineering strategies have been developed for these metabolites. Microalgal biofuels have also drawn great attention recently, resulting in efforts for improving the production of hydrogen and photosynthates, particularly triacylglycerides. Metabolic pathways of microalgae have also been manipulated in order to improve photosynthetic growth under specific conditions and for achieving trophic conversion. Although these pathways are not strictly related to secondary metabolites, the synthetic biology approaches could potentially be translated to this field and will also be discussed.

Characterization and Functional Identification of a Gene Encoding Geranylgeranyl Diphosphate Synthase from Dunaliella bardawil

Geranylgeranyl diphosphate synthase (GGPS) catalyzes the biosynthesis of geranylgeranyl diphosphate, a key precursor for carotenoid biosynthesis. In this study, a full-length cDNA encoding GGPS from Dunaliella bardawil (DbGGPS) was isolated by rapid amplification of cDNA ends (RACE) for the first time. The full-length cDNA of DbGGPS was 1814 bp, containing a 1074 bp ORF encoding 357 amino acids with a calculated mass of 38.88 kDa. Analysis of DbGGPS genomic DNA revealed that it contained 10 exons and 9 introns. It was predicted that DbGGPS possessed a 48 amino acid transit peptide at its N terminus. Bioinformatic analysis revealed that DbGGPS was a member of a group of polyprenyltransferases with five conserved domains and two highly conserved aspartate-rich motifs. Using heterologous expression, carotenoid complementation assay, and gene deletion analysis, it was shown that the coding region of DbGGPS encodes a functional GGPS. This provides new gene sources for carotenoid genetic engineering.

Production of carotenoids by microalgae: achievements and challenges

Carotenoids are a wide group of lipophylic isoprenoids synthesized by all photosynthetic organisms and also by some non-photosynthetic bacteria and fungi. Animals, which cannot synthesize carotenoids de novo, must include them in their diet to fulfil essential provitamin, antioxidant, or colouring requirements. Carotenoids are indispensable in light harvesting and energy transfer during photosynthesis and in the protection of the photosynthetic apparatus against photooxidative damage. In this review, we outline the factors inducing carotenoid accumulation in microalgae, the knowledge acquired on the metabolic pathways responsible for their biosynthesis, and the recent achievements in the genetic engineering of this pathway. Despite the considerable progress achieved in understanding and engineering algal carotenogenesis, many aspects remain to be elucidated. The increasing number of sequenced microalgal genomes and the data generated by high-throughput technologies will enable a better understanding of carotenoid biosynthesis in microalgae. Moreover, the growing number of industrial microalgal species genetically modified will allow the production of novel strains with enhanced carotenoid contents.

Interaction of Temperature and Photoperiod Increases Growth and Oil Content in the Marine Microalgae Dunaliella viridis

Eukaryotic marine microalgae like Dunaliella spp. have great potential as a feedstock for liquid transportation fuels because they grow fast and can accumulate high levels of triacylgycerides with little need for fresh water or land. Their growth rates vary between species and are dependent on environmental conditions. The cell cycle, starch and triacylglycerol accumulation are controlled by the diurnal light:dark cycle. Storage compounds like starch and triacylglycerol accumulate in the light when CO₂ fixation rates exceed the need of assimilated carbon and energy for cell maintenance and division during the dark phase. To delineate environmental effects, we analyzed cell division rates, metabolism and transcriptional regulation in Dunaliella viridis in response to changes in light duration and growth temperatures. Its rate of cell division was increased under continuous light conditions, while a shift in temperature from 25°C to 35°C did not significantly affect the cell division rate, but increased the triacylglycerol content per cell several-fold under continuous light. The amount of saturated fatty acids in triacylglycerol fraction was more responsive to an increase in temperature than to a change in the light regime. Detailed fatty acid profiles showed that Dunaliella viridis incorporated lauric acid (C12:0) into triacylglycerol after 24 hours under continuous light. Transcriptome analysis identified potential regulators involved in the light and temperature-induced lipid accumulation in Dunaliella viridis.

Exploring the potential of using algae in cosmetics

The applications of microalgae in cosmetic products have recently received more attention in the treatment of skin problems, such as aging, tanning and pigment disorders. There are also potential uses in the areas of anti-aging, skin-whitening, and pigmentation reduction products. While algae species have already been used in some cosmetic formulations, such as moisturizing and thickening agents, algae remain largely untapped as an asset in this industry due to an apparent lack of utility as a primary active ingredient. This review article focuses on integrating studies on algae pertinent to skin health and beauty, with the purpose of identifying serviceable algae functions in practical cosmetic uses.

Marine Algae: a Source of Biomass for Biotechnological Applications

Biomass derived from marine microalgae and macroalgae is globally recognized as a source of valuable chemical constituents with applications in the agri-horticultural sector (including animal feeds and health and plant stimulants), as human food and food ingredients as well as in the nutraceutical, cosmeceutical, and pharmaceutical industries. Algal biomass supply of sufficient quality and quantity however remains a concern with increasing environmental pressures conflicting with the growing demand. Recent attempts in supplying consistent, safe and environmentally acceptable biomass through cultivation of (macro- and micro-) algal biomass have concentrated on characterizing natural variability in bioactives, and optimizing cultivated materials through strain selection and hybridization, as well as breeding and, more recently, genetic improvements of biomass. Biotechnological tools including metabolomics, transcriptomics, and genomics have recently been extended to algae but, in comparison to microbial or plant biomass, still remain underdeveloped. Current progress in algal biotechnology is driven by an increased demand for new sources of biomass due to several global challenges, new discoveries and technologies available as well as an increased global awareness of the many applications of algae. Algal diversity and complexity provides significant potential provided that shortages in suitable and safe biomass can be met, and consumer demands are matched by commercial investment in product development.

Highly valuable microalgae: biochemical and topological aspects

The past decade has seen a surge in the interest in microalgae culture for biodiesel production and other applications as renewable biofuels as an alternative to petroleum transport fuels. The development of new technologies for the culture of these photosynthetic microorganisms and improved knowledge of their biochemical composition has spurred innovation in the field of high-value biomolecules. These developments are only economically viable if all the microalgae fractions are valorized in a biorefinery strategy. Achieving this objective requires an understanding of microalgae content and the cellular localization of the main biomolecular families in order to develop efficient harvest and sequential recovery technologies. This review summarizes the state of the art in microalgae compositions and topologies using some examples of the main industrially farmed microalgae.

Carotenoid metabolism in mammals, including man: formation, occurrence, and function of apocarotenoids

Vitamin A was recognized as an essential nutrient 100 years ago. In the 1930s, it became clear that dietary β-carotene was cleaved at its central double to yield vitamin A (retinal or β-apo-15'-carotenal). Thus a great deal of research has focused on the central cleavage of provitamin A carotenoids to form vitamin A (retinoids). The mechanisms of formation and the physiological role(s) of noncentral (eccentric) cleavage of both provitamin A carotenoids and nonprovitamin A carotenoids has been less clear. It is becoming apparent that the apocarotenoids exert unique biological activities themselves. These compounds are found in the diet and thus may be absorbed in the intestine, or they may form from enzymatic or nonenzymatic cleavage of the parent carotenoids. The mechanism of action of apocarotenoids in mammals is not fully worked out. However, as detailed in this review, they have profound effects on gene expression and work, at least in part, through the modulation of ligand-activated nuclear receptors. Understanding the interactions of apocarotenoids with other lipid-binding proteins, chaperones, and metabolizing enzymes will undoubtedly increase our understanding of the biological roles of these carotenoid metabolites.

Molecular clone and expression of a NAD+-dependent glycerol-3-phosphate dehydrogenase isozyme gene from the halotolerant alga Dunaliella salina

Glycerol is an important osmotically compatible solute in Dunaliella. Glycerol-3-phosphate dehydrogenase (G3PDH) is a key enzyme in the pathway of glycerol synthesis, which converts dihydroxyacetone phosphate (DHAP) to glycerol-3-phosphate. Generally, the glycerol-DHAP cycle pathway, which is driven by G3PDH, is considered as the rate-limiting enzyme to regulate the glycerol level under osmotic shocks. Considering the peculiarity in osmoregulation, the cDNA of a NAD⁺-dependent G3PDH was isolated from D. salina using RACE and RT-PCR approaches in this study. Results indicated that the length of the cDNA sequence of G3PDH was 2,100 bp encoding a 699 amino acid deduced polypeptide whose computational molecular weight was 76.6 kDa. Conserved domain analysis revealed that the G3PDH protein has two independent functional domains, SerB and G3PDH domains. It was predicted that the G3PDH was a nonsecretory protein and may be located in the chloroplast of D. salina. Phylogenetic analysis demonstrated that the D. salina G3PDH had a closer relationship with the G3PDHs from the Dunaliella genus than with those from other species. In addition, the cDNA was subsequently subcloned in the pET-32a(+) vector and was transformed into E. coli strain BL21 (DE3), a expression protein with 100 kDa was identified, which was consistent with the theoretical value.

Toxicity evaluation of two typical surfactants to Dunaliella bardawil, an environmentally tolerant alga

Sodium dodecyl benzene sulfonate (SDBS) and cetyl trimethyl ammonium chloride (CTAC) are two kinds of surfactants widely applied in various industries. The tremendous direct discharge of these surfactants into natural waters has posed a significant threat to ecosystems. Dunaliella bardawil was employed in the present research to test the toxic effects of SDBS, CTAC, and their mixture on cell growth, cellular morphology, β-carotene accumulation, and enzymatic activities of superoxide dismutase (SOD) and catalase (CAT). The results showed that SDBS at 200, 550, 900, 1,350, 1,800, and 2,400 mg/L and CTAC at 0.4, 0.7, 1.0, 1.3, 2.8, and 3.5 mg/L inhibited algal growth and β-carotene accumulation, both of which declined and then increased. In particular, CTAC (median inhibitory concentration at 10 days [IC50](10 d)  = 2.8 ± 1.49 mg/L) was more hazardous than SDBS (IC50(10 d)  = 2,044 ± 637.3 mg/L). The additive index (AI) calculated from carotene content data was (-4.10, -1.67) < 0, indicating an antagonistic effect between SDBS and CTAC. Algae cultivated at level 6 of the binary system showed hormesis due to the mitigated toxicity; SDBS at 2,400 mg/L, CTAC at 3.5 mg/L, and combined surfactants at level 6 exerted lethal effects on D. bardawil. Both SOD and CAT activities showed similar associations with varied concentrations of surfactants: SOD was significantly promoted by 550 to 1,800 mg/L SDBS, 0.7 to 1.3 mg/L CTAC, and mixtures at levels 2 to 4; CAT was clearly promoted by 900 mg/L SDBS, 0.4 to 1.3 mg/L CTAC, and mixtures at levels 2 to 4.

Enhancement of carotenoid biosynthesis in the green microalga Dunaliella salina with light-emitting diodes and adaptive laboratory evolution

There is a particularly high interest to derive carotenoids such as β-carotene and lutein from higher plants and algae for the global market. It is well known that β-carotene can be overproduced in the green microalga Dunaliella salina in response to stressful light conditions. However, little is known about the effects of light quality on carotenoid metabolism, e.g., narrow spectrum red light. In this study, we present UPLC-UV-MS data from D. salina consistent with the pathway proposed for carotenoid metabolism in the green microalga Chlamydomonas reinhardtii. We have studied the effect of red light-emitting diode (LED) lighting on growth rate and biomass yield and identified the optimal photon flux for D. salina growth. We found that the major carotenoids changed in parallel to the chlorophyll b content and that red light photon stress alone at high level was not capable of upregulating carotenoid accumulation presumably due to serious photodamage. We have found that combining red LED (75 %) with blue LED (25 %) allowed growth at a higher total photon flux. Additional blue light instead of red light led to increased β-carotene and lutein accumulation, and the application of long-term iterative stress (adaptive laboratory evolution) yielded strains of D. salina with increased accumulation of carotenoids under combined blue and red light.

Analysis of Anabaena vaginicola and Nostoc calcicola from Northern Iran, as rich sources of major carotenoids

Four major carotenoids of high nutritional significance, including β-carotene, lycopene, lutein and zeaxanthin were determined in three isolates of heterocystous cyanobacteria, belonging to the genera Anabaena and Nostoc, isolated from Iranian terrestrial and aquatic ecosystems, for the first time. The ultrasonically extracted carotenoids were identified and quantified by a rapid and sensitive isocratic HPLC method and identification was further confirmed by spiking authentic standards and the pattern of the UV-Vis spectra obtained from photo-diode array detector. The results showed that these isolates contain large amounts of four major carotenoids, especially lycopene (up to 24,570 μg/g dry weight, DW) which appears to be the highest reported amount until present; and β-carotene (up to 8133 μg/g DW) which is comparable with the best natural sources of β-carotene. Meanwhile, they are rich in the cis-isomers of lycopene and β-carotene which is important in their bioavailability and health benefits.

Pathways of carotenoid biosynthesis in bacteria and microalgae

The carotenoids, a subfamily of the isoprenoids, are among the most widespread, ancient, diverse, and rich class of all natural products and biomolecules. Microorganisms, as well as microalgae and bacteria synthesize isoprenoids from isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). For long time, mevalonic acid was assumed to be the only natural precursor for IPP and DMAPP in the cytosolic acetate/mevalonate pathway for the biosynthesis of sterols, sesquiterpenes, triterpenoids, and carotenoids. At present, it is accepted that the relatively new route, the methylerythritol 4-phosphate (MEP), or 1-deoxy-D: -xylulose-5-phosphate (DOXP) is the main pathway for the biosynthesis of plastidic isoprenoids, such as carotenoids, phytol (a side chain of chlorophylls), plastoquinone-9, isoprene, mono-, and diterpenes. Cytosolic isoprenoids (sterols) biosynthesized by MEP have been reported in eubacteria and algae (Chlorella, Chlamydomonas, Scenedesmus, and Dunaliella). This review summarizes current knowledge of the biosynthetic pathways leading to the formation of different isoprenoids and carotenoids in bacteria and microalgae. Particular attention was given to the last early steps of the biosynthesis of the key C(5)-precursor and the final steps of the biosynthesis of carotenoids including selected examples in microalgae and bacteria as well as the recent advances in genomics and metabolic engineering.

Microbial carotenoids

Carotenoids are among the most widely distributed pigments in nature, and they are exclusively synthesized by plants and microorganisms. These compounds may serve a protective role against many chronic diseases such as cancers, age-related macular degeneration, and cardiovascular diseases and also act as an excellent antioxidant system within cells. Recent advances in the microbial genome sequences and increased understanding about the genes involved in the carotenoid biosynthetic pathways will assist industrial microbiologists in their exploration of novel microbial carotenoid production strategies. Here we present an overview of microbial carotenogenesis from biochemical, proteomic, and biotechnological points of view.

ANALYSIS OF EXPRESSED SEQUENCE TAGS FROM THE GREEN ALGA DUNALIELLA SALINA (CHLOROPHYTA)(1)

The unicellular green alga Dunaliella salina (Dunal) Teodor. is a novel model photosynthetic eukaryote for studying photosystems, high salinity acclimation, and carotenoid accumulation. In spite of such significance, there have been limited studies on the Dunaliella genome transcriptome and proteome. To further investigate D. salina, a cDNA library was constructed and sequenced. Here, we present the analysis of the 2,282 expressed sequence tags (ESTs) generated together with 3,990 ESTs from dbEST. A total of 4,148 unique sequences (UniSeqs) were identified, of which 56.1% had sequence similarity with Uniprot entries, suggesting that a large number of unique genes may be harbored by Dunaliella. Additionally, protein family domains were identified to further characterize these sequences. Then, we also compared EST sequences with different complete eukaryotic genomes from several animals, plants, and fungi. We observed notable differences between D. salina and other organisms. This EST collection and its annotation provided a significant resource for basic and applied research on D. salina and laid the foundation for a systematic analysis of the transcriptome basis of green algae development and diversification.

Quantitative tracking of the growth of individual algal cells in microdroplet compartments

In this paper we introduce a simple droplet-based microfluidic system consisting of two separate devices to encapsulate and culture microalgae, in contrast to cultivation in bulk liquid medium. This microdroplet technology has been used to monitor the growth of individual microalgal cells in a constant environment for extended periods of time. Single cells from three species of green microalgae, (two freshwater species Chlamydomonas reinhardtii and Chlorella vulgaris, and one saline species Dunaliella tertiolecta), were encapsulated and incubated in microdroplet compartments of diameter of ∼80 μm, and their growth analysed over 10 days. In all cases, the doubling time of microalgae grown in microdroplets was similar to growth in bulk. The growth of C. reinhardtii in microdroplets of varying diameters and with different initial cell numbers per droplet was investigated, as well as the effect of varying medium conditions such as pH and nitrogen concentration. This methodology offers the opportunity to study characteristics over time of individual cells and colonies, as well as to screen large numbers of them.

Toxic effects of chemical pesticides (trichlorfon and dimehypo) on Dunaliella salina

Dunaliella salina, a unicellular green alga of environmental tolerance, was employed as test organism to investigate the toxicity effects of trichlorfon and dimehypo widely used in agriculture and veterinary as pesticides. The influences of trichlorfon and dimehypo on cell growth, β-carotene level, cell morphology changes, and activities of superoxide dismutase (Sod) and catalase (Cat) were investigated. At the concentrations less than 0.050 g L(-1) trichlorfon or 0.0005 g L(-1) dimehypo, cell responses were similar to control. When treated with 0.075-0.100 g L(-1) trichlorfon or 0.001-0.004 g L(-1) dimehypo, cell growth and β-carotene levels declined at first and then revived. When concentrations were higher than 0.125 g L(-1) trichlorfon or 0.005 g L(-1) dimehypo, both cell growth and β-carotene levels decreased until they were undetectable. The 10-d IC50 of trichlorfon and dimehypo on D. salina were 0.179 g L(-1) and 0.032 g L(-1). Both pollutants could stimulate the increase of Cat activity at a low concentration. Tolerance of D. salina to trichlorfon was obviously higher than that of dimehypo.

In silico analysis of phytoene synthase and its promoter reveals hints for regulation mechanisms of carotenogenesis in Duanliella bardawil

MOTIVATION

Previous researches showed that phytoene synthase (Psy) from Dunaliella bardawil is the first regulatory point in carotenogenesis. We hypothesize certain interactions between the environmental stress factors and the regulatory sequences of Psy in D.bardawil (DbPsy). Consequently, LA PCR-based genomic walking approach was performed for isolation of psy promoter and terminator, respectively. The obtained nucleic acid sequences and the corresponding protein structure of DbPsy were analyzed and predicted using various bioinformatics tools. Finally, we presented some hints for the regulation mechanisms of DbPsy at the molecular level according to the computed results.

RESULTS

LA PCR-based genomic walking results showed that the isolated sequences are the promoter and terminator of psy, correspondingly. Computational analysis demonstrated several candidate motifs of the promoter exhibiting hypothetic UV-B-, norglurzon- and salt-induced characteristics, as well as some typical domains universally discovered in promoter sequences, such as TATA-box, CCAAT-box and GATA-box, etc. Furthermore, the structure of Psy was also predicted and aligned along with many counterparts at the protein level. Low homology of N-terminus was found in D.bardawil, while a relatively conserved C-terminus was predicted to be involved in the catalytic activity and substrate recognization/binding. Phylogenic analysis classified the DbPsy into a cluster with other algae. These results implied that Psy may share similar regulation mechanisms among algae with respect to their C-termini; while the diversity in N-terminus among Psys, along with the predicted inducible motifs in psy promoter from D.bardawil, may confer the fine tuning differences between D.bardawil and other algae.

CONCLUSION

By means of computer techniques, we found in D.barawali that two interesting conserved motifs of psy promoter may involve in UV-B, norglurzon and salt regulation correspondingly; and that the diversity of Psy protein mainly lies in the N-termini among algae. These results indicate some hints for regulation mechanisms of carotenogenesis in D.bradawil.

CONTACT

jgjiang@scut.edu.cn.

Microalgae as sources of carotenoids

Marine microalgae constitute a natural source of a variety of drugs for pharmaceutical, food and cosmetic applications-which encompass carotenoids, among others. A growing body of experimental evidence has confirmed that these compounds can play important roles in prevention (and even treatment) of human diseases and health conditions, e.g., cancer, cardiovascular problems, atherosclerosis, rheumatoid arthritis, muscular dystrophy, cataracts and some neurological disorders. The underlying features that may account for such favorable biological activities are their intrinsic antioxidant, anti-inflammatory and antitumoral features. In this invited review, the most important issues regarding synthesis of carotenoids by microalgae are described and discussed-from both physiological and processing points of view. Current gaps of knowledge, as well as technological opportunities in the near future relating to this growing field of interest, are also put forward in a critical manner.

Effects of salinities on the gene expression of a (NAD+)-dependent glycerol-3-phosphate dehydrogenase in Dunaliella salina

Glycerol-3-phosphate dehydrogenase (G3pdh) is a key enzyme in the pathway of glycerol synthesis, which converts dihydroxyacetone phosphate (DHAP) to glycerol-3-phosphate. In this study, the effects of salinity changes on variation of cell shape and single cell glycerol content of Dunaliella salina were observed, and the effects of salinity changes on the gene expressions of a (NAD+)-dependent G3pdh (EC1.1.1.8) among G3pdh isozymes in D. salina were detected by real-time quantitative PCR. Results showed that the changes of shape and volume of D. salina cell cultured chronically at various salinities were minor, but when the salinity was changed rapidly, the variations of cell shape and cell volume of D. salina were significant, which were recovered basically after 2h except treating by high salinity. Also, it was found some lipid globules in the surface of D. salina cells when the salinity increased from 2.0 to 4.0-5.0 M NaCl rapidly. When D. salina was cultured chronically at various salinities, the accumulation of single cell glycerol increased with increased salinity, and D. salina also could rapidly decrease or increase single cell glycerol contents to adapt to hypoosmotic or hyperosmotic shock. The expression level of G3pdh in D. salina grown at various salinities was significantly inversely correlated to the salinity, but there was no significant correlation between the expression level of G3pdh and salinity after 2 h of treatment by hyperosmotic or hypoosmotic shock.

Carotenoids and their cleavage products: biosynthesis and functions

This review focuses on plant carotenoids, but it also includes progress made on microbial and animal carotenoid metabolism to better understand the functions and the evolution of these structurally diverse compounds with a common backbone. Plants have evolved isogenes for specific key steps of carotenoid biosynthesis with differential expression profiles, whose characteristic features will be compared. Perhaps the most exciting progress has been made in studies of carotenoid cleavage products (apocarotenoids) with an ever-expanding variety of novel functions being discovered. This review therefore covers structural, molecular genetic and functional aspects of carotenoids and apocarotenoids alike. Apocarotenoids are specifically tailored from carotenoids by a family of oxidative cleavage enzymes, but whether there are contributions to their generation from chemical oxidation, photooxidation or other mechanisms is largely unknown. Control of carotenoid homeostasis is discussed in the context of biosynthetic and degradative reactions but also in the context of subcellular environments for deposition and sequestration within and outside of plastids. Other aspects of carotenoid research, including metabolic engineering and synthetic biology approaches, will only be covered briefly.

Analysis of an essential carotenogenic enzyme: ζ-carotene desaturase from unicellular Alga Dunaliella salina

The green alga Dunaliella has become a valuable model organism for understanding the interesting mechanism of massive carotenoid accumulation. Previously, DNA sequences of several carotenogenic enzymes were obtained from Dunaliella. In this study, the cDNA of zds was isolated from Dunaliella salina using a polymerase chain reaction approach. The full-length cDNA sequence was 2178 base pairs (bp) containing a 1731 bp putative open reading frame which coded a 576 amino acid deduced polypeptide whose molecular weight was 63.9 kDa computationally. A complete homologous search displayed that the nucleotide and putative protein sequence have sequence identities of 69% and 66% with those of green alga Chlamydomonas reinhardtii, respectively. It was predicted that this ζ-carotene desaturase (Zds) may be located in the chloroplast of D. salina. Phylogenetic analysis demonstrated that the D. salina Zds had a closer relationship with the Zds of algae and higher plants than with those of other species.