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Mercury Toxicity
Published in Edgardo R. Donati, Heavy Metals in the Environment, 2018
Mohammed H. Abu-Dieyeh, Kamal Usman, Haya Alduroobi, Mohammad Al-Ghouti
Microorganisms have developed an extensive defense mechanism against mercury toxicity (Fig. 1). “Mercuric reductase” is an essential bacterial cytoplasmic enzyme that transforms ionic mercury (Hg2+) to elemental mercury (Hg0), which is then diffused out of the bacterial cell (Wagner-Döbler, 2003). The most widely studied defense mechanism is the degradative enzymatic pathways of organic to inorganic and ionic form (Hg2+). It is reduced to metallic mercury (Hg0), removed from the cells, and eventually volatized to the atmosphere (Chien et al., 2012). However, Wagner-Döbler (2003) reports that instead of releasing metallic mercury back to the atmospheric environment, it can be accumulated in a mechanical bioreactor. The sequestration of mercury by microbial cell surface components and dead cells is also a common occurrence (François et al., 2012). Adsorption of ionic mercury (Hg2+) by the secretion of exo-polymers under suitable condition have been reported in some resistant microbes (Mahbub et al., 2016).
Genetic Strategies for Strain Improvement
Published in Daphne L. Stoner, Biotechnology for the Treatment of Hazardous Waste, 2017
One mechanism of microbial mercury resistance is encoded by a small operon, called the mer operon, carrying genes for mercuric reductase and mercury uptake whose expression is induced by mercury. This enzyme system catalyzes the reduction of Hg (II) to mercury metal Hg (0), which is volatile, and causes removal of the mercury from the local environment. The first step in mercury metabolism is an active transport, mercury-inducible mechanism comprising three to four genes.57 An active transport system for mercury can cause accumulation of this toxic heavy metal in the cells and in instances where either the accompanying mercuric reductase activity has been deleted by cloning or mutation, or the over-expression of uptake proteins causes excessive mercury uptake, the cells display a mercury hypersensitive phenotype.58 Once mercury is transported into the cell, it becomes a substrate for mercuric reductase (the mer A gene product). The inducible NADPH-dependent mercuric reductase converts Hg (II) to Hg (0) causing the volatilization of this metal.
Role of Enzymes in the Bioremediation of Refractory Pollutants
Published in Maulin P. Shah, Removal of Refractory Pollutants from Wastewater Treatment Plants, 2021
Viresh R. Thamke, Ashvini U. Chaudhari, Kisan M. Kodam, Jyoti P. Jadhav
Heavy metal pollutants like Cr, As, Hg, Cd, etc., cause major environmental concern because of their higher toxicity. Chromate is a serious environmental toxicant due to the wide use of chromium compounds in industries such as tanning, corrosion control, plating, pigment manufacturing, and nuclear weapons production. The enzymatic detoxification of Cr(VI) to Cr(III) has been reported by chromate reductase isolated from Escherichia coli ATCC 33456, Lysinibacillus sp., Pseudomonas putida, and Lysinibacillus sphaericus G1 (Bae et al. 2005; Chaudhari et al. 2013). Chromate reductase converts the highly toxic and soluble hexavalent chromium to its less toxic trivalent form having much lower solubility; thereby the reduction by the enzymes affords a means of chromate bioremediation. Chromate reductase from Pseudomonas putida and YieF from Escherichia coli both reduced Cr(VI) (kcat/Km = ~2 × 104 M−1 • s−1) as well as reduced quinones, potassium ferricyanide, and 2,6-dichloroindophenol. YieF protein effectively reduces high-valence metals like V(V), Mo(VI), and methylene blue. Thus, chromate reductases from Pseudomonas putida and YieF from Escherichia coli appears to have a broader substrate range (Ackerley et al. 2004). Nitroreductases from Escherichia coli DH5 and Vibrio harveyi KCTC 2720 show nitrofurazone and 2,4,6-trinitrotoluene as well as chromate reductase activity (Kwak et al. 2003). The involvement of Cytochrome C7 in the reduction of Cr(VI) to Cr(III) from Desulfuromonas acetoxidans has also been demonstrated. Mercury can exist in three forms in the environment: metallic Hg(0), inorganic Hg(I)/Hg(II), and organic Hg. Mercuric reductase is an important enzyme involved in the detoxification of mercury by the reduction of Hg(II) to metallic Hg in effluents. Mercuric reductases from Lysinibacillus sphaericus, Pseudomonas aeruginosa PA09501, and Bacillus sp. strain RC607 have been reported (Schiering et al. 1991; Bafana et al. 2013).
Field trials of phytomining and phytoremediation: A critical review of influencing factors and effects of additives
Published in Critical Reviews in Environmental Science and Technology, 2020
Liuwei Wang, Deyi Hou, Zhengtao Shen, Jin Zhu, Xiyue Jia, Yong Sik Ok, Filip M. G. Tack, Jörg Rinklebe
Phytovolatilization: volatilization of Hg by mer operon modified plants has been investigated. Studies by Haque, Zeyaullah, Nabi, Srivastava, and Ali (2010) and Hussein, Ruiz, Terry, and Daniell (2007) used transgenic tobacco plants with merA gene to volatilize Hg from soils. The mercuric reductase enzyme (MerA) encoded by merA gene transformed divalent mercury [Hg(II)] to elemental mercury [Hg(0)], the latter of which could then volatize rapidly.