Physiological characterization and stress-induced metabolic responses of Dunaliella salina isolated from salt pan

Co-metabolism of microorganisms: A study revealing the mechanism of antibiotic removal, progress of biodegradation transformation pathways

The widespread use of antibiotics has resulted in large quantities of antibiotic residues entering aquatic environments, which can lead to the development of antibiotic-resistant bacteria and antibiotic-resistant genes, posing a potential environmental risk and jeopardizing human health. Constructing a microbial co-metabolism system has become an effective measure to improve the removal efficiency of antibiotics by microorganisms. This paper reviews the four main mechanisms involved in microbial removal of antibiotics: bioaccumulation, biosorption, biodegradation and co-metabolism. The promotion of extracellular polymeric substances for biosorption and extracellular degradation and the regulation mechanism of enzymes in biodegradation by microorganisms processes are detailed therein. Transformation pathways for microbial removal of antibiotics are discussed. Bacteria, microalgae, and microbial consortia's roles in antibiotic removal are outlined. The factors influencing the removal of antibiotics by microbial co-metabolism are also discussed. Overall, this review summarizes the current understanding of microbial co-metabolism for antibiotic removal and outlines future research directions.

Antimicrobial Activity of Arthrospira (Former Spirulina) and Dunaliella Related to Recognized Antimicrobial Bioactive Compounds

With the increasing rate of the antimicrobial resistance phenomenon, natural products gain our attention as potential drug candidates. Apart from being used as nutraceuticals and for biotechnological purposes, microalgae and phytoplankton have well-recognized antimicrobial compounds and proved anti-infectious potential. In this review, we comprehensively outline the antimicrobial activity of one genus of cyanobacteria (Arthrospira, formerly Spirulina) and of eukaryotic microalgae (Dunaliella). Both, especially Arthrospira, are mostly used as nutraceuticals and as a source of antioxidants for health supplements, cancer therapy and cosmetics. Their diverse bioactive compounds provide other bioactivities and potential for various medical applications. Their antibacterial and antifungal activity vary in a broad range and are strain specific. There are strains of Arthrospira platensis with very potent activity and minimum inhibitory concentrations (MICs) as low as 2-15 µg/mL against bacterial fish pathogens including Bacillus and Vibrio spp. Arthrospira sp. has demonstrated an inhibition zone (IZ) of 50 mm against Staphylococcus aureus. Remarkable is the substantial amount of in vivo studies of Arthrospira showing it to be very promising for preventing vibriosis in shrimp and Helicobacter pylori infection and for wound healing. The innovative laser irradiation of the chlorophyll it releases can cause photodynamic destruction of bacteria. Dunaliella salina has exhibited MIC values lower than 300 µg/mL and an IZ value of 25.4 mm on different bacteria, while Dunaliella tertiolecta has demonstrated MIC values of 25 and 50 μg/mL against some Staphylococcus spp. These values fulfill the criteria for significant antimicrobial activity and sometimes are comparable or exceed the activity of the control antibiotics. The bioactive compounds which are responsible for that action are fatty acids including PUFAs, polysaccharides, glycosides, peptides, neophytadiene, etc. Cyanobacteria, such as Arthrospira, also particularly have antimicrobial flavonoids, terpenes, alkaloids, saponins, quinones and some unique-to-them compounds, such as phycobiliproteins, polyhydroxybutyrate, the peptide microcystin, etc. These metabolites can be optimized by using stress factors in a two-step process of fermentation in closed photobioreactors (PBRs).

Establishment of a Halophilic Bloom in a Sterile and Isolated Hypersaline Mesocosm

Extreme environments, including hypersaline pools, often serve as biogeographical islands. Putative colonizers would need to survive transport across potentially vast distances of inhospitable terrain. Hyperhalophiles, in particular, are often highly sensitive to osmotic pressure. Here, we assessed whether hyperhalophiles are capable of rapidly colonizing an isolated and sterile hypersaline pool and the order of succession of the ensuing colonizers. A sterile and isolated 1 m3 hypersaline mesocosm pool was constructed on a rooftop in Charleston, SC. Within months, numerous halophilic lineages successfully navigated the 20 m elevation and the greater than 1 km distance from the ocean shore, and a vibrant halophilic community was established. All told, in a nine-month period, greater than a dozen halophilic genera colonized the pool. The first to arrive were members of the Haloarchaeal genus Haloarcula. Like a weed, the Haloarcula rapidly colonized and dominated the mesocosm community but were later supplanted by other hyperhalophilic genera. As a possible source of long-distance inoculum, both aerosol and water column samples were obtained from the Great Salt Lake and its immediate vicinity. Members of the same genus, Haloarcula, were preferentially enriched in the aerosol sample relative to the water column samples. Therefore, it appears that a diverse array of hyperhalophiles are capable of surviving aeolian long-distance transport and that some lineages, in particular, have possibly adapted to that strategy.

Overexpression of S-R enhances the accumulation of biomass, fatty acids, and β-carotene in Schizochytrium

Strategies for enhancing biomass accumulation and increasing the production of fatty acids and β-carotene in Schizochytrium are hindered by the lack of suitable targets. In this study, S-R, a RING (really interesting new gene) finger domain-containing protein, was identified in Schizochytrium, with homologs found in the family Thraustochytriaceae. Transgenic strains overexpressing S-R showed a minor improvement in cell growth but a significant increase in total fatty acids content by 1.29- to 1.36-fold. Almost all individual saturated fatty acids exhibited significant increases, with the greatest increase observed in the C14:0 content, by 1.52- to 1.78-fold. Additionally, the β-carotene content of S-R strains was significantly upregulated. Overexpression of s-r conferred hypersaline tolerance in Schizochytrium, with a significant increase in dry cell weight, total fatty acids and β-carotene, likely due to the upregulation of glycerol and proline. This study provides a feasible strategy to engineer Thraustochytriaceae for efficient biomass and biochemical production.

Physiological and Biochemical Responses in Microalgae Dunaliella salina, Cylindrotheca closterium and Phormidium versicolor NCC466 Exposed to High Salinity and Irradiation

Dunaliella salina (Chlorophyceae), Phormidium versicolor (Cyanophyceae), and Cylindrotheca closterium (Bacillariophyceae) were isolated from three ponds in the solar saltern of Sfax (Tunisia). Growth, pigment contents, and photosynthetic and antioxidant enzyme activities were measured under controlled conditions of three light levels (300, 500, and 1000 µmol photons m^-2 s^-1) and three NaCl concentrations (40, 80, and 140 g L^-1). The highest salinity reduced the growth of D. salina and P. versicolor NCC466 and strongly inhibited that of C. closterium. According to ΦPSII values, the photosynthetic apparatus of P. versicolor was stimulated by increasing salinity, whereas that of D. salina and C. closterium was decreased by irradiance rise. The production of carotenoids in D. salina and P. versicolor was stimulated when salinity and irradiance increased, whereas it decreased in the diatom. Catalase (CAT), Superoxide dismutase (SOD), and Ascorbate peroxidase (APX) activities were only detected when the three species were cultivated under E1000. The antioxidant activity of carotenoids could compensate for the low antioxidant enzyme activity measured in D. salina. Salinity and irradiation levels interact with the physiology of three species that have mechanisms of more or less effective stress resistance, hence different resistance to environmental stresses according to the species. Under these stress-controlled conditions, P. versicolor and C. closterium strains could provide promising sources of extremolyte for several purposes.

Transcriptome Analysis Reveals the Mechanisms of Tolerance to High Concentrations of Calcium Chloride Stress in Parachlorella kessleri

Salt stress is one of the abiotic stress factors that affect the normal growth and development of higher plants and algae. However, few research studies have focused on calcium stress, especially in algae. In this study, the mechanism of tolerance to high calcium stress of a Parachlorella kessleri strain was explored by the method of transcriptomics combined with physiological and morphological analysis. Concentrations of CaCl₂ 100 times (3.6 g/L) and 1000 times (36 g/L) greater than the standard culture were set up as stresses. The results revealed the algae could cope with high calcium stress mainly by strengthening photosynthesis, regulating osmotic pressure, and inducing antioxidant defense. Under the stress of 3.6 g/L CaCl₂, the algae grew well with normal cell morphology. Although the chlorophyll content was significantly reduced, the photosynthetic efficiency was well maintained by up-regulating the expression of some photosynthesis-related genes. The cells reduced oxidative damage by inducing superoxide dismutase (SOD) activities and selenoprotein synthesis. A large number of free amino acids were produced to regulate the osmotic potential. When in higher CaCl₂ stress of 36 g/L, the growth and chlorophyll content of algae were significantly inhibited. However, the algae still slowly grew and maintained the same photosynthetic efficiency, which resulted from significant up-regulation of massive photosynthesis genes. Antioxidant enzymes and glycerol were found to resist oxidative damage and osmotic stress, respectively. This study supplied algal research on CaCl₂ stress and provided supporting data for further explaining the mechanism of plant salt tolerance.

Changes in Carbon Partitioning and Pattern of Antioxidant Enzyme Activity Induced by Arginine Treatment in the Green Microalga Dunaliella salina Under Long-Term Salinity

In this work, the effects of arginine (Arg) on biochemical responses and antioxidant enzyme activity in the green microalga Dunaliella salina grown at different salt concentrations were investigated. Suspensions adapted with the concentrations of 1, 2, and 3 M NaCl were treated at the exponential growth phase with a concentration of 5 mM Arg. Salt stress was associated with a large decrease in the number of cells and non-reducing sugar levels but accumulated higher amounts of chlorophyll, β-carotene, reducing sugar, starch, total protein, free amino acid, and glycerol. Increased levels of protein carbonylation, lipid peroxidation, proteolysis, hydrogen peroxide, and antioxidant enzyme activity also occurred during salinity. Arg treatment changed the pattern of biochemical responses in the cells grown at high salinity by directing carbon flow to the biosynthesis of non-reducing sugars instead of starch, lowering levels of hydrogen peroxide, and downregulating antioxidant enzyme activity, but the levels of lipid peroxidation, glycerol, and β-carotene remained nearly unchanged. These results suggest that Arg treatment alleviates salinity-induced oxidative stress in D. salina cells by modifying carbon partitioning and inducing signaling molecules rather than antioxidant enzymes.

Progress in microalgal mediated bioremediation systems for the removal of antibiotics and pharmaceuticals from wastewater

Worldwide demand for antibiotics and pharmaceutical products is continuously increasing for the control of disease and improvement of human health. Poor management and partial metabolism of these compounds result in the pollution of aquatic systems, leading to hazardous effects on flora, fauna, and ecosystems. In the past decade, the importance of microalgae in micropollutant removal has been widely reported. Microalgal systems are advantageous as their cultivation does not require additional nutrients: they can recover resources from wastewater and degrade antibiotics and pharmaceutical pollutants simultaneously. Bioadsorption, degradation, and accumulation are the main mechanisms involved in pollutant removal by microalgae. Integration of microalgae-mediated pollutant removal with other technologies, such as biodiesel, biochemical, and bioelectricity production, can make this technology more economical and efficient. This article summarizes the current scenario of antibiotic and pharmaceutical removal from wastewater using microalgae-mediated technologies.

Trehalose treatment alters carbon partitioning and reduces the accumulation of individual metabolites but does not affect salt tolerance in the green microalga Dunaliella bardawil

The effects of trehalose (Tre), a non-reducing disaccharide, on metabolic changes, antioxidant status, and salt tolerance in Dunaliella bardawil cells were investigated. Algal suspensions containing 1, 2, and 3 M NaCl were treated with 5 mM Tre. While the content of pigments, reducing sugars, proteins, glycerol, and ascorbate pool accumulated with increasing salinity, the content of non-reducing sugars, starch, amino acids, proline, hydrogen peroxide, and lipid peroxidation level decreased significantly. Tre-treated cells showed a decrease in pigments content, reducing sugars, starch, proteins, amino acids, proline, glycerol, and the activity of non-specific peroxidase and polyphenol oxidase, but an increase in non-reducing sugars, oxidized ascorbate, and ascorbate peroxidase activity occurred unchanged in the ascorbate pool. However, the density and fresh weight of the cells remained statistically unchanged in all Tre-treated and untreated cultures. These results suggest that D. bardawil cells potentially tolerate different salt levels by accumulating metabolites, whereas Tre treatment changes carbon partitioning and significantly reduces beneficial metabolites without altering salt tolerance. Therefore, the regulation of carbon partitioning rather than the amount of assimilated carbon may play an important role in inducing salinity tolerance of D. bardawil. However, Tre is not able to enhance the salt tolerance of halotolerants and is even economically damaging due to the reduction of unique metabolites such as glycerol and β-carotene.

The Ecophysiological Performance and Traits of Genera within the Stichococcus-like Clade (Trebouxiophyceae) under Matric and Osmotic Stress

Changes in water balance are some of the most critical challenges that aeroterrestrial algae face. They have a wide variety of mechanisms to protect against osmotic stress, including, but not limited to, downregulating photosynthesis, the production of compatible solutes, spore and akinete formation, biofilms, as well as triggering structural cellular changes. In comparison, algae living in saline environments must cope with ionic stress, which has similar effects on the physiology as desiccation in addition to sodium and chloride ion toxicity. These environmental challenges define ecological niches for both specialist and generalist algae. One alga known to be aeroterrestrial and euryhaline is Stichococcus bacillaris Nägeli, possessing the ability to withstand both matric and osmotic stresses, which may contribute to wide distribution worldwide. Following taxonomic revision of Stichococcus into seven lineages, we here examined their physiological responses to osmotic and matric stress through a salt growth challenge and desiccation experiment. The results demonstrate that innate compatible solute production capacity under salt stress and desiccation tolerance are independent of one another, and that salt tolerance is more variable than desiccation tolerance in the Stichococcus-like genera. Furthermore, algae within this group likely occupy similar ecological niches, with the exception of Pseudostichococcus.

Limitation of Microbial Processes at Saturation-Level Salinities in a Microbial Mat Covering a Coastal Salt Flat

Hypersaline microbial mats are dense microbial ecosystems capable of performing complete element cycling and are considered analogs of early Earth and hypothetical extraterrestrial ecosystems. We studied the functionality and limits of key biogeochemical processes, such as photosynthesis, aerobic respiration, and sulfur cycling, in salt crust-covered microbial mats from a tidal flat at the coast of Oman. We measured light, oxygen, and sulfide microprofiles as well as sulfate reduction rates at salt saturation and in flood conditions and determined fine-scale stratification of pigments, biomass, and microbial taxa in the resident microbial community. The salt crust did not protect the mats against irradiation or evaporation. Although some oxygen production was measurable at salinities of ≤30% (wt/vol) in situ, at saturation-level salinity (40%), oxygenic photosynthesis was completely inhibited and only resumed 2 days after reducing the porewater salinity to 12%. Aerobic respiration and active sulfur cycling occurred at low rates under salt saturation and increased strongly upon salinity reduction. Apart from high relative abundances of Chloroflexi, photoheterotrophic Alphaproteobacteria, Bacteroidetes, and Archaea, the mat contained a distinct layer harboring filamentous Cyanobacteria, which is unusual for such high salinities. Our results show that the diverse microbial community inhabiting this salt flat mat ultimately depends on periodic salt dilution to be self-sustaining and is rather adapted to merely survive salt saturation than to thrive under the salt crust. IMPORTANCE Due to their abilities to survive intense radiation and low water availability, hypersaline microbial mats are often suggested to be analogs of potential extraterrestrial life. However, even the limitations imposed on microbial processes by saturation-level salinity found on Earth have rarely been studied in situ. While abundance and diversity of microbial life in salt-saturated environments are well documented, most of our knowledge on process limitations stems from culture-based studies, few in situ studies, and theoretical calculations. In particular, oxygenic photosynthesis has barely been explored beyond 5 M NaCl (28% wt/vol). By applying a variety of biogeochemical and molecular methods, we show that despite abundance of photoautotrophic microorganisms, oxygenic photosynthesis is inhibited in salt-crust-covered microbial mats at saturation salinities, while rates of other energy generation processes are decreased several-fold. Hence, the complete element cycling required for self-sustaining microbial communities only occurs at lower salt concentrations.

Antioxidant Production in Dunaliella

Microalgae have become an attractive natural source of a diverse range of biomolecules, including enzymatic and non-enzymatic antioxidants; nevertheless, economically sustainable production of such compounds from microalgae biomass is still challenging. The main hurdles are: (a) increasing microalgae yield; (b) achieving optimal cultivation conditions; (c) energy-efficient and cost-effective downstream processing (extraction and purification); (d) optimal storage of post-processed antioxidant molecules. This review provides a detailed overview of enzymatic and non-enzymatic antioxidants in the cellular metabolism of the commercially important microalgae Dunaliella, industrial applications of antioxidant enzymes, strategies to enhanced antioxidant accumulation in cells, and the opportunities and limitations of current technologies for antioxidant enzymes production from microalgae biomass as an alternative to common microbial sources.

Salinization and wastewater effects on the growth and some cell contents of Scenedesmus bijugatus

The aim of this study is to determine the effect of salinization and wastewater stresses on the growth, some cellular contents (total soluble proteins, total soluble carbohydrates, nucleic acids, and amino acids composition) and ultrastructure using TEM of unicellular green alga Scenedesmus bijugatus. Treatment of S. bijugatus by NaCl at 10 and 50 mg L-1 significantly increased the growth of this alga and its cellular macro-molecules. While, treatment above this concentration with NaCl significantly inhibited the growth and cellular macro-molecules. On the other hand, treatment by NaCl at the pre-lethal concentration (300 mg L-1) had different effects on its detected amino acids. Whereas, Asp. Acid, Pro, Cys, Val, Iso-leu, leu, Phe.ala and Lys were slightly stimulated with salinization treatment. On contrast the levels of amino acids: Thr, Ser, Glu.acid, Gly, Ala, Mth, His and Arg were markedly inhibited. Ultrastructure examination of treated S. bijugatus by 300 mg L-1 of NaCl for 8 days showed increase of starch granules, shrinkage of cell contents and thickening of cell wall. The recorded data indicated also that treatment by wastewater with all concentrations led to stimulatory effects on their growth and cellular macro-molecules except at 100% wastewater which had inhibitory effects on Asp., Gly., Thr., Ser., Pro., Glu., Ala., Meth., and Cyst., of S. bijugatus. Also, wastewater induced a slight change in the treated S. bijugatus as elevation in starch granules and presence of thylakoid membranes although not clear as in the control.

Effect of Nitrate Ions on Acidithiobacillus ferrooxidans-Mediated Bio-oxidation of Ferrous Ions and Pyrite

Exploring the effects of nitrate ions (NO₃^-) on the bio-oxidation of Fe²⁺ and pyrite will help reveal the actual mechanism of acid mine drainage (AMD) production. Long period shaking flask experiments were carried out in order to assess the effect of NO₃^- on the Acidithiobacillus ferrooxidans LX5 (A. ferrooxidans LX5)-mediated bio-oxidation of Fe²⁺ and pyrite. In Fe²⁺ bio-oxidation systems, A. ferrooxidans LX5 had stronger Fe²⁺ oxidation capabilities in a NO₃^--loaded solution than in a NO₃^--free solution after 24 days, and the Fe²⁺ bio-oxidation capacity of A. ferrooxidans LX5 acclimatized in solutions containing low concentrations (8.2-32.9 mmol/L) of NO₃^- was greater than when it was acclimatized in high NO₃^- concentration solutions (49.4-65.8 mmol/L). In pyrite bio-oxidation systems, in comparison with the system without NO₃^-, pyrite bio-oxidation efficiency was significantly increased when the NO₃^- concentration in the system was 8.2-16.5 mmol/L, and that the pyrite bio-oxidation efficiency in the system containing 8.2 mmol/L of NO₃^- was greater than that for the system with 16.5 mmol/L of NO₃^-. The pyrite bio-oxidation efficiency was inhibited when the NO₃^- concentration was above 32.9 mmol/L. The results from this study can be used to reveal the actual control behavior of NO₃^- on AMD production.

Two splice variants of the DsMEK1 mitogen-activated protein kinase kinase (MAPKK) are involved in salt stress regulation in Dunaliella salina in different ways

BACKGROUND

Dunaliella salina can produce glycerol under salt stress, and this production can quickly adapt to changes in external salt concentration. Notably, glycerol is an ideal energy source. In recent years, it has been reported that the mitogen-activated protein kinase cascade pathway plays an important role in regulating salt stress, and in Dunaliella tertiolecta DtMAPK can regulate glycerol synthesis under salt stress. Therefore, it is highly important to study the relationship between the MAPK cascade pathway and salt stress in D. salina and modify it to increase the production of glycerol.

RESULTS

In our study, we identified and analysed the alternative splicing of DsMEK1 (DsMEK1-X1, DsMEK1-X2) from the unicellular green alga D. salina. DsMEK1-X1 and DsMEK1-X2 were both localized in the cytoplasm. qRT-PCR assays showed that DsMEK1-X2 was induced by salt stress. Overexpression of DsMEK1-X2 revealed a higher increase rate of glycerol production compared to the control and DsMEK1-X1-oe under salt stress. Under salt stress, the expression of DsGPDH2/3/5/6 increased in DsMEK1-X2-oe strains compared to the control. This finding indicated that DsMEK1-X2 was involved in the regulation of DsGPDH expression and glycerol overexpression under salt stress. Overexpression of DsMEK1-X1 increased the proline content and reduced the MDA content under salt stress, and DsMEK1-X1 was able to regulate oxidative stress; thus, we hypothesized that DsMEK1-X1 could reduce oxidative damage under salt stress. Yeast two-hybrid analysis showed that DsMEK1-X2 could interact with DsMAPKKK1/2/3/9/10/17 and DsMAPK1; however, DsMEK1-X1 interacted with neither upstream MAPKKK nor downstream MAPK. DsMEK1-X2-oe transgenic lines increased the expression of DsMAPKKK1/3/10/17 and DsMAPK1, and DsMEK1-X2-RNAi lines decreased the expression of DsMAPKKK2/10/17. DsMEK1-X1-oe transgenic lines did not exhibit increased gene expression, except for DsMAPKKK9.

CONCLUSION

Our findings demonstrate that DsMEK1-X1 and DsMEK1-X2 can respond to salt stress by two different pathways. The DsMEK1-X1 response to salt stress reduces oxidative damage; however, the DsMAPKKK1/2/3/9/10/17-DsMEK1-X2-DsMAPK1 cascade is involved in the regulation of DsGPDH expression and thus glycerol synthesis under salt stress.

Salinity Stress Responses and Adaptation Mechanisms in Eukaryotic Green Microalgae

High salinity is a challenging environmental stress for organisms to overcome. Unicellular photosynthetic microalgae are especially vulnerable as they have to grapple not only with ionic imbalance and osmotic stress but also with the generated reactive oxygen species (ROS) interfering with photosynthesis. This review attempts to compare and contrast mechanisms that algae, particularly the eukaryotic Chlamydomonas microalgae, exhibit in order to immediately respond to harsh conditions caused by high salinity. The review also collates adaptation mechanisms of freshwater algae strains under persistent high salt conditions. Understanding both short-term and long-term algal responses to high salinity is integral to further fundamental research in algal biology and biotechnology.

Metabolic responses of the green microalga Dunaliella salina to silver nanoparticles-induced oxidative stress in the presence of salicylic acid treatment

In the present study, the biochemical responses and antioxidant enzymes activity of the Dunaliella salina, a green microalga, to the interaction of silver nanoparticles (AgNPs) and salicylic acid (SA) were investigated. Algal suspensions in the phase of logarithmic growth were subjected to the concentrations of 0, 5, 15, and 25 pM AgNPs with or without 1 mM SA. AgNPs level of 25 pM declined cell division but highly accumulated levels of chlorophyll, β-carotene, proteins, free amino acid, carbohydrates, and hydrogen peroxide, which was associated with enhanced the activity of proteolysis, lipid peroxidation, and antioxidant enzymes. SA-treated cells at 25 pM AgNPs improved cell growth but declined the activities of antioxidant enzymes and proteolytic along with a lower accumulation of metabolites except β-carotene relative to untreated controls. These results suggest that AgNPs treatment induce oxidative stress in D. salina cells, which tolerated by alga through the metabolic modifications and accumulating β-carotene, while SA induces AgNPs tolerance by the mechanisms that direct carbon flux to growth and β-carotene biosynthesis rather than the antioxidant enzymes or osmoprotectant metabolites.

Effect of betaine and selenium on the growth and photosynthetic pigment production in Dunaliella salina as biostimulants

The aim of our study was to establish the effect of selenium and betaine on the growth of D. salina, accumulation of photosynthetic pigments and antioxidant activity of the hydrophobic fraction. This approach was an attempt to demonstrate 'microalgae biostimulant' effects, similar to 'plant biostimulant' effects, i.e. increased tolerance to abiotic stress and enhanced accumulation of bioactive compounds. A high-throughput assay was done in 24-well microplates, at 15% NaCl and different concentrations of sodium selenite (0, 0.5, 2 and 8 µM) or betaine (0, 5, 50 and 500 µM). Both selenium and betaine induced a slight delay in algae growth during the actively growing stage but the final density reached similar values to the control. Betaine significantly enhanced (50%-100%) carotenoids and chlorophyll a accumulation, in a concentration depending manner. Antioxidant activity increased almost 3-fold in extracts of algae treated with 50 µM betaine. Selenium had a much more discrete effect than betaine on pigments biosynthesis. The antioxidant activity of the extracts increased 2-fold in the presence of Se compared to the control. Our work proves that it is possible to enhance production and activity of bioactive compounds from microalgae by using ingredients, which already proved to act as plant biostimulants.

Biochemical and physiological characterization of a halotolerant Dunaliella salina isolated from hypersaline Sambhar Lake, India

The objective of the present study was to characterize intrinsic physiological and biochemical properties of the wall-less unicellular cholorophyte Dunaliella salina isolated from a hypersaline Sambhar Lake. The strain grew optimally at 0.5 M NaCl and 16:8 h L:D photoperiod along with maintaining low level of intracellular Na⁺ even at higher salinity, emphasizing special features of its cell membranes. It was observed that the cells experienced stress beyond 2 M NaCl as evidenced by increased intracellular reactive oxygen species and antioxidative enzymes, nevertheless proline and malondialdehyde content declined sharply accompanied by higher neutral lipid accumulation. Salinity exceeding 2 M resulted decrease in photosynthetic quantum yield (Fv/Fm) and enhanced glycerol synthesis accompanied by leakage. Super oxide dismutase seemed to play a pivotal role in antioxidative defense as eight isoforms were expressed differentially while catalase and glutathione peroxidase showing no significant change in their expression at higher salinity. The ability of D. salina to grow in range of salinities by sustaining healthy photosynthetic apparatus along with accumulation of valuable products made this alga an ideal organism that can be exploited as resource for biofuel and commercial products.

Delineating the molecular responses of a halotolerant microalga using integrated omics approach to identify genetic engineering targets for enhanced TAG production

BACKGROUND

Harnessing the halotolerant characteristics of microalgae provides a viable alternative for sustainable biomass and triacylglyceride (TAG) production. Scenedesmus sp. IITRIND2 is a fast growing fresh water microalga that has the capability to thrive in high saline environments. To understand the microalga's adaptability, we studied its physiological and metabolic flexibility by studying differential protein, metabolite and lipid expression profiles using metabolomics, proteomics, real-time polymerase chain reaction, and lipidomics under high salinity conditions.

RESULTS

On exposure to salinity, the microalga rewired its cellular reserves and ultrastructure, restricted the ions channels, and modulated its surface potential along with secretion of extrapolysaccharide to maintain homeostasis and resolve the cellular damage. The algal-omics studies suggested a well-organized salinity-driven metabolic adjustment by the microalga starting from increasing the negatively charged lipids, up regulation of proline and sugars accumulation, followed by direction of carbon and energy flux towards TAG synthesis. Furthermore, the omics studies indicated both de-novo and lipid cycling pathways at work for increasing the overall TAG accumulation inside the microalgal cells.

CONCLUSION

The salt response observed here is unique and is different from the well-known halotolerant microalga; Dunaliella salina, implying diversity in algal response with species. Based on the integrated algal-omics studies, four potential genetic targets belonging to two different metabolic pathways (salt tolerance and lipid production) were identified, which can be further tested in non-halotolerant algal strains.

Transcriptome sequencing and annotation of the halophytic microalga Dunaliella salina

The unicellular green alga Dunaliella salina is well adapted to salt stress and contains compounds (including β-carotene and vitamins) with potential commercial value. A large transcriptome database of D. salina during the adjustment, exponential and stationary growth phases was generated using a high throughput sequencing platform. We characterized the metabolic processes in D. salina with a focus on valuable metabolites, with the aim of manipulating D. salina to achieve greater economic value in large-scale production through a bioengineering strategy. Gene expression profiles under salt stress verified using quantitative polymerase chain reaction (qPCR) implied that salt can regulate the expression of key genes. This study generated a substantial fraction of D. salina transcriptional sequences for the entire growth cycle, providing a basis for the discovery of novel genes. This first full-scale transcriptome study of D. salina establishes a foundation for further comparative genomic studies.

The induction of salt stress tolerance by propyl gallate treatment in green microalga Dunaliella bardawil, through enhancing ascorbate pool and antioxidant enzymes activity

The effect of propyl gallate (PG), a synthetic antioxidant, on antioxidant responses and salinity tolerance was investigated in the cells of the green microalga, Dunaliella bardawil. Algal suspensions grown at three salinity levels of 1, 2, and 3 M NaCl were incubated with 1 mM of PG. The number of cells was significantly lower in all PG-treated cells compared to untreated controls. Despite PG-induced cell death, the fresh weight of all PG-treated cells was considerably higher than controls. PG-treated cells had enhanced antioxidant capacity because of increased levels of Chlorophyll a, β-carotene, reduced ascorbate, protein, and enzymatic activities, but accumulated lower levels of malonyldialdehyde and hydrogen peroxide compared to untreated cells. The results suggest that PG acts as a signal molecule both directly by reducing of free radical oxidants and indirectly by augmenting ascorbate pool levels, β-carotene production, and antioxidant enzymes activity to boost the capacity of antioxidant systems and radical oxygen species scavenging. Therefore, induction of salt stress tolerance by PG in D. bardawil is associated with metabolic adjustments through activation or synthesis of both enzymatic and non-enzymatic molecules involved in antioxidant systems.

Accumulation of phytoene, a colorless carotenoid by inhibition of phytoene desaturase (PDS) gene in Dunaliella salina V-101

The aim of this work was to study the accumulation of phytoene in Dunaliella salina V-101 by down-regulating its phytoene desaturase (PDS) gene expression using RNA interference and Antisense technology. RNAi and antisense constructs were introduced into the Dunaliella cells by Agrobacterium-mediated transformation. Among thirty-two transformants, six showed positive down-regulation of PDS expression with RNAi construct and five positive transformants were obtained using antisense construct. Characterization of PDS suppression was carried out using semi-quantitative RT-PCR and quantitative determination of phytoene as well as other carotenoids by HPLC. Both the RNAi and antisense lines showed a significant decrease in the expression levels of phytoene desaturase and carotenoid content compared to wild type cells. The RNAi line #5 showed maximum Phytoene content (108.34±22.34µg/100mg DCW) compared to other transgenic lines. These phytoene-accumulating phenotypes exhibited slower growth rates and were found to be sensitive to high light conditions.

Analysis of carotenogenic genes promoters and WRKY transcription factors in response to salt stress in Dunaliella bardawil

The unicellular alga Dunaliella bardawil is a highly salt-tolerant organism, capable of accumulating glycerol, glycine betaine and β-carotene under salt stress, and has been considered as an excellent model organism to investigate the molecular mechanisms of salt stress responses. In this study, several carotenogenic genes (DbCRTISO, DbZISO, DbLycE and DbChyB), DbBADH genes involved in glycine betaine synthesis and genes encoding probable WRKY transcription factors from D. bardawil were isolated, and promoters of DbCRTISO and DbChyB were cloned. The promoters of DbPSY, DbLycB, DbGGPS, DbCRTISO and DbChyB contained the salt-regulated element (SRE), GT1GMSCAM4, while the DbGGPS promoter has another SRE, DRECRTCOREAT. All promoters of the carotenogenic genes had light-regulated elements and W-box cis-acting elements. Most WRKY transcription factors can bind to the W-box, and play roles in abiotic stress. qRT-PCR analysis showed that salt stress up-regulated both carotenogenic genes and WRKY transcription factors. In contrast, the transcription levels of DbBADH showed minor changes. In D. bardawil, it appears that carotenoid over-accumulation allows for the long-term adaptation to salt stress, while the rapid modulation of glycine betaine biosynthesis provides an initial response.

BnSIP1-1, a Trihelix Family Gene, Mediates Abiotic Stress Tolerance and ABA Signaling in Brassica napus

The trihelix family genes have important functions in light-relevant and other developmental processes, but their roles in response to adverse environment are largely unclear. In this study, we identified a new gene, BnSIP1-1, which fell in the SIP1 (6b INTERACTING PROTEIN1) clade of the trihelix family with two trihelix DNA binding domains and a fourth amphipathic α-helix. BnSIP1-1 protein specifically targeted to the nucleus, and its expression can be induced by abscisic acid (ABA) and different stresses. Overexpression of BnSIP1-1 improved seed germination under osmotic pressure, salt, and ABA treatments. Moreover, BnSIP1-1 decreased the susceptibility of transgenic seedlings to osmotic pressure and ABA treatments, whereas there was no difference under salt stress between the transgenic and wild-type seedlings. ABA level in the transgenic seedlings leaves was higher than those in the control plants under normal condition. Under exogenous ABA treatment and mannitol stress, the accumulation of ABA in the transgenic plants was higher than that in the control plants; while under salt stress, the difference of ABA content before treatment was gradually smaller with the prolongation of salt treatment time, then after 24 h of treatment the ABA level was similar in transgenic and wild-type plants. The transcription levels of several general stress marker genes (BnRD29A, BnERD15, and BnLEA1) were higher in the transgenic plants than the wild-type plants, whereas salt-responsive genes (BnSOS1, BnNHX1, and BnHKT) were not significantly different or even reduced compared with the wild-type plants, which indicated that BnSIP1-1 specifically exerted different regulatory mechanisms on the osmotic- and salt-response pathways in seedling period. Overall, these findings suggested that BnSIP1-1 played roles in ABA synthesis and signaling, salt and osmotic stress response. To date, information about the involvement of the Brassica napus trihelix gene in abiotic response is scarce. Here, we firstly reported abiotic stress response and possible function mechanisms of a new trihelix gene in B. napus.

Proteomic analysis of halotolerant proteins under high and low salt stress in Dunaliella salina using two-dimensional differential in-gel electrophoresis

Dunaliella salina, a single-celled marine alga with extreme salt tolerance, is an important model organism for studying fundamental extremophile survival mechanisms and their potential practical applications. In this study, two-dimensional differential in-gel electrophoresis (2D-DIGE) was used to investigate the expression of halotolerant proteins under high (3 M NaCl) and low (0.75 M NaCl) salt concentrations. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF/TOF MS) and bioinformatics were used to identify and characterize the differences among proteins. 2D-DIGE analysis revealed 141 protein spots that were significantly differentially expressed between the two salinities. Twenty-four differentially expressed protein spots were successfully identified by MALDI-TOF/TOF MS, including proteins in the following important categories: molecular chaperones, proteins involved in photosynthesis, proteins involved in respiration and proteins involved in amino acid synthesis. Expression levels of these proteins changed in response to the stress conditions, which suggests that they may be involved in the maintenance of intracellular osmotic pressure, cellular stress responses, physiological changes in metabolism, continuation of photosynthetic activity and other aspects of salt stress. The findings of this study enhance our understanding of the function and mechanisms of various proteins in salt stress.

A Review on the Assessment of Stress Conditions for Simultaneous Production of Microalgal Lipids and Carotenoids

Microalgal species are potential resource of both biofuels and high-value metabolites, and their production is growth dependent. Growth parameters can be screened for the selection of novel microalgal species that produce molecules of interest. In this context our review confirms that, autotrophic and heterotrophic organisms have demonstrated a dual potential, namely the ability to produce lipids as well as value-added products (particularly carotenoids) under influence of various physico-chemical stresses on microalgae. Some species of microalgae can synthesize, besides some pigments, very-long-chain polyunsaturated fatty acids (VL-PUFA,>20C) such as docosahexaenoic acid and eicosapentaenoic acid, those have significant applications in food and health. Producing value-added by-products in addition to biofuels, fatty acid methyl esters (FAME), and lipids has the potential to improve microalgae-based biorefineries by employing either the autotrophic or the heterotrophic mode, which could be an offshoot of biotechnology. The review considers the potential of microalgae to produce a range of products and indicates future directions for developing suitable criteria for choosing novel isolates through bioprospecting large gene pool of microalga obtained from various habitats and climatic conditions.

The effect of salinity on waste activated sludge alkaline fermentation and kinetic analysis

The effect of salinity on sludge alkaline fermentation at low temperature (20°C) was investigated, and a kinetic analysis was performed. Different doses of sodium chloride (NaCl, 0-25g/L) were added into the fermentation system. The batch-mode results showed that the soluble chemical oxygen demand (SCOD) increased with salinity. The hydrolysate (soluble protein, polysaccharide) and the acidification products (short chain fatty acids (SCFAs), NH4(+)-N, and PO4(3-)-P) increased with salinity initially, but slightly declined respectively at higher level salinity (20g/L or 20-25g/L). However, the hydrolytic acidification performance increased in the presence of salt compared to that without salt. Furthermore, the results of Haldane inhibition kinetics analysis showed that the salt enhanced the hydrolysis rate of particulate organic matter from sludge particulate and the specific utilization of hydrolysate, and decreased the specific utilization of SCFAs. Pearson correlation coefficient analysis indicated that the importance of polysaccharide on the accumulation of SCFAs was reduced with salt addition, but the importance of protein and NH4(+)-N on SCFA accumulation was increased.

The potential of transgenic green microalgae; a robust photobioreactor to produce recombinant therapeutic proteins

Microalgae have been used in food, cosmetic, and biofuel industries as a natural source of lipids, vitamins, pigments and antioxidants for a long time. Green microalgae, as potent photobioreactors, can be considered as an economical expression system to produce recombinant therapeutical proteins at large-scale due to low cost of production and scaling-up capitalization owning to the inexpensive medium requirement, fast growth rate, and the ease of manipulation. These microalgae possess all benefit eukaryotic expression systems including the ability of post-translational modifications required for proper folding and stability of active proteins. Among the many items regarded as recombinant protein production, this review compares the different expression systems with green microalgae like Dunaliella by viewing the nuclear/chloroplast transformation challenges/benefits, related selection markers/reporter genes, and crucial factors/strategies affecting the increase of foreign protein expression in microalgae transformants. Some important factors were discussed regarding the increase of protein yielding in microalgae transformants including: transformation-associated genotypic modifications, endogenous regulatory factors, promoters, codon optimization, enhancer elements, and milking of recombinant protein.

Hypoosmotic expression of Dunaliella bardawil ζ-carotene desaturase is attributed to a hypoosmolarity-responsive element different from other key carotenogenic genes

Some key carotenogenic genes (crts) in Dunaliella bardawil are regulated in response to salt stress partly due to salt-inducible cis-acting elements in their promoters. Thus, we isolated and compared the ζ-carotene desaturase (Dbzds) promoter with other crts promoters including phytoene synthase (Dbpsy), phytoene desaturase (Dbpds), and lycopene β-cyclase1 (DblycB1) to identify salt-inducible element(s) in the Dbzds promoter. In silico analysis of the Dbzds promoter found several potential cis-acting elements, such as abscisic acid response element-like sequence, myelocytomatosis oncogene1 recognition motif, AGC box, anaerobic motif2, and activation sequence factor1 binding site. Remarkably, instead of salt-inducible elements, we found a unique regulatory sequence architecture in the Dbzds promoter: a hypoosmolarity-responsive element (HRE) candidate followed by a potential hypoosmolarity-inducible factor GBF5 binding site. Deletion experiments demonstrated that only HRE, but not the GBF5 binding site, is responsible for hypoosmotic expression of the fusion of Zeocin resistance gene (ble) to the enhanced green fluorescent protein (egfp) chimeric gene under salt stress. Dbzds transcripts were in accordance with those of ble-egfp driven by the wild-type Dbzds promoter. Consequently, Dbzds is hypoosmotically regulated by its promoter, and HRE is responsible for this hypoosmotic response. Finally, the hypoosmolarity mechanism of Dbzds was studied by comparing transcript profiles and regulatory elements of Dbzds with those of Dbpsy, Dbpds, DblycB1, and DblycB2, revealing that different induction characteristics of crts may correlate with regulatory sequence architecture.

Effect of salinity on extracellular polymeric substances of activated sludge from an anoxic-aerobic sequencing batch reactor

The effect of salinity on extracellular polymeric substances (EPS) of activated sludge was investigated in an anoxic-aerobic sequencing batch reactor (SBR). The contents of loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) were positively correlated with the salinity. The polysaccharide (PS) and protein (PN) contents in both LB-EPS and TB-EPS increased with the increase of salinity. With the increase of salinity from 0.5% to 6%, the PN/PS ratios in LB-EPS and TB-EPS decreased from 4.8 to 0.9 and from 2.9 to 1.4, respectively. The four fluorescence peaks in both LB-EPS and TB-EPS identified by three-dimensional excitation-emission matrix fluorescence spectroscopy are attributed to PN-like substances and humic acid-like substances. The Fourier transform infrared spectra of the LB-EPS and TB-EPS appeared to be very similar, but the differences across the spectra were apparent in terms of the relative intensity of some bands with the increase of salinity. The sludge volume index showed a linear correlation with LB-EPS (R(2)=0.9479) and TB-EPS (R(2)=0.9355) at different salinities, respectively.

High-pH-induced flocculation-flotation of the hypersaline microalga Dunaliella salina

Natural autoflocculation was not observed in a Dunaliella salina hypersaline culture and the microalgae did not float without destabilization of the algal suspension. High-pH-induced flocculation by sodium hydroxide addition was chosen to induce flotation. Recovery efficiencies greater than 90% and concentration factors of around 20 were reached. An autoflocculation mechanism, with precipitation of magnesium hydroxide, is proposed to explain a sweeping flotation of D. salina cells. The influence of the flow rate of sodium hydroxide addition was also studied to anticipate the constraints related to the industrialization of this process. The flow rate of sodium hydroxide addition had no effect on the recovery efficiency and reduced the concentration factor only for abrupt injections. Natural increase of culture pH by photosynthetic activity could reduce the amount of base consumed. Non-harvested cells remained viable during pH increase and could be used as inoculum for a new culture.

OPTIMIZATION OF SALT CONCENTRATIONS FOR A HIGHER CAROTENOID PRODUCTION IN DUNALIELLA SALINA (CHLOROPHYCEAE)(1)

Dunaliella salina (Dunal) Teodor, when treated over 25 d with a wide range of NaCl salinities (0.6-4.5 M), showed its maximal growth potentialities at 1.5-3.0 M NaCl and was able to survive even at 4.5 M NaCl. Sodium concentrations increased significantly at the supraoptimal salinities, reaching up to 5 mmol · g(-1) dry weight (dwt) at 4.5 M NaCl. Interestingly, ability of D. salina to take up essential mineral nutrients was not impaired by increased salinity. As for growth, chl concentrations were maximal in the 1.5-3.0 M NaCl range. Interestingly, carotenoid concentrations increased with the increasing salinity. The highest values of total antioxidant activity (5.2-6.9 mg gallic acid equivalents [GAE] · g(-1) dwt), antiradical activity, and reducing power were measured at 1.5-3.0 M NaCl. As a whole, these results showed that at 1.5-3.0 M NaCl, D. salina produce appreciable antioxidant level. But, once it reaches its growth maximum, a salt addition up to 4.5 M could enhance its carotenoid yield.

Comparison of three Chlamydomonas strains which show distinctive oxidative stress tolerance

Methyl viologen (MV) causes severe oxidative stress by generating superoxide in the photosystem. The marine Chlamydomonas strain W80 is highly tolerant to MV (inhibitory concentration 50% [IC₅₀]=110 μM), and another marine Chlamydomonas strain HS5 shows also relatively a high tolerance (IC₅₀=12 μM). These two marine strains and a freshwater Chlamydomonas reinhardtii, which is highly sensitive to MV (IC₅₀=0.03 μM), were compared with respect to their reactive oxygen species (ROS) eliminating enzymes (superoxide dismutase, catalase, glutathione peroxidase, and ascorbate peroxidase), intracellular free amino acids, and antioxidant activities of the cell extracts. The marked difference between the marine Chlamydomonas strains and C. reinhardtii is the much higher (more than 5 fold) ascorbate peroxidase (APX) activity in the marine strains. The marine strains also kept the high APX activities (more than 100% of non-stressed condition) under the MV stressed condition, while the APX activity in C. reinhardtii was significantly decreased (36% of non-stressed condition) under the stressed condition, indicating that APX activity potentially contributes to the oxidative stress tolerance in Chlamydomonas. In addition, the levels of intracellular free proline, which is supposed to ameliorate oxidative stress, were several tens of times higher in the marine Chlamydomonas strains than in C. reinhardtii.

Expressed sequence tag (EST) profiling in hyper saline shocked Dunaliella salina reveals high expression of protein synthetic apparatus components

The unicellular, halotolerant, green alga, Dunaliella salina (Chlorophyceae) has the unique ability to adapt and grow in a wide range of salt conditions from about 0.05 to 5.5M. To better understand the molecular basis of its salinity tolerance, a complementary DNA (cDNA) library was constructed from D. salina cells adapted to 2.5M NaCl, salt-shocked at 3.4M NaCl for 5h, and used to generate an expressed sequence tag (EST) database. ESTs were obtained for 2831 clones representing 1401 unique transcripts. Putative functions were assigned to 1901 (67.2%) ESTs after comparison with protein databases. An additional 154 (5.4%) ESTs had significant similarity to known sequences whose functions are unclear and 776 (27.4%) had no similarity to known sequences. For those D. salina ESTs for which functional assignments could be made, the largest functional categories included protein synthesis (35.7%), energy (photosynthesis) (21.4%), primary metabolism (13.8%) and protein fate (6.8%). Within the protein synthesis category, the vast majority of ESTs (80.3%) encoded ribosomal proteins representing about 95% of the approximately 82 subunits of the cytosolic ribosome indicating that D. salina invests substantial resources in the production and maintenance of protein synthesis. The increased mRNA expression upon salinity shock was verified for a small set of selected genes by real-time, quantitative reverse-transcription-polymerase chain reaction (qRT-PCR). This EST collection also provided important new insights into the genetic underpinnings for the biosynthesis and utilization of glycerol and other osmoprotectants, the carotenoid biosynthetic pathway, reactive oxygen-scavenging enzymes, and molecular chaperones (heat shock proteins) not described previously for D. salina. EST discovery also revealed the existence of RNA interference and signaling pathways associated with osmotic stress adaptation. The unknown ESTs described here provide a rich resource for the identification of novel genes associated with the mechanistic basis of salinity stress tolerance and other stress-adaptive traits.

Dunaliella as an attractive candidate for molecular farming

Pharmaceutical recombinant proteins are widely used in human healthcare. At present, several protein expression systems are available to generate therapeutic proteins. These conventional systems have distinct advantages and disadvantages in protein yielding; in terms of ease of manipulation, the time required from gene transformation to protein purification, cost of production and scaling-up capitalization, proper folding and stability of active proteins. Depending on the research goal and priorities, a special system may be selected for protein expression. However, considering the limited variety of organisms currently used and their usage restrictions, there are still much more pharmaceutical proteins waiting to be economically and efficiently produced. Distinguished biological and technical features of microalgae Dunaliella such as inexpensive medium requirement, fast growth rate, the ease of manipulation, easy scaling up procedure, facility of milking in bioreactors and the ability of post-translational modifications make this microorganism an attractive candidate for molecular farming.