Abstract
An awful consequence of metal contamination in environment is one of the global problems posing severe hazardous and toxic impacts in microorganisms. The objective of the present chapter is to elucidate how metals cause toxicity at biochemical and molecular levels in microorganisms. The excess concentration of metals is responsible for causing various toxicity reactions in microbial cell, such as, over production of reactive oxygen species; protein and enzyme dysfunction, destruction of thiol and iron-sulfide cluster, metal substitution and inhibition of nutrient assimilation; lipid peroxidation; and DNA damage. Consequently, toxicity causes mutagenicity effects and/or cell death that lead to immeasurable damage in microorganisms and microbial community. The biochemical and molecular mechanisms of metal toxicity may be helpful to depth metal toxicity study in microbes and other organisms for controlling and treating the metal toxicity in further. Moreover, metal-resistant microbes have potential significance in environmental and human health perspectives.
TopIntroduction
Fast progress technologies are primarily responsible for causing tremendous environmental pollution. Metal pollution is one of the prime problems worldwide posing severe damage in environmental and human health. Metals1 are naturally occurring major constituting elements in the earth’s crust. Although, many of them play vital role in daily human life and various physiological process of organisms, it becomes potential contaminant in the environment under certain condition. Anthropogenic and geogenic activities generate massive amount of metals and its derivative chemicals and discharge as pollutants, which are awfully responsible for causing the environmental metal pollution. Therefore, metal pollution is a growing concern worldwide during last few decades due to posing severely hazardous and toxic impacts in all forms of life on earth especially microorganisms by bioconcentration, bioaccumulation and biomagnifications phenomena through food chain and food web (Jillian, Robert, & Rajakaruna, 2015, Bhakta, 2016). Microorganisms (such as bacteria, protists, fungi, yeast, algae, etc.) are omnipresent vital ecosystem components playing pivotal roles in various biogeochemical cycling process of environment (Gadd, 2010). Microbial communities are tremendously affected and intoxicated by environmental metal contamination (Bhakta, Ohnishi, Munekage, & Iwasaki, 2010; Bhakta, Munekage, Ohnishi, & Jana, 2012a; Bhakta, Ohnishi, Munekage, Iwasaki, & Wei, 2012b; Olaniran, Balgobind, & Pillay, 2013; Lenart-Boroń & Boroń, 2014; Bhakta, Munekage, Ohnishi, Jana, & Balcazar, 2014; Kuperman, Siciliano, Römbke, & Oorts, 2014).
Certain metals are crucial for structure and function of cell, and hence indispensable for the biochemical process of life. Approximately half of all known proteins are predicted to be dependent on metal atoms for their structure and their participation in key cellular processes, such as electron transfer and catalysis (Waldron & Robinson, 2009; Andreini, Bertini, & Rosato, 2004). Several metal elements e.g. Na, Mg, K, Ca, Mn, Fe, Co, Ni, Zn, Mo, etc. are essential for functioning the life. These “essential metal” elements are present in a certain concentration range in biological systems and take part in various biochemical reactions in cell. They act as essential cofactors for structural and catalytic roles in enzymes and proteins such as, metalloenzyme and metalloproteins and for stabilizing biological molecules. They are also used in electron transfer and utilization of dioxygen and osmotic balance (Bruins, Kapil, & Oehme, 2000). The essential metal elements can alter (decrease/increase) metabolic activity and lead to develop an adverse situation at too low or insufficient/high concentrations. Besides, there are a number of metals, for example Al, Au, Ag, Bi, Cd, Cr, Hg, Pb, Sn, Ti etc. (metals) and As, Sb, Te (metalloids), those have no known positive biological role and showed hazardous toxic impacts on life are commonly known as “non-essential” metals. In addition, there are some other metals those have no known biological role and may be toxic to microbes with little known biological interactions, these are Be, Cs, Li and Sr. Non-essential metals are generally toxic at very low concentrations and inhibit metabolic activity at certain concentrations in organism. Some of them are referred to as “heavy metals”. These metals are generally called as “toxic metals” under certain conditions. Such, different conditions of metals, (i) insufficient levels of essential metal ions, (ii) excess levels of essential metal ions and (iii) the presence of toxic metal ions, can cause cellular stress conditions.
Key Terms in this Chapter
Reactive Oxygen Species (ROS) Stress: Reactive Oxygen Species (ROS) is referred to oxygen containing reactive molecules and free radicals derived from metabolism of oxygen which is responsible for causing significant damages in cell. These elevated ROS mediated adverse consequences in cell can be termed known as “ROS stress”.
Metal Speciation: A metal can exist as different chemical forms/species [e.g. As(III) and As(V) species of arsenic (As)] in any single milieu which is known as metal speciation.
Ion Mimicry: The ionic mimicry is referred to the phenomena by which an unbound, cationic species of non-essential metal can behave or serve as a structural and/or functional homolog or mimic of another (usually an essential) element at the site of a carrier protein, ion channel, enzyme, structural protein, transcription factor and/or metal-binding protein.
Hard–Soft Acid Base Theory: It is an empirically derived theory of chemistry that helps us to explain the stability of metal complexes and the mechanisms of their reactions in complex mixtures of inorganic and organic reactants.
Metal Genotoxicity: It refers to the property of metals that damages the structure and function of genetic materials in a cell causing genetic mutations.
Antioxidant Depletion: It refers to the deficiency condition of antioxidant (such as, thiols, ascorbic acid, polyphenols, etc.) used in inhibiting the oxidation process of other molecules in cell.
Thiol: It is a carbon-bonded sulfhydryl (–C–SH or R–SH) group containing organosulfur compound that has capability to reduce some metal species.
Free Radical: Free radical is highly reactive atomic or group of atom or molecular with unpaired (odd) number of electrons that have independent existence and capable to start a chain of reactions. Many transition metals are considered as free radical.
Reduction Potential: Reduction potential or redox potential or oxidation/reduction potential (i.e., ORP or pE or e, or E h ) is a measure [by volts (V) or millivolts (mV)] of the tendency of a chemical species to acquire electrons and thereby be reduced.
Donor Atom Selectivity: It is a specific principle of the atom of donor ligand that shows affinity to select specific metals and forms metal-ligand coordination complex.
Competitive Inhibition: In this chapter, it refers to the phenomenon by which the entry of essential metal ions into cell is interfered by other non-essential metal ions due to having ion mimic property that results in nutrient starvation of cell.
Iron-Sulfur (Fe–S) Cluster: Iron-sulfur (Fe-S) clusters, consisting of iron and elemental sulfur, are generally characteristically present in proteins and enzymes at various molar ratios which play important role to avoid toxicity in cell.
Fenton Reaction: Fenton reaction, a process of oxidation of organic molecules by Fe(II) and hydrogen peroxide (H 2 O 2 ), is very common in biological system first described by H. J. H. Fenton in 1894.