Recent advancements in cadmium-microbe interactive relations and their application for environmental remediation: a mechanistic overview

The toxic and persistent nature of cadmium (Cd) in the environment has become a matter of concern with its drastic increase in the concentrations over past few decades. Among the various techniques, the microbial remediation has been accepted as an effective decontamination tool for environmental applications, which is sustainable over a period of time. The Cd decontamination potential of the microbes depends on various internal and external factors that play a crucial role in selection of the microbes for application in a particular environment. Thus, it is important to understand the role of these factors for optimal application of the microbes. This study provides an insight into the mechanisms involved between the microbes and the environmental Cd. The study also briefly reviews the mathematical models that have been used to predict the remediation potential of the microbes and the kinetics involved during the process. A critical analysis of the recent advancements in the techniques for use of bacteria, fungi, and algal cells to remove Cd has been also presented in the manuscript.

Oleic Acid Facilitates Cd Excretion by Increasing the Abundance of Burkholderia in Cd-Exposed Mice

As a global pollutant, cadmium (Cd) can easily enter the body through food chains, threatening human health. Most Cd is initially absorbed in the gut, with the gut microbiota playing a pivotal role in reducing Cd absorption and accumulation. This study assessed the effects of three fatty acids on Cd accumulation and toxicity in Cd-exposed mice. The results showed that oleic acid (OA) was the most effective in facilitating Cd excretion in mice among these fatty acids. The use of OA led to reduced Cd accumulation in the organs and increased Cd content in the feces. The metagenomic analysis of the gut microbiota showed that the genus Burkholderia was the most significantly restored by OA in Cd-exposed mice. Burkholderia cepacia, as the type species for the genus Burkholderia, also exhibited strong Cd tolerance after treatment with OA. Furthermore, the electron microscopy analysis showed that most of the Cd was adsorbed on the surface of B. cepacia, where the extracellular polymeric substances (EPSs) secreted by B. cepacia play a key role, displaying a strong capacity for Cd adsorption. The peak at 2355 cm^-1 and the total sulfhydryl group content of EPSs showed significant increases following co-treatment with Cd and OA. The results demonstrated the potential roles that gut Burkholderia may play in OA-mediated Cd excretion in mice.

Altered proteome in translation initiation fidelity defective eIF5G31R mutant causes oxidative stress and DNA damage

The recognition of the AUG start codon and selection of an open reading frame (ORF) is fundamental to protein biosynthesis. Defect in the fidelity of start codon selection adversely affect proteome and have a pleiotropic effect on cellular function. Using proteomic techniques, we identified differential protein abundance in the translation initiation fidelity defective eIF5^G31R mutant that initiates translation using UUG codon in addition to the AUG start codon. Consistently, the eIF5^G31R mutant altered proteome involved in protein catabolism, nucleotide biosynthesis, lipid biosynthesis, carbohydrate metabolism, oxidation–reduction pathway, autophagy and re-programs the cellular pathways. The utilization of the upstream UUG codons by the eIF5^G31R mutation caused downregulation of uridylate kinase expression, sensitivity to hydroxyurea, and DNA damage. The eIF5^G31R mutant cells showed lower glutathione levels, high ROS activity, and sensitivity to H₂O₂.

Molecular Biology of Cadmium Toxicity in Saccharomyces cerevisiae

Cadmium (Cd) is a toxic heavy metal mainly originating from industrial activities and causes environmental pollution. To better understand its toxicity and pollution remediation, we must understand the effects of Cd on living beings. Saccharomyces cerevisiae (budding yeast) is an eukaryotic unicellular model organism. It has provided much scientific knowledge about cellular and molecular biology in addition to its economic benefits. Effects associated with copper and zinc, sulfur and selenium metabolism, calcium (Ca²⁺) balance/signaling, and structure of phospholipids as a result of exposure to cadmium have been evaluated. In yeast as a result of cadmium stress, "mitogen-activated protein kinase," "high osmolarity glycerol," and "cell wall integrity" pathways have been reported to activate different signaling pathways. In addition, abnormalities and changes in protein structure, ribosomes, cell cycle disruption, and reactive oxygen species (ROS) following cadmium cytotoxicity have also been detailed. Moreover, the key OLE1 gene that encodes for delta-9 FA desaturase in relation to cadmium toxicity has been discussed in more detail. Keeping all these studies in mind, an attempt has been made to evaluate published cellular and molecular toxicity data related to Cd stress, and specifically published on S. cerevisiae.

Comparative Study of Cytotoxicity, DNA Damage and Oxidative Stress Induced by Heavy Metals Cd(II), Hg(II) and Cr(III) in Yeast

Wide range of applications of heavy metals and improperly discarded their castoffs possess serious threats to environment and human health. In this study, cytotoxicity, DNA damage and oxidative stress induced by Cd(II), Hg(II) and Cr(III) were comparatively studied in yeast Saccharomyces cerevisiae. Cd(II), Hg(II), and Cr(III) all produced strong cytotoxicity resulting in growth inhibition and cell mortality to varying degrees (Hg(II) > Cd(II) > Cr(III)). Hg(II) produced more oxidative stress. Cr(III) caused more serious DNA damage in vitro. Cd(II) also caused both obvious DNA damage and oxidative stress at higher concentration, but not as efficiently as Cd(II) and Hg(II). A further null mutation sensitivity assay showed that the relative sensitivity of rad1∆ to the metals was Cr(III) > Cd(II) > Hg(II), and that of trx1∆ to the metals was Hg(II) > Cd(II) > Cr(III). These data provide a clear evidence that the Cr(III) can cause significant DNA damage and potential genotoxicity; Hg(II) can strongly inhibit SOD activity, produce lipid peroxidation and cause serious membrane injury, suggesting these heavy metals can cause different toxic effects in different ways.

Ascorbic acid mitigates cadmium-induced stress, and contributes to ionome stabilization in fission yeast

Cadmium is a highly toxic environmental pollutant which through enhancement of reactive oxygen species (ROS) production triggers oxidative stress to the cell. Cell growth, a fundamental feature of all living organisms is closely connected to the cell shape and homeostasis. As these processes largely depend on cell fitness status and environmental conditions we have analyzed, the impact of different cadmium concentrations and the effect of ascorbic acid (ascorbate, AsA) supplementation on cell growth parameters, cell morphology, and ionome balance maintenance in Schizosaccharomyces pombe. We show that cadmium causes membrane lipid peroxidation resulting in cell shape alterations leading to growth impairment and through mineral elements disequilibrium affects ionome homeostasis in a dose- and time-dependent manner. AsA recognized as one of the most prominent antioxidants, when overdosed, displays considerable pro-oxidant activity, though precise dosing of its supplementation is desired. We present here that AsA under efficacious concentration largely improves cell condition affected by cadmium. Although, we clearly demonstrate the beneficial feature of AsA, further studies are required to fully understand its protective nature on cell homeostasis maintenance under conditions of the broken environment.

Protective role of l-threonine against cadmium toxicity in Saccharomyces cerevisiae

Environment and food contamination with cadmium (Cd) can cause serious toxicity, posing a severe threat to agricultural production and human health. However, how amino acids contribute to defenses against oxidative stress caused by Cd in cells is not fully understood. As a model eukaryote with a relatively clear genetic background, Saccharomyces cerevisiae has been commonly used in Cd toxicity research. To gain insight into Cd toxicity and cell defenses against it, 20 amino acids were screened for protective roles against Cd stress in S. cerevisiae. The results showed that threonine (Thr, T) had the strongest protective effect against Cd-induced mortality and membrane damage in the cells. Compared to the antioxidant vitamin C (VC), Thr exhibited a higher efficacy in restoring the superoxide dismutase (SOD) activity that was inhibited by Cd but not by H₂ O₂ in vivo. Thr exhibited evident DPPH (2,2-diphenyl-1-picrylhydrazyl) activity but weak ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-9 sulfonic acid)) scavenging activity, giving it a weaker effect against Cd-induced lipid peroxidation and superoxide radical O^2- , compared to VC. More importantly, compared to the chelating agent EDTA, Thr showed stronger chelation of Cd, giving it a stronger protective effect on SOD against Cd than VC in vitro. The results of the in vivo and in vitro experiments revealed that the role Thr plays in cell defenses against Cd may be attributed to its protection of the SOD enzyme, predominantly through the preferential chelation of Cd. Our results provide insights into the protective mechanisms of amino acid Thr that ameliorate Cd toxicity and suggest that a supplement of Thr might help to reduce Cd-induced oxidative damage.

Microbial Cd(II) and Cr(VI) resistance mechanisms and application in bioremediation

The heavy metals cadmium (Cd) and chromium (Cr) are extensively used in industry and result in water and soil contamination. The highly toxic Cd(II) and Cr(VI) are the most common soluble forms of Cd and Cr, respectively. They enter the human body through the food chain and drinking water and then cause serious illnesses. Microorganisms can adsorb metals or transform Cd(II) and Cr(VI) into insoluble or less bioavailable forms, and such strategies are applicable in Cd and Cr bioremediation. This review focuses on the highlighting of novel achievements on microbial Cd(II) and Cr(VI) resistance mechanisms and their bioremediation applications. In addition, the knowledge gaps and research perspectives are also discussed in order to build a bridge between the theoretical breakthrough and the resolution of Cd(II) and Cr(VI) contamination problems.

Cadmium-Induced Cell Homeostasis Impairment is Suppressed by the Tor1 Deficiency in Fission Yeast

Cadmium has no known physiological function in the body; however, its adverse effects are associated with cancer and many types of organ system damage. Although much has been shown about Cd toxicity, the underlying mechanisms of its responses to the organism remain unclear. In this study, the role of Tor1, a catalytic subunit of the target of rapamycin complex 2 (TORC2), in Cd-mediated effects on cell proliferation, the antioxidant system, morphology, and ionome balance was investigated in the eukaryotic model organism Schizosaccharomyces pombe. Surprisingly, spectrophotometric and biochemical analyses revealed that the growth rate conditions and antioxidant defense mechanisms are considerably better in cells lacking the Tor1 signaling. The malondialdehyde (MDA) content of Tor1-deficient cells upon Cd treatment represents approximately half of the wild-type content. The microscopic determination of the cell morphological parameters indicates the role for Tor1 in cell shape maintenance. The ion content, determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), showed that the Cd uptake potency was markedly lower in Tor1-depleted compared to wild-type cells. Conclusively, we show that the cadmium-mediated cell impairments in the fission yeast significantly depend on the Tor1 signaling. Additionally, the data presented here suggest the yet-undefined role of Tor1 in the transport of ions.

Effect of cadmium on mRNA mistranslation in Saccharomyces cerevisiae

Although highly accurate molecular processes and various messenger RNA (mRNA) quality control and ribosome proofreading mechanisms are used by organisms to transcribe their genes and maintain the fidelity of genetic information, errors are inherent in all biological systems. Low-level translation errors caused by an imbalance of homologous and nonhomologous amino acids caused by stress conditions are particularly common. Paradoxically, advantageous phenotypic diversity can be generated by such errors in eukaryotes through unknown molecular processes. Here, we found that the significant cadmium-resistant phenotype was correlated with an increased mistranslation rate of the mRNA in Saccharomyces cerevisiae. This phenotypic change was also related to endogenous sulfur amino acid starvation. Compared with the control, the mistranslation rate caused by cadmium was significantly increased (p < .01). With the increase of cysteine contents in medium, the mistranslation rate of WT(BY4742a) decreased significantly (p < .01). This demonstrates that cadmium treatment and sulfur amino acid starvation both can induce translation errors. Although cadmium uptake is independent of the Sul1 transporter, cadmium-induced mRNA mistranslation is dependent on the sulfate uptake of the Sul1p transporter. Furthermore, cadmium-induced translation errors depend on methionine biosynthesis. Taken together, cadmium causes endogenous sulfur starvation, leading to an increase in the mRNA mistranslation, which contributes to the resistance of yeast cells to cadmium. We provide a new pathway mediating the toxicity of cadmium, and we propose that altering mRNA mistranslation may portray a different form of environmental adaptation.

Oleic Acid Alleviates Cadmium-Induced Oxidative Damage in Rat by Its Radicals Scavenging Activity

Toxic heavy metal cadmium wildly pollutes the environment and threats the human health. Effective treatment of cadmium-induced toxicity and organ damage is an important issue. Cadmium causes organ damage through inducing oxidative stress. Our previous study also found oleic acid (OA) synthesis-related gene can confer resistance to cadmium and alleviate cadmium-induced stress in yeast. However, its alleviation mechanism on cadmium stress especially in animals is still unclear. In this study, the alleviative effects of OA on cadmium and cadmium-induced oxidative stress in rats were investigated. Oral administration of 10, 20, and 30 mg/kg/day OA can significantly increase the survival rate of rats intraperitoneally injected with 30 mg/kg/day cadmium continuously for 7 days. Similar to ascorbic acid (AA), OA can significantly reduce the cadmium-induced lipid peroxidation in multiple organs of rats. The investigation of OA on superoxide dismutase (SOD) activity showed that OA increased the SOD activity of cadmium-treated rat organs. More important, OA reduced the level of superoxide radical O^2- of cadmium-treated rat organs. And OA exhibited a strong DPPH radicals scavenging activity at dose of 10, 20 and 30 mg/mL, which may contributed to alleviating cadmium-induced oxidative stress. This study revealed that OA could significantly alleviate cadmium stress via reducing cadmium-induced lipid peroxidation and SOD activity inhibition through its radicals scavenging activity.