Cadmium sensitivity, uptake, subcellular distribution and thiol induction in a marine diatom: exposure to cadmium

Mitigating Effect of Trans-Zeatin on Cadmium Toxicity in Desmodesmus armatus

Phytohormones, particularly cytokinin trans-zeatin (tZ), were studied for their impact on the green alga Desmodesmus armatus under cadmium (Cd) stress, focusing on growth, metal accumulation, and stress response mechanisms. Using atomic absorption spectroscopy for the Cd level and high-performance liquid chromatography for photosynthetic pigments and phytochelatins, along with spectrophotometry for antioxidants and liquid chromatography-mass spectrometry for phytohormones, we found that tZ enhances Cd uptake in D. armatus, potentially improving phycoremediation of aquatic environments. Cytokinin mitigates Cd toxicity by regulating internal phytohormone levels and activating metal tolerance pathways, increasing phytochelatin synthase activity and phytochelatin accumulation essential for Cd sequestration. Treatment with tZ and Cd also resulted in increased cell proliferation, photosynthetic pigment and antioxidant levels, and antioxidant enzyme activities, reducing oxidative stress. This suggests that cytokinin-mediated mechanisms in D. armatus enhance its capacity for Cd uptake and tolerance, offering promising avenues for more effective aquatic phycoremediation techniques.

Heterologous Expression of the Phytochelatin Synthase CaPCS2 from Chlamydomonas acidophila and Its Effect on Different Stress Factors in Escherichia coli

Phytochelatins (PCs) are cysteine-rich small peptides, enzymatically synthesized from reduced glutathione (GSH) by cytosolic enzyme phytochelatin synthase (PCS). The open reading frame (ORF) of the phytochelatin synthase CaPCS2 gene from the microalgae Chlamydomonas acidophila was heterologously expressed in Escherichia coli strain DH5α, to analyze its role in protection against various abiotic agents that cause cellular stress. The transformed E. coli strain showed increased tolerance to exposure to different heavy metals (HMs) and arsenic (As), as well as to acidic pH and exposure to UVB, salt, or perchlorate. In addition to metal detoxification activity, new functions have also been reported for PCS and PCs. According to the results obtained in this work, the heterologous expression of CaPCS2 in E. coli provides protection against oxidative stress produced by metals and exposure to different ROS-inducing agents. However, the function of this PCS is not related to HM bioaccumulation.

Mechanisms underlying silicon-dependent metal tolerance in the marine diatom Phaeodactylum tricornutum

Anthropogenic activities have significantly changed the stoichiometry and concentrations of nutrients in coastal waters. Silicon (Si) has become a potential limiting nutrient due to disproportionate nitrogen, phosphorus, and silicate inputs into these areas. The disrupted nutrient ratios can cause changes to metal sensitivity and accumulation in marine diatoms, an important group of eukaryotic phytoplankton that requires silicon for growth. In this study, we examined the effects of Si availability on the metal sensitivity in the diatom P. tricornutum. We found that Si starvation dramatically compromised its cadmium, copper, and lead tolerances. Interestingly, multiple lines of evidence indicated that Si-enriched cells had higher metal adsorption and influx rates than Si-starved cells. Yet Si-enriched cells also had a greater ability to respond to and counteract metal toxicity via elevated expression of membrane and vacuolar metal transporters and greater antioxidant activities which scavenge reactive oxygen species created by metal stress.

Mitigation effects of CO2-driven ocean acidification on Cd toxicity to the marine diatom Skeletonema costatum

Ocean acidification (OA) is a global problem to marine ecosystems. Cadmium (Cd) is a typical metal pollutant, which is non-essential but extremely toxic to marine organisms. The combined effects of marine pollution and climate-driven ocean changes should be considered for the effective marine ecosystem management of coastal areas. Previous reports have separately investigated the influences of OA and Cd pollution on marine organisms. However, little is known of the potential combined effects of OA and Cd pollution on marine diatoms. We investigated the sole and combined influences of OA (1500 ppm CO₂) and Cd exposure (0.4 and 1.2 mg/L) on the coastal diatom Skeletonema costatum. Our results clearly showed that OA significantly alleviated the toxicity of Cd to S. costatum growth and mitigated the oxidant stress, although the intercellular Cd accumulation still increased. OA partially rescued S. costatum from the inhibition of photosynthesis and pyruvate metabolism caused by Cd exposure. It also upregulated genes involved in gluconeogenesis, glycolysis, the citrate cycle (TCA), Ribonucleic acid (RNA) metabolism, and especially the biosynthesis of non-protein thiol compounds. These changes might contribute to algal growth and Cd resistance. Overall, this study demonstrates that OA can alleviate Cd toxicity to S. costatum and explores the potential underlying mechanisms at both the cellular and molecular levels. These results will ultimately help us understand the impacts of combined stresses of climate change and metal pollution on marine organisms and expand the knowledge of the ecological risks of OA.

Combined effects of CO2-driven ocean acidification and Cd stress in the marine environment: Enhanced tolerance of Phaeodactylum tricornutum to Cd exposure

Ocean acidification (OA) and heavy metals are common stress factors for marine ecosystems subject to anthropogenic impacts. OA coupled with the heavy metal is likely to affect marine species. This study investigated the single and combined effects of OA (1500 ppm) and cadmium (Cd; 0.4, 1.2 mg/L) on the marine diatom Phaeodactylum tricornutum under 7 d exposure. The results clearly indicated that either OA or Cd stress (1.2 mg/L) alone inhibited the growth of P. tricornutum. However, under the combined OA-Cd stress, the growth inhibition disappeared, and the intracellular oxidative damage was mitigated. These results indicated a significantly enhanced tolerance of P. tricornutum to Cd while under OA conditions, which could be beneficial to the survival of this diatom. This study will ultimately help us understand the responses of marine organisms to multiple stressors and have broad implications for the potential ecological risks of Cd under future OA conditions.

Predicting cadmium toxicity with the kinetics of phytochelatin induction in a marine diatom

Phytochelatin (PC) synthesis is thought to be a rapid and specific response to metal exposure in marine phytoplankton, but its potential as a predictor of metal toxicity is far from conclusive. Thus this research examines the bioaccumulation, PC induction, and toxicity of Cadmium (Cd) in Thalassiosira weissflogii, a coastal diatom under varying nutrient conditions. Nitrogen limitation strongly inhibited Cd uptake and PC induction at the same [Cd²⁺] level, and increased metal sensitivity. Conversely, phosphorus limitation had little influence on Cd accumulation and PC induction, yet also enhanced metal effect on growth. Differential growth inhibitions were correlated with [Cd²⁺], intracellular Cd concentration, PC concentration, the kinetics of Cd uptake and PC induction, respectively. It was found that stronger interrelations existed between kinetic rates (both Cd uptake and PC synthesis) and Cd sensitivity than between the static concentrations (Cd and PC) and growth inhibition. Moreover, according to the calculated median inhibition concentration (IC50), median effective uptake rate of Cd, as well as median effective induction rate of PCs, the latter two showed the smallest variation when nutrients were varied (1.4-1.9 fold). Our study set out the first step toward considering the use of PC synthesis kinetics to predict metal toxicity for phytoplankton.

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.

Quantitative Relationship between Cadmium Uptake and the Kinetics of Phytochelatin Induction by Cadmium in a Marine Diatom

Heavy metals activate the synthesis of phytochelatins (PCs), while the induced PCs might affect metal uptake via chelating intracellular free metals. However, the relationship of PCs to metal uptake is poorly understood. In this study, we examined the kinetics of cadmium (Cd) accumulation and the synthesis of PCs in a marine diatom, Thalassiosira weissflogii, under different irradiance levels. Irradiance alone could not change the concentrations of PCs in the Cd-free treatments, while higher irradiance accelerated the induction of intracellular PCs at the same [Cd²⁺] level. PC-SH (2 × PC₂ + 3 × PC₃ + 4 × PC₄) was bound with Cd at a stoichiometric ratio of 2 to 49 in our short-term uptake experiments, indicating that PC induction is sufficient to serve as the first line of defense against Cd stress. A positive linear correlation between the induction rate of PCs and the Cd uptake rate was observed, while the ratio of the PC content to intracellular Cd varied greatly when the irradiance was increased several fold. Because metal uptake has been successfully used in predicting acute metal toxicity, our findings are helpful for understanding the role of PCs in metal detoxification and developing PCs as biomarkers for metal sensitivity.

Intraspecific differences in cadmium tolerance of Nitzschia palea (Kützing) W. Smith: a biochemical approach

Intraspecific variability occurs in all types of organisms and is a driving force to speciation, conferring genotypic and phenotypic differences that enable adaptive responses to sub-lethal stimuli such as exposure to pollutants (including cadmium, Cd). Thus, differences in biochemical parameters are expected among isolates of the same species. Studying the extent of these differences throughout a stress range, will provide information for the development of approaches to mitigate habitat contamination. This work was designed to identify possible differences in Cd tolerance of five isolates of the freshwater diatom Nitzschia palea from different sampling sites. Each isolate was exposed to five increasing Cd concentrations during 10 days. Growth inhibition was assessed and intracellular accumulation of Cd was quantified. Superoxide dismutase and catalase activities were determined. Glutathione as well as lipid peroxidation (LPO) and intracellular protein content were quantified. The results obtained identified intraspecific differences among isolates. These differences were associated with different approaches of coping with Cd stress. Higher intracellular Cd concentrations induced lower tolerance in isolates, since antioxidant mechanisms were unable to fight effectively against higher oxidative stress. Reversely, lower intracellular accumulation of Cd induced lower oxidative damage and allowed cells to better tolerate exposure to Cd. LPO emerged as an excellent marker of oxidative stress in N. palea and its use can differentiate isolates according to their tolerance.

The response and detoxification strategies of three freshwater phytoplankton species, Aphanizomenon flos-aquae, Pediastrum simplex, and Synedra acus, to cadmium

The response and detoxification mechanisms of three freshwater phytoplankton species (the cyanobacterium Aphanizomenon flos-aquae, the green alga Pediastrum simplex, and the diatom Synedra acus) to cadmium (Cd) were investigated. The cell growth of each species was measured over 10 days, and chlorophyll a fluorescence, Cd bioaccumulation (including surface-adsorbed and intracellular Cd), and phytochelatin (PC) synthesis were determined after 96-h exposures. The growth of the three phytoplankton species was significantly inhibited when Cd concentrations were ≥5 mg L(-1). Compared with P. simplex, greater growth inhibition in S. acus and A. flos-aquae occurred. The changes in chlorophyll fluorescence parameters including the maximal quantum yield of PSII (Fv/Fm) and relative variable fluorescence of the J point (Vj) demonstrated that the increase in Cd concentration damaged PSII in all three species. After 96-h exposures, the accumulation of surface-adsorbed Cd and intracellular Cd increased significantly in all three species, with the increase of Cd concentrations in the media; total cadmium accumulation was 245, 658, and 1670 times greater than that of the control in A. flos-aquae, P. simplex, and S. acus, respectively, after exposure to 10 mg L(-1). Total thiols exhibited a similar trend to that of Cd accumulation. PC3 was found in A. flos-aquae and P. simplex in all Cd treatments. Glutathione (GSH) and PC2 were also produced in response to exposure to high concentrations of Cd. PC4 was only discovered at exposure concentrations of 10 mg L(-1) Cd and only in S. acus. The intracellular Cd/PCs ratio increased in all three phytoplankton with an increase in Cd concentrations, and a linear relationship between the ratio and the growth inhibition rates was observed with P. simplex and S. acus. Our results have demonstrated that metal detoxification mechanisms were dependent on the species. This study suggested that the variance of metal detoxification strategies, such as cadmium accumulation and PCs, might be an explanation why algal species have different sensitivity to Cd at various levels.

Subcellular cadmium distribution and antioxidant enzymatic activities in the leaves of two castor (Ricinus communis L.) cultivars exhibit differences in Cd accumulation

The aims of this study were: (1) the study of cadmium (Cd) accumulation and toxicity in different castor cultivars (Ricinus communis L.); (2) to investigate changes in antioxidant enzymatic activities and the subcellular distribution of Cd in young and old leaves from two different castor cultivars, after exposure to two different Cd concentrations, and explore the underlying mechanism of Cd detoxification focusing on antioxidant enzymes and subcellular compartmentalization. The Cd concentration, toxicity, and subcellular distribution, as well as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities were measured in Zibo-3 and Zibo-9 cultivars after exposure to two different concentrations of Cd (2mg/L and 5mg/L) for 10 days. This research revealed Cd accumulation characteristics in castor are root>stem>young leaf>old leaf. Castor tolerance was Cd dose exposure and the cultivars themselves dependent. Investigation of subcellular Cd partitioning showed that Cd accumulated mainly in the heat stable protein (HSP) and cellular debris fractions, followed by the Cd rich granule (MRG), heat denatured protein (HDP), and organelle fractions. With increasing Cd concentration in nutrient solution, the decreased detoxified fractions (BDM) and the increased Cd-sensitive fractions (MSF) in young leaves may indicate the increased Cd toxicity in castor cultivars. The BDM-Cd fractions or MSF-Cd in old leaves may be linked with Cd tolerance of different cultivars of castor. The antioxidant enzymes that govern Cd detoxification were not found to be active in leaves. Taken together, these results indicate Cd tolerance and toxicity in castor can be explained by subcellular partitioning.

António D, Rossi F, Lettieri T. Detection of silver nanoparticles inside marine diatom Thalassiosira pseudonana by electron microscopy and focused ion beam

In the following article an electron/ion microscopy study will be presented which investigates the uptake of silver nanoparticles (AgNPs) by the marine diatom Thalassiosira pseudonana, a primary producer aquatic species. This organism has a characteristic silica exoskeleton that may represent a barrier for the uptake of some chemical pollutants, including nanoparticles (NPs), but that presents a technical challenge when attempting to use electron-microscopy (EM) methods to study NP uptake. Here we present a convenient method to detect the NPs interacting with the diatom cell. It is based on a fixation procedure involving critical point drying which, without prior slicing of the cell, allows its inspection using transmission electron microscopy. Employing a combination of electron and ion microscopy techniques to selectively cut the cell where the NPs were detected, we are able to demonstrate and visualize for the first time the presence of AgNPs inside the cell membrane.

Efficiency of cadmium chelation by phytochelatins in Nitzschia palea (Kützing) W. Smith

Phytochelatins (PCs) are thiol-rich peptides, enzymatically synthesized by plants and algae under exposure to certain metals (Cd, Pb, Zn, Ag, As, Cu). Due to their ability to bind metal ions, they play an important role in the cellular detoxification, forming stable metal-PC complexes that minimize the intracellular deleterious effects of metals. The aim of the present work was to evaluate the efficiency of PC-Cd chelation in the freshwater diatom Nitzschia palea under 0, 0.1 and 0.2 mg Cd L(-1), which induced different levels of oxidative stress. This objective was accomplished by the isolation of PC-Cd complexes through size exclusion chromatography. Two peaks were identified, corresponding to high molecular weight (HMW) and low molecular weight (LMW) complexes. In each of the complexes, Cd was quantified by inductively coupled plasma-mass spectrometry, thiol composition was determined by HPLC analysis and the efficiency of Cd chelation calculated by -SH/Cd ratios in HMW and LMW complexes at both Cd concentrations. Results showed that the majority of intracellular Cd was complexed with PCs (75.2-91.2 %). PCs-binding efficiency in this diatom species was higher at HMW than at LMW complexes and enhanced with the increase of Cd concentration exposure. Our work evidenced the important role of PCs as the main intracellular tolerance mechanism in this species. The efficiency increase of Cd-PC binding is related to the increment of PCs synthesis and to the number of Cd ions coordinately bonded to -SH groups in LMW and HMW complexes.

Ameliorating effects of extracellular polymeric substances excreted by Thalassiosira pseudonana on algal toxicity of CdSe quantum dots

Quantum dots (QDs) are engineered nanoparticles (ENs) that have found increasing applications and shown great potential in drug delivery, biological imaging and industrial products. Knowledge of their stability, fate and transport in the aquatic environment is still lacking, including details of how these nanomaterials interact with marine phytoplankton. Here, we examined the toxicity of functionalized CdSe/ZnS QDs (amine- and carboxyl-) by exposing them for five days to Thalassiosira pseudonana (marine diatom) grown under different nutrient-conditions (enriched versus nitrogen-limited media). The released polysaccharides and proteins, the major components of extracellular polymeric substances (EPS), were measured to assess their potential effects on the interactions between QDs and T. pseudonana. The partitioning of QDs was analyzed by monitoring the concentration of Cd in different size fractions of the cultures (i.e., filtrate, <0.22 μm and permeate, <3 kDa). We found that the Cd release of QDs in the T. pseudonana culture was dependent on the nutrient conditions and nature of QDs' surface coating. Both amine- and carboxyl-functionalized QDs exhibited higher rates of Cd release in N-limited cultures than in nutrient enriched cultures. The results also showed that amine-functionalized QDs aggregate with minimal Cd release, independent of nutrient conditions. Laser scanning confocal microscopy images confirmed that aggregates are composed of QDs and the culture matrix (EPS). In addition, both types of QDs showed limited toxicity to T. pseudonana. The increasing production of proteins induced by QDs suggests that extracellular proteins might be involved in the detoxification of QDs to T. pseudonana via the Cd release of QDs. Our results here demonstrated that EPS can play an ameliorating role in QD toxicity, fate and transport in the aquatic environment.

Reduced translocation of cadmium from roots is associated with increased production of phytochelatins and their precursors

Cadmium (Cd) is a non-essential trace element and its environmental concentrations are approaching toxic levels, especially in some agricultural soils. Understanding how and where Cd is stored in plants is important for ensuring food safety. In this study, we examined two plant species that differ in the distribution of Cd among roots and leaves. Lettuce and barley were grown in nutrient solution under two conditions: chronic (4 weeks) exposure to a low, environmentally relevant concentration (1.0 μM) of Cd and acute (1 h) exposure to a high concentration (5.0 mM) of Cd. Seedlings grown in solution containing 1.0 μM CdCl₂ did not show symptoms of toxicity and, at this concentration, 77% of the total Cd was translocated to leaves of lettuce, whereas only 24% of the total Cd was translocated to barley leaves. We tested the hypothesis that differential accumulation of Cd in roots and leaves is related to differential concentrations of phytochelatins (PCs), and its precursor peptides. The amounts of PCs and their precursor peptides in the roots and shoots were measured using HPLC. Each of PC₂₋₄ was synthesized in the barley root upon chronic exposure to Cd and did not increase further upon acute exposure. In the case of lettuce, no PCs were detected in the root given either Cd treatment. The high amounts of PCs produced in barley root could have contributed to preferential retention of Cd in barley roots.

Acute and chronic toxicity of polychlorinated biphenyl 126 to Tigriopus japonicus: effects on survival, growth, reproduction, and intrinsic rate of population growth

The harpacticoid copepod Tigriopus japonicus has a wide geographical distribution and is considered as a suitable model species for the assessment of toxicity of marine pollutants. The aim of the present study was to test the impacts of polychlorinated biphenyl (PCB) 126 (3,3',4,4',5-pentachlorobiphenyl) on the growth, development, and reproduction of T. japonicus in two successive generations. We first quantified the 96-h 50% lethal concentration (2.83 mg/L; all reported concentrations are nominal values), the no-observed-effect concentration (0.6 mg/L), and the lowest-observed-effect concentration (LOEC; 1.2 mg/L) of PCB126 in the nauplii. Nauplii were more sensitive than the adults, which still survived at the highest tested PCB126 concentration (8 mg/L). In the chronic toxicity testing, 10 life history traits were quantified for T. japonicus. No obvious effect on any of these traits was observed in the first generation (F0) at tested concentrations (<100 µg/L) lower than the LOEC. During the second generational life-cycle exposure (F1), however, PCB126 had an obvious toxic effect on the reproduction (>1 µg/L) and growth (>0.1 µg/L). Thus, copepods became more sensitive to PCB126 exposure as generations developed. Among the different traits tested, body size was the most sensitive parameter. Reproduction (fecundity, number of clutches, nauplii/clutch) and intrinsic population growth were also significantly impacted by PCB exposure. The survivorship, sex ratio, hatching time, and development were not affected. Environmental risk assessment of contaminants must therefore be based on a long-term multigenerational exposure to provide a realistic measurement of the influences of pollutants on aquatic life.

Genome-wide profiling of responses to cadmium in the diatom Phaeodactylum tricornutum

The only group of organisms in which a biological function for cadmium has been shown is the diatoms, which are unicellular phytoplankton. Yet diatoms exhibit similar sensitivity to Cd as other groups of phytoplankton. We have investigated responses of Cd on molecular, metabolic, and physiological levels in the diatom Phaeodactylum tricornutum. P. tricornutum apparently has a high tolerance to Cd; only minor responses were observed on growth, pigment, and transcriptional changes at cadmium concentrations of 123 μg/L. No significant changes in chlorophyll and xanthophyll levels were observed, and the very few transcripts affected strongly indicate that the cells were able to respond to the increased Cd(2+) levels without changing proteins levels. At 10 times this concentration, 1230 μg/L, a much clearer response was observed, including transcripts encoding proteins involved in metal transport, cell signaling, and detoxification processes. Our results point toward putative pathways for the removal or detoxification of Cd and its metabolites as well as a possible Cd uptake mechanism. We predict that ATPase5-1B is involved in removal of Cd by pumping Cd(2+) ions out of the cell, whereas VIT1/CCC1 sequesters Cd(2+) in the vacuole.

Cadmium sensitivity, uptake, subcellular distribution and thiol induction in a marine diatom: Recovery from cadmium exposure

Studies in the recovery from metal stress and the tolerance development to metal exposure of aquatic organisms are important for the understanding of epidemic pollution. In this study, the responses of a marine diatom, Thalassiosira nordenskioeldii, following recovery from environmental cadmium (Cd) stress were investigated. The diatoms were exposed to different concentrations of Cd for 7 days, and were then allowed different periods of time to recover. The Cd sensitivity increased after recovery from Cd stress, followed by a gradual restoration. The extent of restoration depended on both the recovery time and the environmental Cd stress during the exposure period. A complete restoration of Cd tolerance proved to be impossible for cells pre-exposed to High-Cd. The Cd cellular burden and subcellular Cd concentration decreased to the control level within the first day of recovery, indicating that the elevated sensitivity may have been due to the accumulation of functional damage caused by Cd exposure instead of a result of physical Cd accumulation. The rapid change in phytochelatins (PC) to both the increase in and the withdrawal of environmental Cd stress made it a good quantitative bioindicator of environmental Cd contamination. However, the relationships between Cd distribution in the metal sensitive fraction (MSF-Cd) or intracellular Cd to thiol ratio (intra-Cd/PC-SH) and the relative change in the median inhibition [Cd(2+)] ([Cd(2+)]-based-IC(50), i.e., Cd sensitivity) differed for the various exposure and recovery periods tested. Our study suggests that more attention should be given to the recovery of aquatic organisms from episodic metal exposure.