Regulation of Two Photosynthetic Pigment-related Genes During Stress-induced Pigment Formation in the Green Alga, Dunaliella salina

Ethanol as a carotenoid production stimulator in Dunaliella salina CCAP 19/18

Dunaliella salina is a rich source of carotenoids. Carotenoid production is induced under specific conditions, i.e., high light intensity, high salt concentration, nutrient limitation, and suboptimal temperatures in this microalga. The control of environmental factors is vital for high productivity of carotenoids. In this paper, the effect of different ethanol concentrations in combination with nitrogen deficiency was investigated to induce carotenoid production in D. salina CCAP 19/18. Also, some biochemical and molecular parameters were investigated in response to ethanol in the cells. It was shown that ethanol at 0.5% concentration increased cell number but, at 5% concentration, reduced cell viability compared to the control. The highest carotenoid production was achieved at 3% ethanol concentration, which was 1.46 fold higher than the nitrogen deficiency condition. Investigation of the 3 carotenoid biosynthesis genes revealed that their expression levels increased at 3% ethanol concentration, and the phytoene synthase gene was the most upregulated one. Lipid peroxidation increased at both 3% and 5% ethanol concentrations. At 3% concentration, the activity of catalase and superoxide dismutase increased, but no significant changes were seen at 5% ethanol concentration. Peroxidase activity reduced at both 3% and 5% concentrations. Moreover, proline and reducing sugar content increased at 3% concentration while decreased at 5% ethanol concertation. The results showed that at 3% ethanol concentration, higher carotenoid productivity was associated with an increase in other intracellular responses (molecular and biochemical). Ethanol as a controllable element may be beneficial to increase carotenoid production even under inappropriate environmental conditions in D. salina.

Disparity of the carotenoids antioxidant properties of wild-type and D-PSY-transgenic Dunaliella parva strains under three environmental stresses

Two strains of the halophilic alga Dunaliella parva, a wild type (WT) and a transgenic strain (D-PSY) containing an exogenous phytoene synthase gene (PSY), were used to investigate the growth, carotenoid accumulation, and carotenoid antioxidant properties under nitrogen starvation, cobalt and biochar treatments. D-PSY had higher carotenoid content (1.8 times) compared to the WT. The applied stressors stimulated the carotenoid content of both WT and D-PSY especially. The carotenoids were assayed for the potential antioxidant activities by five different assays. Generally, the antioxidant activities of D-PSY carotenoids were superior to that of WT. The biochar and nitrogen treatments generally enhanced the antioxidant activities of the carotenoids, whereas cobalt came third in this respect. The D-PSY transgenic algal strain has both high carotenoids content and antioxidant properties which enhanced under the relatively lower concentrations of the applied stressors. The results have shown to lead to an accurate application of the transgenic alga as a source of potent antioxidant compounds.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01077-0.

Effects of different culture media on physiological features and laboratory scale production cost of Dunaliella salina

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Comparison among well-known marine microalgae culture media was performed.

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Conway medium increased cellular growth of Dunaliella salina.

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Johnson medium improved accumulation of photosynthetic pigments.

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Microalgal biomass showed a cost variation of US$4.64–301.61 per kg in lab scale.

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Monitoring of pigment profile proved to be essential in microalgal parameters.

Microalgal cultivation system depends on optimal parameters obtained from laboratory conditions to overcome high-value compounds production and decrease cost. In this study, the laboratory-scale cultivation of Dunaliella salina was performed to explore the cost and biochemical effects on biomass cultivated using F/2, Conway, and Johnson media. D. salina cultivation was monitored by cell counting, dry biomass measurements and biochemical analyses. Photosynthetic pigment profiles were identified and quantified through high-performance liquid chromatography (HPLC) whereas economic evaluation was based on media cost. Cellular growth parameters were significantly improved by Conway medium, whereas maximum accumulation of pigments, especially violaxanthin (0.855 mg g⁻¹) and chlorophyll a (14.255 mg g⁻¹) were observed when Johnson medium was used. Conway-biomass showed 43 % of total lipid content. This value represents 1.5 and 1.4 times higher than lipid content found in Johnson- and F/2-biomass, respectively. Furthermore, media cost presented a variation of US$1.17–49.62 for each 10³ L of media mainly due to NaNO₃, KNO_3, and cyanocobalamin supplementation. Thus, biomass production showed a cost variation of US$4.64–301.61 per kg on a dry weight basis. These outcomes emphasize the suitability of laboratory parameters and cost comparison among biomass produced by different media.

Carotenoids Overproduction in Dunaliella Sp.: Transcriptional Changes and New Insights through Lycopene β Cyclase Regulation

Dunaliella is a green microalga known for its ability to produce high levels of carotenoids under well-defined growing conditions. Molecular responses to the simultaneous effect of increasing salinity, light intensity and decrease of nitrogen availability were investigated in terms of their effect on different metabolic pathways (isoprenoids synthesis, glycolysis, carbohydrate use, etc.) by following the transcriptional regulation of enolase (ENO), 1-deoxy-D-xylulose 5-phosphate synthase (DXS), lycopene β-cyclase (LCYB), carotene globule protein (CGP), chloroplast-localized heat shock protein (HSP70), and chloroplast ribulose phosphate-3-epimerase (RPE) genes. The intracellular production of carotenoid was increased five times in stressed Dunaliella cells compared to those grown in an unstressed condition. At transcriptional levels, ENO implicated in glycolysis, and revealing about polysaccharides degradation, showed a two-stage response during the first 72 h. Genes directly involved in β-carotene accumulation, namely, CGP and LCYB, revealed the most important increase by about 54 and 10 folds, respectively. In silico sequence analysis, along with 3D modeling studies, were performed to identify possible posttranslational modifications of CGP and LCYB proteins. Our results described, for the first time, their probable regulation by sumoylation covalent attachment as well as the presence of expressed SUMO (small ubiquitin-related modifier) protein in Dunaliella sp.

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.

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.

The induction of menadione stress tolerance in the marine microalga, Dunaliella viridis, through cold pretreatment and modulation of the ascorbate and glutathione pools

The effect of cold pretreatment on menadione tolerance was investigated in the cells of the marine microalga, Dunaliella viridis. In addition, the involvement of ascorbate and glutathione in the response to menadione stress was tested by treating cell suspensions with l-galactono-1,4-lactone, an ascorbate precursor, and buthionine sulfoximine, an inhibitor of glutathione synthesis. Menadione was highly toxic to non cold-pretreated cells, and caused a large decrease in cell number. Cold pretreatment alleviated menadione toxicity and cold pretreated cells accumulated lower levels of reactive oxygen species, and had enhanced antioxidant capacity due to increased levels of β-carotene, reduced ascorbate and total glutathione compared to non cold-pretreated cells. Cold pretreatment also altered the response to l-galactono-1,4-lactone and buthionine sulfoximine treatments. Combined l-galactono-1,4-lactone and menadione treatment was lethal in non-cold pretreated cells, but in cold-pretreated cells it had a positive effect on cell numbers compared to menadione alone. Overall, exposure of Dunaliella cells to cold stress enhanced tolerance to subsequent oxidative stress induced by menadione.

Transcriptome analysis in tardigrade species reveals specific molecular pathways for stress adaptations

Tardigrades have unique stress-adaptations that allow them to survive extremes of cold, heat, radiation and vacuum. To study this, encoded protein clusters and pathways from an ongoing transcriptome study on the tardigrade Milnesium tardigradum were analyzed using bioinformatics tools and compared to expressed sequence tags (ESTs) from Hypsibius dujardini, revealing major pathways involved in resistance against extreme environmental conditions. ESTs are available on the Tardigrade Workbench along with software and databank updates. Our analysis reveals that RNA stability motifs for M. tardigradum are different from typical motifs known from higher animals. M. tardigradum and H. dujardini protein clusters and conserved domains imply metabolic storage pathways for glycogen, glycolipids and specific secondary metabolism as well as stress response pathways (including heat shock proteins, bmh2, and specific repair pathways). Redox-, DNA-, stress- and protein protection pathways complement specific repair capabilities to achieve the strong robustness of M. tardigradum. These pathways are partly conserved in other animals and their manipulation could boost stress adaptation even in human cells. However, the unique combination of resistance and repair pathways make tardigrades and M. tardigradum in particular so highly stress resistant.

Carotenoid and fatty acid metabolism in light-stressed Dunaliella salina

beta-Carotene is overproduced in the alga Dunaliella salina in response to high light intensities. We have studied the effects of a sudden light increase on carotenoid and fatty acid metabolism using a flat panel photobioreactor that was run in turbidostat mode to ensure a constant light regime throughout the experiments. Upon the shift to an increased light intensity, beta-carotene production commenced immediately. The first 4 h after induction were marked by constant intracellular levels of beta-carotene (2.2 g LCV(-1)), which resulted from identical increases in the production rates of cell volume and beta-carotene. Following this initial phase, beta-carotene productivity continued to increase while the cell volume productivity dropped. As a result, the intracellular beta-carotene concentration increased reaching a maximum of 17 g LCV(-1) after 2 days of light stress. Approximately 1 day before that, the maximum beta-carotene productivity of 30 pg cell(-1) day(-1) (equivalent to 37 mg LRV(-1) day(-1)) was obtained, which was about one order of magnitude larger than the average productivity reported for a commercial beta-carotene production facility, indicating a vast potential for improvement. Furthermore, by studying the light-induced changes in both beta-carotene and fatty acid metabolism, it appeared that carotenoid overproduction was associated with oil globule formation and a decrease in the degree of fatty acid unsaturation. Our results indicate that cellular beta-carotene accumulation in D. salina correlates with accumulation of specific fatty acid species (C16:0 and C18:1) rather than with total fatty acid content.

Molecular and functional characterization of a cDNA encoding 4-hydroxy-3-methylbut-2-enyl diphosphate reductase from Dunaliella salina

In green algae, the final step of the plastidial methylerythritol phosphate (MEP) pathway is catalyzed by 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (HDR; EC: 1.17.1.2), an enzyme proposed to play a key role in the regulation of isoprenoid biosynthesis. Here we report the isolation and functional characterization of a 1959-bp Dunaliella salina HDR (DsHDR) cDNA encoding a deduced polypeptide of 474 amino acid residues. Phylogenetic analysis implied a cyanobacterial origin for plant and algal HDR genes. Steady-state DsHDR transcript levels were higher in D. salina cells submitted to nutritional depletion, high salt and/or high light, suggesting that DsHDR may respond to the same environmental cues as genes involved in carotenoid biosynthesis.

Biomarkers in aquatic plants: selection and utility

This review emphasizes the predictive ability, sensitivity and specificity of aquatic plant biomarkers as biomonitoring agents of exposure and effect. Biomarkers of exposure are those that provide functional measures of exposure that are characterized at a sub-organism level. Biomarkers of effect require causal linkages between the biomarker and effects, measured at higher levels of biological organization. With the exception of pathway specific metabolites, the biomarkers assessed in this review show variable sensitivity and predictive ability that is often confounded by variations in growth conditions, rendering them unsuitable as stand alone indicators of environmental stress. The use of gene expression for detecting pollution has been, and remains immature; this immaturity derives from inadequate knowledge on predictive ability, sensitivity and specificity. Moreover, the ability to the detect mode of action of unknown toxicants using gene expression is not as clear-cut as initially hypothesized. The principal patterns in gene expression is not as clear-cut as initially hypothesized. The principal patterns in gene expression are generally derived from stress induced genes, rather than on ones that respond to substances with known modes of action (Baerson et al. 2005). Future developments in multivariate statistics and chemometric methods that enhance pattern analyses in ways that could produce a "fingerprint", may improve methods for discovering modes of action of unknown toxicants. Pathway specific metabolites are unambiguous, sensitive, correlate well to growth effects, and are relatively unaffected by growth conditions. These traits make them excellent biomarkers under both field and laboratory conditions. Changes in metabolites precede visible growth effects; therefore, measuring changes in metabolite concentrations (Harring et al. 1998; Shaner et al. 2005). The metabolic phase I enzymes (primarily associated with P-450 activity) are non-specific biomarkers, and few studies relate them to growth parameters. P-450 activity both increases and decreases in response to chemical stress, often confounding interpretation of experimental results. Alternatively, phase II metabolic enzymes (e.g., glutathione S-transferases; GST's) appear to be sensitive biomarkers of exposure, and potentially effect. Some GST's are affected by growth factors, but others may only be induced by xenobiotics. Measuring xenobiotic-induced GST's, or their gene expression patterns, are good candidates for future biomarkers of the cumulative load of chemical stress, both in the laboratory and under field conditions. Phytochelatins respond to some but not all metal ions, and may therefore be used as biomarkers of exposure to identify the presence and bioavailability of ions to which they respond. However, more data on their specificity to, and interactions with growth factors, in more species are needed. The flavenoids are only represented by one heavy metal exposure study; therefore their use as biomarkers is currently difficult to judge. Stress proteins tend to be specific for toxicants that affect protein function. Growth factors are known to affect the level of stress proteins; hence, the use of stress proteins as biomarkers will be confined to experiments performed under controlled growth conditions, where they can be excellent indicators of proteotoxicity. Reactive oxygen species (ROS), ROS scavenging enzymes, changes in pigment content, photosynthesis and chlorophyll fluorescence are all affected by growth factors, particularly light and nutrient availability. Therefore, these biomarkers are best suited to investigate the mode of action of toxicants under controlled growth conditions. These biomarkers are sensitive to xenobiotic stressors that affect various processes in the photosynthetic apparatus, and can be used to diagnose which photosynthetic process or processes are primarily affected. Chlorophyll fluorescence is a non-destructive measure, and is thereby well suited for repeated measures of effect and recovery (Abbaspoor and Streibig 2005; Abbaspoor et al. 2006; Cedergreen et al. 2004). Bi-phasic responses (over time and with dose) are probably major sources of variation in sensitivity for many biomarkers. Metabolic enzymes, stress proteins, ROS and their corresponding scavenging enzymes increase in a time-frame and at doses in which plant cell damage is still repairable. However, when toxicity progresses to the point of cell damage, the concentration/activity of the biomarker either stabilizes or decreases. Examples of this response pattern are given in Lei et al. (2006); Pflugmacher et al. (2000b); Teisseire et al. (1998); and Teisseire and Guy (2000). Gene expression is also a time-dependent phenomenon varying several fold within a few hour. Therefore, bi-phasic response patterns make timing and dose-range, within which the biomarkers can be used as measures of both exposure and effect, extremely important. As a result, most biomarkers are best suited for situations in which the time and dose dependence of the biomarker, in the investigated species, are established. Notwithstanding the previously mentioned limitations, all assessed biomarkers provide valuable information on the physiological effects of specific stressors, and are valuable tools in the search for understanding xenobiotic modes of action. However, the future use of aquatic plant biomarkers will probably be confined to laboratory studies designed to assess toxicant modes of action, until further knowledge is gained regarding the time, dose and growth-factor dependence of biomarkers, in different species. No single biomarker is viable in gaining a comprehensive understanding of xenobiotic stress. Only through the concomitant measurement of a suite of appropriate biomarkers will our diagnostic capacity be enhanced and the field of ecotoxicology, as it relates to aquatic plants, advanced.

Nutrient limitation is the main regulatory factor for carotenoid accumulation and for Psy and Pds steady state transcript levels in Dunaliella salina (Chlorophyta) exposed to high light and salt stress

Dunaliella salina (Dunal) Teodoresco (1905) is a green unicellular alga able to withstand severe salt, light, and nutrient stress, adaptations necessary to grow in harsh environments such as salt ponds. In response to such growth conditions, this microalga accumulates high amounts of beta-carotene in its single chloroplast. In this study, we show that carotenoid accumulation is consistently inhibited in cells grown in nutrient-supplemented media and exposed either to high-light or medium-low-light conditions. Likewise, carotenogenesis in cells shifted to higher salinity (up to 27% NaCl) under medium-low-light conditions is inhibited by the presence of nutrients. The steady-state levels of transcripts encoding phytoene synthase and phytoene desaturase increased substantially in D. salina cells shifted to high light or high salt under nutrient-limiting conditions, whereas the presence of nutrients inhibited this response. The regulatory effect of nutrient availability on the accumulation of carotenoids and messenger RNA levels of the first two enzymes committed to carotenoid biosynthesis is discussed.

Exploring and exploiting carotenoid accumulation in Dunaliella salina for cell-factory applications

The unicellular alga Dunaliella salina is the most interesting cell factory for the commercial production of beta-carotene because this species accumulates carotenoids to high concentrations. Nevertheless, little is known about the underlying mechanisms of carotenoid accumulation. Here, we review the regulatory mechanisms involved in beta-carotene overproduction in D. salina. The potential roles of reactive oxygen species and the plastoquinone redox state in signal sensing are discussed, together with available evidence on transcriptional and (post)translational regulation. Moreover, future directions that might further our knowledge in this area are given. Ultimately, a better understanding of the regulatory mechanisms involved in beta-carotene overproduction will facilitate innovative production of specific carotenoids and other products in D. salina and in related organisms.

Biosynthesis and regulation of carotenoids in Dunaliella: progresses and prospects

Natural carotenoids are high in demand in global market owing to their widespread applications in nutrition, medicine, food coloring agent and cosmetic, as well as to the natural and healthy preference of consumers today. Some strains of Dunaliella are well known for their talent of massive beta-carotene accumulation. Content of the high bioavailability stereoisomer of beta-carotene, the 9-cis stereoisomer, is highest in Dunaliella among all the natural carotenoids sources. These valuable algae have been exploited commercially for beta-carotene-rich Dunaliella powder and natural beta-carotene in many countries since 1980s. However, drawbacks of traditional production methods have hampered the worldwide promotion of carotenoids production with Dunaliella. To shake off the dilemma, complete understanding of carotenogenic mechanism is urgent. Carotenogenic mechanism in Dunaliella is described in present paper, including carotenogenic pathway and its regulation. Generally, it seems that carotenogenic pathway in Dunaliella is close to the one of higher plants. It is known that reactive oxygen species (ROS) were involved in signal transduction for gene activation. Induction of ROS is in parallel with the enhanced beta-carotene accumulation in Dunaliella. It is suggested that ROS trigger massive carotenoids accumulation in Dunaliella. It also revealed that relation may exist between enhanced beta-carotene accumulation and lipid metabolism. For the talent of beta-carotene synthesis, it is possible that Dunaliella massively accumulates beta-carotene and other high-value carotenoids by genetic technologies.