Quantitative proteomic analysis of Dunaliella salina upon acute arsenate exposure

Key Proteomics Tools for Fundamental and Applied Microalgal Research

Microscopic, photosynthetic prokaryotes and eukaryotes, collectively referred to as microalgae, are widely studied to improve our understanding of key metabolic pathways (e.g., photosynthesis) and for the development of biotechnological applications. Omics technologies, which are now common tools in biological research, have been shown to be critical in microalgal research. In the past decade, significant technological advancements have allowed omics technologies to become more affordable and efficient, with huge datasets being generated. In particular, where studies focused on a single or few proteins decades ago, it is now possible to study the whole proteome of a microalgae. The development of mass spectrometry-based methods has provided this leap forward with the high-throughput identification and quantification of proteins. This review specifically provides an overview of the use of proteomics in fundamental (e.g., photosynthesis) and applied (e.g., lipid production for biofuel) microalgal research, and presents future research directions in this field.

Using wastewater as a cultivation alternative for microalga Dunaliella salina: Potentials and challenges

Untreated or poorly treated wastewater still represents environmental issues world-widely. Wastewater, especially saline wastewater treatment, is still primarily associated with high costs from physical and chemical processes, as high salinity hinders biological treatment. One favourable way is to find the suitable biological pathways and organisms to improve the biological treatment efficiency. In this context, halophilic microorganisms could be strong candidates to address the economics and effectiveness of the saline wastewater treatment process. Dunaliella salina is a photoautotrophic microalga that grows in saline environments. It is known for producing marketable bio-compounds such as carotenoids, lipids, and proteins. A biological treatment based on D. salina cultivation offers the opportunity to treat saline wastewater, reducing the threat of possible eutrophication from inappropriate discharge. At the same time, D. salina cultivation could yield compounds of industrial relevance to turn saline wastewater treatment into a profitable and sustainable process. Most research on D. salina has primarily focused on bioproduct generation, leaving thorough reviews of its application in wastewater treatment inadequate. This paper discusses the future challenges and opportunities of using D. salina to treat wastewater from different sources. The main conclusions are (1) D. salina effectively recovers some heavy metals (driven by metal binding capacity and exposure time) and nutrients (driven by pH, their bioavailability, and functional groups in the cell); (2) salinity plays a significant role in bioproducts generation, and (3) wastewater can be combined with the generation of bioproducts.

Utilizing proteomics to identify and optimize microalgae strains for high-quality dietary protein: a review

Algae-derived protein has immense potential to provide high-quality protein foods for the expanding human population. To meet its potential, a broad range of scientific tools are required to identify optimal algal strains from the hundreds of thousands available and identify ideal growing conditions for strains that produce high-quality protein with functional benefits. A research pipeline that includes proteomics can provide a deeper interpretation of microalgal composition and biochemistry in the pursuit of these goals. To date, proteomic investigations have largely focused on pathways that involve lipid production in selected microalgae species. Herein, we report the current state of microalgal proteome measurement and discuss promising approaches for the development of protein-containing food products derived from algae.

Enhancement of arsenic uptake and accumulation in green microalga Chlamydomonas reinhardtii through heterologous expression of the phosphate transporter DsPht1

Arsenate (AsV) is a predominant arsenic contaminant in aerobic water. Microalgae have been recently used in the phytoremediation of arsenic-contaminated water. However, the amount of AsV uptake in microalgae is limited, which hinders the application of microalgae in arsenic-contaminated water treatment. Here, we found that the expression of a novel phosphate transporter DsPht1 in Dunaliella salina was highly upregulated after AsV exposure. Fluorescent protein-tagging analysis showed the plasma membrane location of DsPht1. Furthermore, DsPht1 was overexpressed in a model microalga Chlamydomonas reinhardtii. The DsPht1 transgenetic lines accumulated up to 6.4-fold higher total arsenic than the untransformed line, and the AsV amount in total arsenic increased by 8.3-fold. Moreover, the organoarsenic content was also higher in the transgenetic lines. Overall, the DsPht1 transformants generated in this study increased arsenate uptake and transformation, which are promising for the effective phytoremediation of arsenic-contaminated water.

Assessment of Joint Toxicity of Arsenate and Lead by Multiple Endpoints in Chlamydomonas reinhardtii

In aquatic ecosystems, arsenate (As(V)) and lead (Pb(II)) frequently coexist but their joint toxicity on microalgae remains unknown. In this study, Chlamydomonas reinhardtii was exposed to various levels of combined As(V) and Pb(II) treatments. The cell growth, respiration, pigment synthesis, polysaccharides and protein secretion as well as As speciation of C. reinhardtii were analyzed. The low-level coexistence of As(V) and Pb(II) had a stimulatory effect, as indicated by enhanced cell proliferation. In the middle-level coexistence, the cells resisted the toxicity by significant increasing protein secretion. Under high-level coexistence, the presence of Pb(II) inhibited the efflux of As and caused the decline of cell numbers and occurrence of cell lysis, indicating that the interaction mode between As(V) and Pb(II) switched to synergistic. Taken together, the above findings may deepen the understanding of detoxification mechanisms of algae upon exposure to combined metal(loid)s in aqueous environments.

Physiological and proteomic responses of Chlamydomonas reinhardtii to arsenate and lead mixtures

Arsenic (As) and lead (Pb) are frequently emitted from various sources into environment, but microbial responses to their combined toxicity have not been systematically investigated. In this study, Chlamydomonas reinhardtii was exposed to two levels of arsenate (As (V), 50, 500 μg/L), Pb (II) (500, 5000 μg/L) and their mixture (50 μg/L As (V) + 500 μg/L Pb (II); 500 μg/L As (V) + 5000 μg/L Pb (II)). The growth of C. reinhardtii was inhibited more remarkably by As (V) than by Pb (II). The As stress was alleviated by Pb in the 50 μg/L As (V) + 500 μg/L Pb (II) treatment, but was enhanced upon the 500 μg/L As (V) + 5000 μg/L Pb (II) exposure, with more pronounced changes in a number of physiological parameters of the algal cells. Proteomic results showed that 71 differently expressed proteins (DEPs) in the treatment of 50 μg/L As (V) + 500 μg/L Pb (II), and 167 DEPs were identified in that of 500 μg/L As (V) + 5000 μg/L Pb (II). These proteins were involved in energy metabolism, photosynthetic carbon fixation, reactive oxygen scavenging and defense, and amino acid synthesis. Taken together, these physiological and proteomic data demonstrated that C. reinhardtii could resist the As (V) and Pb (II) combined treatments through extracellular complexation and intracellular pathways.

Proteomics and transcriptomic analysis of Micrococcus luteus strain AS2 under arsenite stress and its potential role in arsenic removal

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M. luteus strain AS2 showed hyper-tolerance against arsenite upto 50 mM.

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Thioredoxin reductase, involved in As-resistance, upregulated 2.8 folds under arsenite stress.

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The maximum metal oxidizing processing ability of the strain AS2 was 90%.

The proteomics and transcriptomic analysis of Micrococcus luteus strain AS2 was carried out through 2D gel electrophoresis and RT-PCR. Seven protein spots were selected randomly from the gel and identified through mass spectrometry. Four proteins including putative metal-dependent hydrolase TatD, thioredoxin reductase, DNA-directed RNA polymerase subunit alpha and chaperone protein DnaK were upregulated while superoxide dismutase [Mn], 3-oxoacyl-[acyl-carrier-protein] reductase FabG, and putative alkyl/aryl-sulfatase YjcS were down-regulated under arsenite stress. No significant difference was observed in aioB gene expression analysis in the presence and absence of arsenite. The optimum arsenite processing ability was determined at 37°C (90%) and at pH 7 (92%). The maximum metal processing ability was determined at 250 mM arsenite/L (90%) while the minimum was estimated at 1250 mM arsenite/L (42%). The maximum arsenite removal ability of strain AS2 determined after 8 days was 68 and 82% from wastewater and distilled water, and the organism can be a good bioresource for green chemistry to eradicate environmental arsenite.

Arsenate-Induced Changes in Bacterial Metabolite and Lipid Pools during Phosphate Stress

Agrobacterium tumefaciens GW4 is a heterotrophic arsenite-oxidizing bacterium with a high resistance to arsenic toxicity. It is now a model organism for studying the processes of arsenic detoxification and utilization. Previously, we demonstrated that under low-phosphate conditions, arsenate [As(V)] could enhance bacterial growth and be incorporated into biomolecules, including lipids. While the basic microbial As(V) resistance mechanisms have been characterized, global metabolic responses under low phosphate remain largely unknown. In the present work, the impacts of As(V) and low phosphate on intracellular metabolite and lipid profiles of GW4 were quantified using liquid chromatography-mass spectroscopy (LC-MS) in combination with transcriptional assays and the analysis of intracellular ATP and NADH levels. Metabolite profiling revealed that oxidative stress response pathways were altered and suggested an increase in DNA repair. Changes in metabolite levels in the tricarboxylic acid (TCA) cycle along with increased ATP are consistent with As(V)-enhanced growth of A. tumefaciens GW4. Lipidomics analysis revealed that most glycerophospholipids decreased in abundance when As(V) was available. However, several glycerolipid classes increased, an outcome that is consistent with maximizing growth via a phosphate-sparing phenotype. Differentially regulated lipids included phosphotidylcholine and lysophospholipids, which have not been previously reported in A. tumefaciens The metabolites and lipids identified in this study deepen our understanding of the interplay between phosphate and arsenate on chemical and metabolic levels.IMPORTANCE Arsenic is widespread in the environment and is one of the most ubiquitous environmental pollutants. Parodoxically, the growth of certain bacteria is enhanced by arsenic when phosphate is limited. Arsenate and phosphate are chemically similar, and this behavior is believed to represent a phosphate-sparing phenotype in which arsenate is used in place of phosphate in certain biomolecules. The research presented here uses a global approach to track metabolic changes in an environmentally relevant bacterium during exposure to arsenate when phosphate is low. Our findings are relevant for understanding the environmental fate of arsenic as well as how human-associated microbiomes respond to this common toxin.

Arsenite Oxidation by Dunaliella salina is Affected by External Phosphate Concentration

Arsenic (As) contamination in terrestrial and aquatic environments is a well-known global environmental problem. The biooxidation of arsenite [As(III)] and subsequent arsenate [As(V)] removal have increasingly been used for remediation of As-polluted groundwater. However, little is known about As(III) oxidation by microalgae, especially those from saltwater environments. In this study, we investigated As(III) toxicity and oxidation in the marine microalga Dunaliella salina in the presence of different phosphate concentrations. The results of the As(III) toxicity experiments showed that D. salina was tolerant to As(III) (5.4 ± 0.31 mg As L^-1 at 72 h of culture). The As(V) percentage in the P-enriched (11.2 mg L^-1) medium was 7.2-fold greater than in the P-deficient one after 24-h exposure, indicating As(III) oxidation by D. salina was more pronounced with increased phosphate levels. Treatment of As(III) with and without 2,4-dinitrophenol (DNP) on the algal cells showed that As(III) oxidation occurred mainly on the cell surface and in the cytoplasm of D. salina. The results of this study suggest that transformation of As(III) into As(V) may be an important pathway of detoxification in D. salina and that phosphate plays a key role in this oxidation process.

Oxidative stress response mechanism of Scenedesmus obliquus to ionic liquids with different number of methyl-substituents

Ionic liquids (ILs) have become persistent contaminants in water because of their good solubility and low biodegradability. The oxidative stress responses of Scenedesmus obliquus to three imidazole ILs with different number of methyl-substituents, i.e., 1-decyl-imidazolium chloride ([C₁₀IM]Cl), 1-decyl-3-methylimidazolium chloride ([C₁₀MIM]Cl), and 1-decyl-2,3-dimethylimidazolium chloride ([C₁₀DMIM]Cl), were studied. There was a positive correlation between ROS level and IL concentration. The activities of antioxidant enzymes, i.e., superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, and glutathione peroxidase, and the content of antioxidants, i.e., ascorbic acid and glutathione, changed in IL treatment with a concentration-dependent effect. Proline accumulation increased with increasing IL concentration. Integrated biomarker response (IBR) index analysis, based on the eight oxidative stress response indicators, revealed that the toxicity order was: [C₁₀IM]Cl < [C₁₀DMIM]Cl < [C₁₀MIM]Cl. Proteomic analysis showed that IL affect the type and distribution of proteins in S. obliquus. Chloroplast and photosystem II were affected as cellular component, and the proteins related to oxidative stress are annotated in GO categories. IBR index and proteomic analysis indicate that oxidative stress response is one of the main biomarkers of IL stress.

Bioengineering of Microalgae: Recent Advances, Perspectives, and Regulatory Challenges for Industrial Application

Microalgae, due to their complex metabolic capacity, are being continuously explored for nutraceuticals, pharmaceuticals, and other industrially important bioactives. However, suboptimal yield and productivity of the bioactive of interest in local and robust wild-type strains are of perennial concerns for their industrial applications. To overcome such limitations, strain improvement through genetic engineering could play a decisive role. Though the advanced tools for genetic engineering have emerged at a greater pace, they still remain underused for microalgae as compared to other microorganisms. Pertaining to this, we reviewed the progress made so far in the development of molecular tools and techniques, and their deployment for microalgae strain improvement through genetic engineering. The recent availability of genome sequences and other omics datasets form diverse microalgae species have remarkable potential to guide strategic momentum in microalgae strain improvement program. This review focuses on the recent and significant improvements in the omics resources, mutant libraries, and high throughput screening methodologies helpful to augment research in the model and non-model microalgae. Authors have also summarized the case studies on genetically engineered microalgae and highlight the opportunities and challenges that are emerging from the current progress in the application of genome-editing to facilitate microalgal strain improvement. Toward the end, the regulatory and biosafety issues in the use of genetically engineered microalgae in commercial applications are described.

Bioaccumulation and biotransformation of arsenic in Leptolyngbya boryana

Leptolyngbya boryana (L. boryana) is a typical filamentous cyanobacterium that is widely distributed in aquatic ecosystems and is considered to play an important role in the arsenic biogeochemical cycle. Our results showed that L. boryana resisted arsenite (As(III)) and arsenate (As(V)) concentrations up to 0.25 mM and 5 mM, respectively. When exposed to 100 μM As(III) or As(V) for 4 weeks, L. boryana accumulated as much arsenic as 570.0 mg kg^-1 and 268.5 mg kg^-1, respectively. After the 4-week exposure to As(III) and As(V), organoarsenicals including dimethylarsenate (DMAs(V)) and oxo-arsenosugar-phosphate (Oxo-PO₄) were detected in the cells of L. boryana, while inorganic arsenic, especially As(V), was still the main species in both the cells and medium. Furthermore, arsenic oxidation was observed to be solely caused by L. boryana cells and was considered the dominant detoxification pathway. In conclusion, due to its powerful arsenic accumulation, biotransformation, and detoxification abilities, L. boryana might play an important role in arsenic remediation in aquatic environments.

Extracellular polymeric substances alter cell surface properties, toxicity, and accumulation of arsenic in Synechocystis PCC6803

Arsenic (As) contamination of water poses severe threats to human health and thus requires effective remediation methods. In this study, Synechocystis PCC6803, a model cyanobacterium common in aquatic environments, was used to investigate the role of extracellular polymeric substances (EPS) in As toxicity, accumulation, and transformation processes. We monitored the growth of Synechocystis with As exposure, measured the zeta potential and binding sites on the cell surface, and analysed As accumulation and speciation in Synechocystis cells with and without EPS. After EPS removal, the binding sites and zeta potential of the cell surface decreased by 44.43% and 31.9%, respectively. The growth of Synechocystis decreased 49.4% and 43.7% with As^(III) and As^(V) exposure, and As accumulation in the cells decreased by 12.8-44.5% and 14-42.7%, respectively. As absorption was enhanced in cells with EPS removed. The oxidation of As^(III) and reduction of As^(V) were significantly greater in cells with intact EPS compared to those with EPS removed. Fourier transform infrared spectroscopy (FTIR) showed that functional groups of EPS and Synechocystis cells, including -NH, -OH, CO, and CC, interacted with As species. Together the results of this work demonstrate that EPS have significant impacts on cell surface properties, thereby affecting As accumulation and transformation in Synechocystis PCC6803. This work provides a basis for using EPS to remedy As pollution in aquatic environments.

Potential of proteomics to probe microbes

An organism exposed to a plethora of environmental perturbations undergoes proteomic changes which enable the characterization of total proteins in it. Much of the proteomic information is obtained from genomic data. Additional information on the proteome such as posttranslational modifications, protein-protein interactions, protein localization, metabolic pathways, and so on are deduced using proteomic tools which genomics and transcriptomics fail to offer. The proteomic analysis allows identification of precise changes in proteins, which in turn solve the complexity of microbial population providing insights into the microbial metabolism, cellular pathways, and behavior of microorganisms in new environments. Furthermore, they provide clues for the exploitation of their special features for biotechnological applications. Numerous techniques for the analysis of microbial proteome such as electrophoretic, chromatographic, mass spectrometric-based methods as well as quantitative proteomics are available which facilitate protein separation, expression, identification, and quantification of proteins. An understanding of the potential of each of the proteomic tools has created a significant impact on diverse microbiological aspects and the same has been discussed in this review.

Toxicity, Physiological, and Ultrastructural Effects of Arsenic and Cadmium on the Extremophilic Microalga Chlamydomonas acidophila

The cytotoxicity of cadmium (Cd), arsenate (As(V)), and arsenite (As(III)) on a strain of Chlamydomonas acidophila, isolated from the Rio Tinto, an acidic environment containing high metal(l)oid concentrations, was analyzed. We used a broad array of methods to produce complementary information: cell viability and reactive oxygen species (ROS) generation measures, ultrastructural observations, transmission electron microscopy energy dispersive x-ray microanalysis (TEM-XEDS), and gene expression. This acidophilic microorganism was affected differently by the tested metal/metalloid: It showed high resistance to arsenic while Cd was the most toxic heavy metal, showing an LC₅₀ = 1.94 µM. Arsenite was almost four-fold more toxic (LC₅₀= 10.91 mM) than arsenate (LC₅₀ = 41.63 mM). Assessment of ROS generation indicated that both arsenic oxidation states generate superoxide anions. Ultrastructural analysis of exposed cells revealed that stigma, chloroplast, nucleus, and mitochondria were the main toxicity targets. Intense vacuolization and accumulation of energy reserves (starch deposits and lipid droplets) were observed after treatments. Electron-dense intracellular nanoparticle-like formation appeared in two cellular locations: inside cytoplasmic vacuoles and entrapped into the capsule, around each cell. The chemical nature (Cd or As) of these intracellular deposits was confirmed by TEM-XEDS. Additionally, they also contained an unexpected high content in phosphorous, which might support an essential role of poly-phosphates in metal resistance.

Identification of Early Salinity Stress-Responsive Proteins in Dunaliella salina by isobaric tags for relative and absolute quantitation (iTRAQ)-Based Quantitative Proteomic Analysis

Salt stress is one of the most serious abiotic factors that inhibit plant growth. Dunaliella salina has been recognized as a model organism for stress response research due to its high capacity to tolerate extreme salt stress. A proteomic approach based on isobaric tags for relative and absolute quantitation (iTRAQ) was used to analyze the proteome of D. salina during early response to salt stress and identify the differentially abundant proteins (DAPs). A total of 141 DAPs were identified in salt-treated samples, including 75 upregulated and 66 downregulated DAPs after 3 and 24 h of salt stress. DAPs were annotated and classified into gene ontology functional groups. The Kyoto Encyclopedia of Genes and Genomes pathway analysis linked DAPs to tricarboxylic acid cycle, photosynthesis and oxidative phosphorylation. Using search tool for the retrieval of interacting genes (STRING) software, regulatory protein⁻protein interaction (PPI) networks of the DAPs containing 33 and 52 nodes were built at each time point, which showed that photosynthesis and ATP synthesis were crucial for the modulation of early salinity-responsive pathways. The corresponding transcript levels of five DAPs were quantified by quantitative real-time polymerase chain reaction (qRT-PCR). These results presented an overview of the systematic molecular response to salt stress. This study revealed a complex regulatory mechanism of early salt tolerance in D. salina and potentially contributes to developing strategies to improve stress resilience.

An alternative analytical method for determining arsenic species in rice by using ion chromatography and inductively coupled plasma-mass spectrometry

Qualitative and quantitative determination of total arsenic content and arsenic species in rice is very important because rice is one of the main sources of human arsenic intake. However, extraction and determination of arsenic species in rice has been very difficult due to severe matrix interference. An alternative analytical method was developed in this study to determine arsenic species in rice by using ion chromatography coupled to inductively coupled plasma-mass spectrometry. Two internal standards were used. The first internal standard was injected before sample introduction to correct signal change with time. The second internal standard was spiked into the sample to reduce matrix interference. Using the developed method, recoveries of dimethylarsinic acid, monomethylarsonic acid, and inorganic arsenic compared to certified values (NIST SRM 1568b rice flour) were 116%, 107%, and 92%, respectively.

Proteomic analysis on roots of Oenothera glazioviana under copper-stress conditions

Proteomic studies were performed to identify proteins involved in the response of Oenothera glazioviana seedlings under Cu stress. Exposure of 28-d-old seedlings to 50 μM CuSO4 for 3 d led to inhibition of shoot and root growth as well as a considerable increase in the level of lipid peroxidation in the roots. Cu absorbed by O. glazioviana accumulated more easily in the root than in the shoot. Label-free proteomic analysis indicated 58 differentially abundant proteins (DAPs) of the total 3,149 proteins in the roots of O. glazioviana seedlings, of which 36 were upregulated and 22 were downregulated under Cu stress conditions. Gene Ontology analysis showed that most of the identified proteins could be annotated to signal transduction, detoxification, stress defence, carbohydrate, energy, and protein metabolism, development, and oxidoreduction. We also retrieved 13 proteins from the enriched Kyoto Encyclopaedia of Genes and Genomes and the protein-protein interaction databases related to various pathways, including the citric acid (CA) cycle. Application of exogenous CA to O. glazioviana seedlings exposed to Cu alleviated the stress symptoms. Overall, this study provided new insights into the molecular mechanisms of plant response to Cu at the protein level in relation to soil properties.

Bioaccumulation kinetics of arsenite and arsenate in Dunaliella salina under different phosphate regimes

Dunaliella salina is a potential candidate for the phycoremediation of saline water contaminated with arsenic (As) due to its strong tolerance of salt and this toxic metalloid. However, the efficiency of As removal by this microalga varies under different phosphate regimes and the underlying mechanisms remain unresolved. Therefore, more detailed studies are needed to optimize As remediation using D. salina. Here, we investigated the dynamic processes of arsenite (As(III)) and arsenate (As(V)) uptake, transformation, and excretion by D. salina under phosphate-deficient (-P) and phosphate-enriched (+P) conditions through short-term and long-term uptake experiments. In the short-term uptake experiment, the absorption of As(III) or As(V) by D. salina was significantly suppressed by an increased phosphate supply. The V _max values for As(III) and As(V) decreased by 2- and 2.5-fold, respectively, under +P conditions, although the Michaelis constants (K _m ) were similar irrespective of the phosphate supply. Long-term uptake experiments also revealed enhanced As(III)/As(V) absorption and efflux rates and As(V) reduction by D. salina under -P conditions. This study quantified the kinetic processes of As metabolism in D. salina. More importantly, the results imply that the optimal As remediation by this microalga may be achieved by regulating the phosphate level in the culture.

Phytochelatin synthesis in Dunaliella salina induced by arsenite and arsenate under various phosphate regimes

This study investigated the dynamic variations in thiol compounds, including cysteine (Cys), glutathione (GSH), and phytochelatins (PCs), in Dunaliella salina samples exposed to arsenite [As(III)] and arsenate [As(V)] under various phosphate (PO₄^3-) regimes. Our results showed that GSH was the major non-protein sulfhydryl compound in D. salina cells. As(III) and As(V) induced PC syntheses in D. salina. PC₂, PC₃, and PC₄ were all found in algal cells; the PC concentrations decreased gradually while exposed to As for 3 d. The synthesis of PC_2-3 was significantly affected by As(III) and As(V) concentrations in the cultures. More PCs were detected in the As(V)-treated algal cells compared with the As(III) treatment. PC levels increased with As(III)/As(V) amount in the medium, but remained stable after 112μgL^-1 As(V) exposure. In contrast, significant (p<0.001) positive correlations were observed between PC synthesis and intracellular As(III) content or As accumulation in As(III)-treated algal cells during the 72-h exposure. PO₄^3- had a significant influence on the PC synthesis in algal cells, irrespective of the As-treated species. Reductions in As uptake and subsequent PC synthesis by D. salina were observed as the PO₄^3- concentration in the growth medium increased. L-Buthionine sulfoximine (BSO) differentially influenced PC synthesis in As-treated D. salina under different extracellular PO₄^3- regimes. Overall, our data demonstrated that the production of GSH and PCs was affected by PO₄^3- and that these thiols played an important role in As detoxification by D. salina.

Metabolic response of Agrobacterium tumefaciens 5A to arsenite

Wide-spread abundance in soil and water, coupled with high toxicity have put arsenic at the top of the list of environmental contaminants. Early studies demonstrated that both concentration and the valence state of inorganic arsenic (arsenite, As(III) vs. arsenate As(V)) can be modulated by microbes. Using genetics, transcriptomic and proteomic techniques, microbe-arsenic detoxification, respiratory As(V) reduction and As(III) oxidation have since been examined. The effect of arsenic exposure on whole-cell intracellular microbial metabolism, however, has not been extensively studied. We combined LC-MS and ¹ H NMR to quantify metabolic changes in Agrobacterium tumefaciens (strain 5A) upon exposure to sub-lethal concentrations of As(III). Metabolomics analysis reveals global differences in metabolite concentrations between control and As(III) exposure groups, with significant perturbations to intermediates shuttling into and cycling within the TCA cycle. These data are most consistent with the disruption of two key TCA cycle enzymes, pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. Glycolysis also appeared altered following As(III) stress, with carbon accumulating as complex saccharides. These observations suggest that an important consequence of As(III) contamination in nature will be to alter microbial carbon metabolism at the microbial community level and thus has the potential to foundationally impact all biogeochemical cycles in the environment.

A symbiotic bacterium differentially influences arsenate absorption and transformation in Dunaliella salina under different phosphate regimes

In this study, we investigated the effects of a symbiotic bacterium and phosphate (PO4(3-)) nutrition on the toxicity and metabolism of arsenate (As(V)) in Dunaliella salina. The bacterium was identified as Alteromonas macleodii based on analysis of its 16S rRNA gene sequence. When no As(V) was added, A. macleodii significantly enhanced the growth of D. salina, irrespective of PO4(3-) nutrition levels, but this effect was reversed after As(V)+PO4(3-) treatment (1.12mgL(-1)) for 3 days. Arsenic (As) absorption by the non-axenic D. salina was significantly higher than that by its axenic counterpart during incubation with 1.12mgL(-1) PO4(3-). However, when the culture was treated with 0.112mgL(-1) PO4(3-), As(V) reduction and its subsequent arsenite (As(III)) excretion by non-axenic D. salina were remarkably enhanced, which, in turn, contributed to lower As absorption in non-axenic algal cells from days 7 to 9. Moreover, dimethylarsinic acid was synthesized by D. salina alone, and the rates of its production and excretion were accelerated when the PO4(3-) concentration was 0.112mgL(-1). Our data demonstrate that A. macleodii strongly affected As toxicity, uptake, and speciation in D. salina, and these impacts were mediated by PO4(3-) in the cultures.

Arsenate toxicity and metabolism in the halotolerant microalga Dunaliella salina under various phosphate regimes

Microalgae play an important role in arsenic (As) biogeochemical cycles as they are capable of accumulating and metabolizing this metalloid efficiently. This study aimed to investigate the toxicity, accumulation and transformation of arsenate (As(v)) in Dunaliella salina, an exceptionally halotolerant microalga, under various phosphate (PO4(3-)) regimes. The results of the 72-h toxicity test showed that D. salina was tolerant to As(v). In addition, the toxicity of As(v) was mitigated by an increased PO4(3-) supply. D. salina resisted the adverse effects of As(v) through the suppression of As uptake, enhancement of As reduction, methylation in the cell and excretion from the cell. Our study revealed that D. salina reduced As(v) toxicity using different strategies, i.e., reduction of As uptake upon acute As stress (24 h) and increase of As efflux following chronic As exposure (9 day). Moreover, PO4(3-) strongly affected the adsorption, uptake and transformation of As(v) in D. salina. As(v) reduction, DMA production and As excretion were enhanced under P-limited conditions (0.112 mg L(-1)) or upon higher As(v) exposure (1120 μg L(-1)). Furthermore, PO4(3-) had a significant influence on the As removal ability of D. salina. A high As removal efficiency (>95.6%) was observed in the 5-day cultures at an initial As concentration of 11.2 μg L(-1) and PO4(3-) of 0.112 and 1.12 mg L(-1). However, only 10.9% of total As was removed under 11.2 mg L(-1) PO4(3-) after 9 days of incubation. The findings of this study illustrate the pivotal roles of extracellular PO4(3-) in As(v) toxicity and metabolism, and the results may be relevant for future research on the minimization of As contamination in algal products as well as on the enhancement of As removal from the environment.