Adaptation strategies of two closely related Desmodesmus armatus (green alga) strains contained different amounts of cadmium: a study with light-induced synchronized cultures of algae

The cadmium tolerance and bioaccumulation mechanism of Tetratostichococcus sp. P1: insight from transcriptomics analysis

Cadmium (Cd) has become a severe issue in relatively low concentration and attracts expert attention due to its toxicity, accumulation, and biomagnification in living organisms. Cd does not have a biological role and causes serious health issues. Therefore, Cd pollutants should be reduced and removed from the environment. Microalgae have great potential for Cd absorption for waste treatment since they are more environmentally friendly than existing treatment methods and have strong metal sorption selectivity. This study evaluated the tolerance and ability of the microalga Tetratostichococcus sp. P1 to remove Cd ions under acidic conditions and reveal mechanisms based on transcriptomics analysis. The results showed that Tetratostichococcus sp. P1 had a high Cd tolerance that survived under the presence of Cd up to 100 µM, and IC50, the half-maximal inhibitory concentration value, was 57.0 μM, calculated from the change in growth rate based on the chlorophyll content. Long-term Cd exposure affected the algal morphology and photosynthetic pigments of the alga. Tetratostichococcus sp. P1 removed Cd with a maximum uptake of 1.55 mg g-1 dry weight. Transcriptomic analysis revealed the upregulation of the expression of genes related to metal binding, such as metallothionein. Group A, Group B transporters and glutathione, were also found upregulated. While the downregulation of the genes were related to photosynthesis, mitochondria electron transport, ABC-2 transporter, polysaccharide metabolic process, and cell division. This research is the first study on heavy metal bioremediation using Tetratostichococcus sp. P1 and provides a new potential microalga strain for heavy metal removal in wastewater.[Figure: see text]Abbreviations:BP: Biological process; bZIP: Basic Leucine Zipper; CC: Cellular component; ccc1: Ca (II)-sensitive cross complementary 1; Cd: Cadmium; CDF: Cation diffusion facilitator; Chl: Chlorophyll; CTR: Cu TRansporter families; DAGs: Directed acyclic graphs; DEGs: Differentially expressed genes; DVR: Divinyl chlorophyllide, an 8-vinyl-reductase; FPN: FerroportinN; FTIR: Fourier transform infrared; FTR: Fe TRansporter; GO: Gene Ontology; IC50: Growth half maximal inhibitory concentration; ICP: Inductively coupled plasma; MF: molecular function; NRAMPs: Natural resistance-associated aacrophage proteins; OD: Optical density; RPKM: Reads Per Kilobase of Exon Per Million Reads Mapped; VIT1: Vacuolar iron transporter 1 families; ZIPs: Zrt-, Irt-like proteins.

Effect of mercury in the influx and efflux of nutrients in the microalga Desmodesmus armatus

Anthropogenic activities such as mining and the metallurgical industry are the main sources of mercury contamination. Mercury is one of the most serious environmental problems in the world. This study aimed to investigate, using experimental kinetic data, the effect of different inorganic mercury (Hg²⁺) concentrations on the response of microalga Desmodesmus armatus stress. Cell growth, nutrients uptake and mercury ions from the extracellular medium, and oxygen production were determined. A Compartment Structured Model allowed elucidating the phenomena of transmembrane transport, including influx and efflux of nutrients, metal ions and bioadsorption of metal ions on the cell wall, which are difficult to determine experimentally. This model was able to explain two tolerance mechanisms against mercury, the first one was the adsorption of Hg²⁺ions onto the cell wall and the second was the efflux of mercury ions. The model predicted a competition between internalization and adsorption with a maximum tolerable concentration of 5.29 mg/L of HgCl₂. The kinetic data and the model showed that mercury causes physiological changes in the cell, which allow the microalga to adapt to these new conditions to counteract the toxic effects. For this reason, D. armatus can be considered as a Hg-tolerant microalga. This tolerance capacity is associated with the activation of the efflux as a detoxification mechanism that facilitates the maintenance of the osmotic balance for all the modeled chemical species. Furthermore, the accumulation of mercury in the cell membrane suggests the presence of thiol groups associated with its internalization, leading to the conclusion that metabolically active tolerance mechanisms are dominant over passive ones.

Metabolomic analysis of Scenedesmus obliquus reveals new insights into the phytotoxicity of imidazolium nitrate ionic liquids

Due to the persistence of ionic liquids (ILs) in aquatic environments, it is necessary to reveal their ecological risk to aquatic organisms. Herein, the biotoxicity of two alkyl-methylimidazolium nitrate ILs ([C₁₀mim]NO₃ and [C₁₂mim]NO₃) against Scenedesmus obliquus were studied. Results showed that the growth inhibition of S. obliquus increased with increasing concentrations of ILs, maximum values of 94.61% at 4 mg/L of [C₁₀mim]NO₃ and 97.34% at 0.8 mg/L of [C₁₂min]NO₃ were observed. The fluorescence parameters of photosystem II, such as light quantum yield and electron transfer rate, showed a negative relationship with the exposure dose. [C₁₂mim]NO₃ had a more significant effect than [C₁₀mim]NO₃. Moreover, the redox homeostasis of algae was disrupted; the accumulation of reactive oxygen species, leading to obvious inhibition of superoxide dismutase and catalase activities was observed. A metabolomic analysis indicated that the contents of most metabolites were reduced significantly, and fructose and galactose decreased significantly by 42.3% and 88.6%, respectively, in the [C₁₀mim]NO₃ treatment compared to those in the control. The inhibition of amino acid biosynthesis and glyoxylate and dicarboxylate metabolism explained the more serious biotoxicity of [C₁₂mim]NO₃ than that of [C₁₀mim]NO₃. This study facilitates a better understanding of the environmental safety and ecological risks of ILs.

Phycoremediation of X-ray developer solution towards silver removal with concomitant lipid production

The present research is mainly focusing on the characterization of X-ray developer solution and its toxic tolerance studies with Desmodesmus armatus towards the phycoremediation studies for removal of pollutants, silver, and concomitant lipid production. The characterization results suggested the presence of 1.229 ± 0.004 g/l BOD, 27.29 ± 0.230 g/l COD with a silver content of 0.01791 ± 0.000 g/l. The tolerance and toxicity limits of with X-ray developer solution reveals the remarkable growth of microalgae in 3:1.dilution ratio of BBM in the X-ray developer solutions. The phycoremediation with 19 days period shown the noticeable results with a relative BOD (20.86%), COD (13.88%), with 57.10% corresponding total phosphorous removal. The phycoremediation also has proven better relative silver removal potential of 44.06% on the 19th day with concomitant 1.392% lipid production. Overall, the present study shows the potential phycoremediation strategy of hazardous X-ray developer solutions with possible concurrent lipid production through a sustainable approach.

Determination of the Ecotoxicity of Herbicides Roundup® Classic Pro and Garlon New in Aquatic and Terrestrial Environments

Herbicides help increase agricultural yields significantly, but they may negatively impact the life of non-target organisms. Modifying the life cycle of primary producers can affect other organisms in the food chain, and consequently in the whole ecosystem. We investigated the effect of common herbicides Roundup® Classic Pro (active substance glyphosate) and Garlon New (triclopyr and fluroxypyr) on aquatic organisms duckweed Lemna minor and green algae Desmodesmus subspicatus, and on the enzymatic activity of soil. We also compared the effects of Roundup® Classic Pro to that of a metabolite of its active substance, aminomethylphosphonic acid (AMPA). The results of an algal growth test showed that AMPA has a 1.5× weaker inhibitory effect on the growth of D. subspicatus than the Roundup formula, and the strongest growth inhibition was caused by Garlon New (IC50Roundup = 267.3 µg/L, IC50Garlon = 21.0 µg/L, IC50AMPA = 117.8 mg/L). The results of the duckweed growth inhibition test revealed that Roundup and Garlon New caused 100% growth inhibition of L. minor even at significantly lower concentrations than the ready-to-use concentration. The total chlorophyll content in the fronds was lowest when Garlon New was used. The highest dehydrogenase activity was observed in soil treated with Garlon New, and the lowest in soil treated with Roundup® Classic Pro. The results of this study showed that all three tested substances were ecotoxic to the tested organisms.

Sensitivity of Chlamydomonas reinhardtii to cadmium stress is associated with phototaxis

Cadmium (Cd) is a common hazardous pollutant to aquatic environments and it easily accumulates in living organisms. The roles of phototactic behavior in Cd tolerance in motile organisms are poorly explored. In this study, two Chlamydomonas reinhardtii strains, a wild type with positive phototaxis (CC125) and a negatively phototactic mutant (agg1), were used to assess the effects of phototaxis on Cd-induced toxicity to algae. Exposure to Cd inhibited the cell growth and photosynthetic activities, reduced the photosynthetic pigment content, and enhanced the intracellular oxidative stress of algae. Well buffered by EDTA in algae medium, the concentrations of Cd causing 50% growth inhibition (EC50) of CC125 and agg1 for 72 h of exposure were 55.96 and 77.20 μM L-1, respectively. Photosystem II activities in CC125 were more sensitive to Cd than agg1 at 60 μM L-1 Cd. In addition, agg1 accumulated less intracellular Cd than CC125. The changes of extracellular polymeric substances and intracellular response to Cd stress might be related to the different tolerances of the two algae to Cd. Taken together, phototaxis was demonstrated to be associated with Cd-induced toxicity to C. reinhardtii.

Acid-tolerant microalgae can withstand higher concentrations of invasive cadmium and produce sustainable biomass and biodiesel at pH 3.5

Two acid-tolerant microalgae, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, originally isolated from non-acidophilic environment, were tested for their ability to withstand higher concentrations of an invasive heavy metal, cadmium (Cd), at an acidic pH of 3.5 and produce biomass rich in biodiesel. The growth analysis, in terms of chlorophyll, revealed that strain MAS1 was tolerant even to 20 mg L^-1 of Cd while strain MAS3 could withstand only up to 5 mg L^-1. When grown in the presence of 2 mg L^-1, a concentration which is 400-fold higher than that usually occurs in the environment, the microalgal strains accumulated >58% of Cd from culture medium at pH 3.5. FTIR analysis of Cd-laden biomass indicated production of significant amounts of biodiesel rich in fatty acid esters. This is the first study that demonstrates the capability of acid-tolerant microalgae to grow well and remove Cd at acidic pH.

An investigation onto Cd toxicity to freshwater microalga Chlorella sorokiniana in mixotrophy and photoautotrophy: A Bayesian approach

Aquatic ecosystems are composed by a myriad of dissolved organic materials that can be assimilated by microalgae, while they can perform photosynthesis, this is refereed as mixotrophy. However, ecotoxicological tests usually consider only the photoautotrophic metabolism. This research investigated the ecotoxicological differences between photoautotrophy and mixotrophy in Chlorella sorokiniana exposed to cadmium (Cd). Chlorophyll a, photosynthetic efficiency (Fv/Fm), cell viability, biochemical composition and pH were used to monitor possible toxic effects at 72 h cultures. Glucose (1 g.L^-1) was used as organic carbon source. To evaluate the probability of the photoautotrophic culture being more affected by Cd than the mixotrophic one, Bayesian statistical analysis was performed. The photoautotrophic cultures were more affected by Cd than the mixotrophic ones, with reduction of all evaluated parameters, except for protein concentration. However, in mixotrophic cultures, no changes in protein concentration and proteins:carbohydrates ratio were observed, and chlorophyll a, Fv/Fm and cell viability were only affected at the high Cd concentrations (range ln -11.5 to -9.4). However, both mixotrophy and photoautotrophy had the same probability of having the carbohydrates concentration affected by Cd. We conclude that the microalgae in mixotrophy were more resistant to the Cd than in photoautotrophy. In addition, we showed that under photoautotrophy Fv/Fm decreased linearly as Cd concentration increased, but in mixotrophy no effect was observed up to 10^-5 molL^-1 Cd, after which it decreased. We rationale that the reduced photosynthetic capacity under mixotrophy can end up reducing the release of oxygen gas, which can compromise the entire aquatic ecosystem.

Physiological changes in Chlamydomonas reinhardtii after 1000 generations of selection of cadmium exposure at environmentally relevant concentrations

Cadmium (Cd) is a nonessential and toxic trace element widely existing in waters through various anthropogenic activities such as mining and waste disposal. The physiological responses of aquatic organisms to long-term Cd exposure at environmentally relevant concentrations are still not well explored. In the present study, two strains of unicellular green algae Chlamydomonas reinhardtii, a walled strain CC125 and a wall-less strain CC406 were selected to investigate the physiological changes of aquatic organisms after long-term Cd exposure at environmentally relevant concentrations (4.92 and 49.2 μg L-1). After about 1000 generations of selection, all of the two strains showed higher intracellular lipid peroxidation and lower photosynthetic activities, and failed to evolve specific adaptation to high levels of Cd (4.92 mg L-1) compared to the control. However, short-term low dose Cd exposure exerted hormetic effects on C. reinhardtii and the hormetic stimulation of growth rate, chlorophyll contents and photochemical activities at the lower concentration of Cd (4.92 μg L-1) groups were more pronounced than those at higher ones (49.2 μg L-1). Taken together, this study confirmed that long-term exposure to Cd at environmentally relevant concentrations which were regarded as nontoxic in acute experiments would produce toxic effects on C. reinhardtii and should be paid more attention.

Impact of humic acid on the accumulation of metals by microalgae

Indirect impact of humic acid (HA) on metal accumulation and toxicity (Cd, Ni, Pb, and Hg; 100 μM; 24 h of exposure) in Scenedesmus quadricauda was studied. Algae were pre-cultured on solid (10 and 100 mg HA/L) or in liquid media (1, 5, and 10 mg HA/L) over 30 days and then exposed to metals mentioned above. Accumulation of applied metals irrespective of pre-culture increased in the order Ni < Cd < Pb < Hg. Algae pre-cultured on solid HA-enriched media accumulated more Cd (+ 46% at 10 mg HA/L), Ni (+ 50 and + 81% at 10 and 100 mg HA/L, respectively), and Pb (+ 15% at 100 mg HA/L) but the impact on Hg amount was not detected. Potassium and calcium decreased in response to all metals (K strongly under Hg excess) and HA had negligible impact. Interestingly, fluorescence microscopy detection of reactive oxygen species/nitric oxide (ROS/NO) balance showed that HA pre-culture suppressed ROS signal and stimulated NO signal in response to Cd (indicating positive impact of HA) while ROS signal in Ni and Pb treatments rather increased but NO signal decreased as expected from elevated Ni and Pb accumulation. Hg had clearly the most toxic impact on the ROS/NO balance. Algae pre-cultured in liquid HA-enriched media showed significantly increased Ni accumulation only (+ 14% at a dose 10 mg HA/L). Present study for the first time showed that humic acid may indirectly affect accumulation of metals and that solid HA-enriched medium used for pre-culture is more suitable to increase accumulation of metals by algae.

Antioxidative Responses of Microalgae to Heavy Metals

Microalgae are unicellular free living entities and therefore their responses to excess of heavy metals must be faster and more efficient than those in vascular plants protected by various types of tissues. Up to date, numerous studies reported metal bioaccumulation potential of algae but metabolic responses have relatively rarely been monitored. Here I provide basic overview of quantitative changes of ascorbic acid (AA), reduced glutathione (GSH), phytochelatins (PCs) and selected related enzymes (ascorbate peroxidase and glutathione reductase) in some common microalgae exposed to various metals (cadmium mainly). Despite various culture and exposure conditions, some common signs of metal toxicity (including e.g. enhancement of phytochelatin biosynthesis) are clearly identifiable in algae. Other metal chelators such as organic acids are also briefly mentioned. Comparison with macroalgae, mosses and vascular plants is discussed in terms of basal values and evolutionary similarities.

Calcium availability but not its content modulates metal toxicity in Scenedesmus quadricauda

Impact of calcium nutrition (pre-culture on solid medium with standard or elevated Ca dose, i. e. 0.17 and 4.40mM marked as low and high Ca) on acute metal toxicity (Cd, Mn and Pb, 24h of exposure to 10µM) in freshwater green alga Scenedesmus quadricauda was studied. Surprisingly, Ca content differed only slightly between low and high Ca samples and applied metals rather suppressed its amount. Na content was higher in metal-exposed high Ca samples, indicating that Ca/Na ratio may affect accumulation of metals. Content of heavy metals increased in order Cd < Mn < Pb and high Ca samples contained less metal than low Ca samples at least in absorbed fraction. Accumulation of ascorbic acid and thiols (GSH - glutathione and PC2 - phytochelatin 2) was affected mainly by Cd, GSH also by Mn and PC2 by Pb with often significant differences between low Ca and high Ca samples. Calcium nutrition also affected responses of algae to metals at the level of antioxidative enzyme activities (SOD, APX, and CAT) and elevated values were typically found in high Ca samples while ROS (hydrogen peroxide and superoxide radical) were mainly depleted in Mn treatment. These data confirm that Ca nutrition affects accumulation of metals in algae and metabolic parameters as observed in vascular plants but, unlike them, rather Ca/Na ratio than absolute Ca content seems to regulate the uptake of metals.

Effect of Nano-Al₂O₃ on the Toxicity and Oxidative Stress of Copper towards Scenedesmus obliquus

Nano-Al₂O₃ has been widely used in various industries; unfortunately, it can be released into the aquatic environment. Although nano-Al₂O₃ is believed to be of low toxicity, it can interact with other pollutants in water, such as heavy metals. However, the interactions between nano-Al₂O₃ and heavy metals as well as the effect of nano-Al₂O₃ on the toxicity of the metals have been rarely investigated. The current study investigated copper toxicity in the presence of nano-Al₂O₃ towards Scenedesmus obliquus. Superoxide dismutase activity and concentration of glutathione and malondialdehyde in cells were determined in order to quantify oxidative stress in this study. Results showed that the presence of nano-Al₂O₃ reduced the toxicity of Cu towards S. obliquus. The existence of nano-Al₂O₃ decreased the growth inhibition of S. obliquus. The accumulation of copper and the level of oxidative stress in algae were reduced in the presence of nano-Al₂O₃. Furthermore, lower copper accumulation was the main factor that mitigated copper toxicity with the addition of nano-Al₂O₃. The decreased copper uptake could be attributed to the adsorption of copper onto nanoparticles and the subsequent decrease of available copper in water.

Influence of sulphate on the reduction of cadmium toxicity in the microalga Chlamydomonas moewusii

Cadmium is considered as one of the most hazardous metals for living organism and ecosystems. Environmental factors play an important role since they alter the toxicity of metals by varying the bioavailability of these elements for the organisms. The aim of the present study was to investigate, using the freshwater microalga Chlamydomonas moewusii, the existence of an interaction between cadmium and sulphate as a factor that varied the toxicity of this metal. Different cell parameters such as cell growth, content of chlorophylls and biosynthesis of phytochelatins (PCs) were determined. A two-way ANOVA showed that the interaction had a significant effect size of 21% (p<0.001) for the growth of this microalga and around of a 6% on the content of chlorophylls/cell. The effect of this inhibition was that when the concentration of sulphate increased, a lower toxic effect of cadmium on the growth and on the content of chlorophylls was observed. In addition, the increase of sulphate concentration allowed the biosynthesis of a higher amount of PCs and/or PCs with higher chain length. This higher biosynthesis was responsible for the reduction of the toxic effect of cadmium and explained the interaction.

Jacks of metal/metalloid chelation trade in plants-an overview

Varied environmental compartments including soils are being contaminated by a myriad toxic metal(loid)s (hereafter termed as "metal/s") mainly through anthropogenic activities. These metals may contaminate food chain and bring irreparable consequences in human. Plant-based approach (phytoremediation) stands second to none among bioremediation technologies meant for sustainable cleanup of soils/sites with metal-contamination. In turn, the capacity of plants to tolerate potential consequences caused by the extracted/accumulated metals decides the effectiveness and success of phytoremediation system. Chelation is among the potential mechanisms that largely govern metal-tolerance in plant cells by maintaining low concentrations of free metals in cytoplasm. Metal-chelation can be performed by compounds of both thiol origin (such as GSH, glutathione; PCs, phytochelatins; MTs, metallothioneins) and non-thiol origin (such as histidine, nicotianamine, organic acids). This paper presents an appraisal of recent reports on both thiol and non-thiol compounds in an effort to shed light on the significance of these compounds in plant-metal tolerance, as well as to provide scientific clues for the advancement of metal-phytoextraction strategies.