Heavy metal–induced stress in eukaryotic algae—mechanisms of heavy metal toxicity and tolerance with particular emphasis on oxidative stress in exposed cells and the role of antioxidant response

Heavy metals is a collective term describing metals and metalloids with a density higher than 5 g/cm3. Some of them are essential micronutrients; others do not play a positive role in living organisms. Increased anthropogenic emissions of heavy metal ions pose a serious threat to water and land ecosystems. The mechanism of heavy metal toxicity predominantly depends on (1) their high affinity to thiol groups, (2) spatial similarity to biochemical functional groups, (3) competition with essential metal cations, (4) and induction of oxidative stress. The antioxidant response is therefore crucial for providing tolerance to heavy metal-induced stress. This review aims to summarize the knowledge of heavy metal toxicity, oxidative stress and antioxidant response in eukaryotic algae. Types of ROS, their formation sites in photosynthetic cells, and the damage they cause to the cellular components are described at the beginning. Furthermore, heavy metals are characterized in more detail, including their chemical properties, roles they play in living cells, sources of contamination, biochemical mechanisms of toxicity, and stress symptoms. The following subchapters contain the description of low-molecular-weight antioxidants and ROS-detoxifying enzymes, their properties, cellular localization, and the occurrence in algae belonging to different clades, as well as the summary of the results of the experiments concerning antioxidant response in heavy metal-treated eukaryotic algae. Other mechanisms providing tolerance to metal ions are briefly outlined at the end.

The Mitochondrial RNA Granule is Necessary for Parkinsonism-Associated Metal Toxicity

Manganese exposure causes a parkinsonian disorder, manganism, which is viewed as a neurodegenerative disorder minimally related to Parkinson s disease. We tested this hypothesis asking if there is phenotypic and mechanistic overlap between two genetic models of these diseases. We targeted for study the plasma membrane manganese efflux transporter SLC30A10 and the mitochondrial Parkinson gene PARK2. We performed comparative molecular systems studies and found that SLC30A10 and PARK2 mutations compromised the mitochondrial RNA granule as well as mitochondrial transcript processing. These shared RNA granule defects led to impaired assembly and function of the mitochondrial respiratory chain. Notably, CRISPR gene editing of subunits of the mitochondrial RNA granule, FASTKD2 and DHX30, or pharmacological inhibition of mitochondrial transcription-translation were protective rather than deleterious for survival of cells acutely exposed to manganese. Similarly, adult Drosophila mutants with defects in the mitochondrial RNA granule component scully were safeguarded from manganese-induced mortality. We conclude that the downregulation of the mitochondrial RNA granule function is a protective mechanism for acute metal toxicity. We propose that initially adaptive mitochondrial dysfunction caused by manganese exposure, when protracted, causes neurodegeneration

Molecular and cellular mechanisms of cytotoxicity in animals

The monograph is devoted to the study of the heavy metal influence on molecular and cellular processes in the animal organism. The state of the art of molecular pathway response to heavy metal toxicity is highlighted. Aspects of the impact of aluminium, cadmium, lead, mercury, and copper on biochemical features of cells are outlined. The relationship between the presence of heavy metal in an organism and disturbances in molecular and cellular progressions is revealed. The scientific monograph is proposed to ecotoxicologists, ecologists, experts interested in molecular biomarkers and bioindication. The book may be useful for students and researchers.

Selected Metal Contents and Phytochemical Profiling of Ficus capreifolia and Mangifera indica Commonly used Antidiabetic Plants from Oil Producing Region of Nigeria

Aim: The study evaluated selected metal contents and phytochemicals present in Ficus capreifolia and Mangifera indica extracts commonly used antidiabetic plants from Aluu and Bodo communities in Rivers State, Nigeria. Place and Duration of Study: University of Port-Harcourt, Choba, Rivers State, Nigeria and its environs was used between June to November, 2020. Methodology: Composite soil samples and the leaves of Ficus capreifolia and Mangifera indica were collected from Bodo and Aluu communities respectively. Atomic absorption spectrophotometer was employed for the analysis of the metals (Zn, Pb, Ni, Mn, Fe, Cu, As and Cr). The plant extracts were screened for the presence of various phytochemicals using spectrophotometric methods. Results: Flavonoid, Saponins, Tannins, Alkaloids. Terpenoid, Glycoside and Carotenoid were detected in Ficus carpreifolia obtained from Bodo and Aluu communities while Magnifera indica obtained from both communities has Alkaloids, Phenols, Flavonoids, Saponins and Tannins detected. Metals such as Zn, Pb, Mn, Fe and As were detected at Bodo and Aluu soil samples. Cu, Ni and Cr were below the detectable limit. Zn, Pb and As were above the permissible limit in Bodo soil samples while only As was above the permissible limit in Aluu community. In the plant samples, only Mn was present and was within the permissible limit in plants samples obtained from Bodo and Aluu communities while Zn, Pb, Ni, Fe, Cu, As and Cr were below the detectable limit. Conclusion: The study shows that Ficus capriefolia and Mangifera indica do not bioaccumulate Fe, Zn, Ni, Pb, As and Cr and this may be attributed to the metal intolerance potential of the plants, hence no fear of metal toxicity when using these plants for medicinal purposes. The presence of saponins, tannins and flavonoid affirmed the hypoglycemic potentials of the plants.

Nitric Oxide Ameliorates Plant Metal Toxicity by Increasing Antioxidant Capacity and Reducing Pb and Cd Translocation

Recently, nitric oxide (NO) has been reported to increase plant resistance to heavy metal stress. In this regard, an in vitro tissue culture experiment was conducted to evaluate the role of the NO donor sodium nitroprusside (SNP) in the alleviation of heavy metal toxicity in a bamboo species (Arundinaria pygmaea) under lead (Pb) and cadmium (Cd) toxicity. The treatment included 200 µmol of heavy metals (Pb and Cd) alone and in combination with 200 µM SNP: NO donor, 0.1% Hb, bovine hemoglobin (NO scavenger), and 50 µM L-NAME, N(G)-nitro-L-arginine methyl ester (NO synthase inhibitor) in four replications in comparison to controls. The results demonstrated that the addition of L-NAME and Hb as an NO synthase inhibitor and NO scavenger significantly increased oxidative stress and injured the cell membrane of the bamboo species. The addition of sodium nitroprusside (SNP) for NO synthesis increased antioxidant activity, protein content, photosynthetic properties, plant biomass, and plant growth under heavy metal (Pb and Cd) toxicity. It was concluded that NO can increase plant tolerance for metal toxicity with some key mechanisms, such as increasing antioxidant activities, limiting metal translocation from roots to shoots, and diminishing metal accumulation in the roots, shoots, and stems of bamboo species under heavy metal toxicity (Pb and Cd).