Abstract
Exposure to lead (Pb), zinc (Zn), cadmium (Cd), copper (Cu), and selenite (SeO3−2) consider the main heavy metals that threat human health. These heavy metals can interfere with the function of vital cellular components. Soil heavy metal contamination represents risks to humans and the ecosystem through drinking of contaminated groundwater, direct ingestion or the food chain, and reduction in food quality. Bioremediation means cleanup of polluted environment via transformation of toxic heavy metals into less toxic form by microbes or its enzymes. Otherwise, bioremediation by microbes has limitations like production of toxic metabolites. The efflux of metal ions outside the cell, biosorption to the cell walls and entrapment in extracellular capsules, precipitation, and reduction of the heavy metal ions to a less toxic state are mechanisms to metals’ resistance.
Keywords
- heavy metals
- bioremediation
- copper
- lead
- cadmium
- selenite
1. Introduction
Since the industrial revolution, heavy metals’ waste has increased rapidly. Toxic metals’ species are mobilized from industrial activities and fossil fuel consumption and eventually are accumulated through the food chain, leading to both ecological and health problems. Some of these metals are taken up as essential nutrients since they are incorporated into enzymes and cofactors. Some heavy metals exert toxic effects on microbial cells (i.e., mercury, lead, cadmium, arsenic, and silver). Mostly, resistance systems have been found on plasmids, whereas bacterial chromosomes contain genes for resistance to many of the same heavy metals’ cations and oxyanions as do plasmids [1, 2]. To survive under metal-stressed conditions, bacteria have evolved several types of mechanisms to tolerate the uptake of heavy metal ions. These mechanisms include biosorption to the cell walls and entrapment in extracellular capsules, precipitation, the efflux of metal ions outside the cell, reduction of heavy metal ions to a less toxic state accumulation, and complexation of metal ions inside the cell [3, 4].
2. Copper bioremediation
In almost all life forms, copper is a metal essential for the normal function. It acts as a cofactor for a number of enzymes involved in respiration and electron transport proteins in plants, animals, and microorganisms. Copper is toxic to cells at high concentrations mainly due to the disruption of the integrity of cell membranes, its interaction with nucleic acids, interference with the energy transport system, and disruption of enzyme active sites [5, 6, 7, 8] . At high cytoplasmic concentrations, copper can compete with other metals for their binding sites in proteins that can lead to dysfunctional proteins. Otherwise, the presence of Cu (I) in cells will react with hydrogen peroxide and produce hydroxyl radicals that will damage DNA, lipids, and other molecules [9, 10]. Resistance to copper in microorganisms is dependent mainly on three different systems:
The periplasmic plasmid-borne copper (pco) resistance system that encodes for PcoA, a multi-copper oxidase protein responsible for oxidation of Cu(I) in the periplasmic space. This system presents only on plasmids and presents high copper resistance [11, 12, 13].
The efflux ATPase pump CopA able to throw copper ions outside [10, 14].
Cus system (copper sensing copper efflux system) belonging to the resistance-nodulation-cell division (RND) family responsible for heavy metal export (HME-RND) that encodes especially for the CusA protein [10, 13, 15].
In agriculture, copper bactericide is considered one of the most important components in environmental contamination with copper especially in programs practiced worldwide in growing areas with citrus [16]. Many species of plant pathogenic bacteria such as
In both
Based on a plate assay, Frankia strains EuI1c, CN3, QA3, and DC12 are tolerant to high levels of copper (MIC values >5 mM), while many other strains tested are very sensitive exhibiting MIC values <0.1 mM [20]. Otherwise, a 24-well growth assay was used to reexamine copper sensitivity of five
The cells grown in the elevated copper levels appeared blue which suggest that copper was accumulating inside of
These smaller structures were about 5 μm in diameter and were also observed as individual structures throughout the hyphae. At higher magnification, the structures have a grooved pattern and appear connected to the hyphae by amorphous material [21]. Similar globular structures were observed with SEM of other copper-resistant
The elemental composition analysis of these structures was investigated by the use of SEM-EDAX. As expected, these structures exhibited an elevated copper content that was represented by a 73-fold more than the control increase in the intensity but also contained an elevated phosphate content that was about 43.88-fold higher intensity level than the control cells. Furthermore, the oxygen content increased 3.5-fold under copper-stressed condition. All three of these elements had nearly the same intensity values under Cu+2 condition. These results suggest that a copper-phosphate compound forms and binds to
In some bacteria and algae, it has been proposed that inorganic polyphosphates and transport of metal-phosphate complexes will participate in heavy metal tolerance [23]. After the
3. Lead bioremediation
Lead enters the cells through Fe2+ and Ca2+ transporters and then exerts its toxicity by displacing these cations at their binding sites in metalloproteins. Heavy metal resistance systems in many bacterial are based on efflux. Two groups of efflux systems have been recognized in gram-negative bacteria which are chemiosmotic pumps, e.g., the three-component divalent-cation efflux systems of
Furthermore, 27 isolates were isolated from some abandoned mining areas in Morocco and found to belong to
Interplay between CBA transporters and P-type ATPases in
A group of transporters, the cation diffusion facilitator family (CDF), can catalyze heavy metal influx or efflux in both prokaryotes and eukaryotes. All characterized CDF proteins to date can transport metals only (such as Fe2+, Co2+, Ni2+, Zn2+ and Cd2+), in contrast to other protein families, such as P-type ATPases or CBA transporters. In
Detoxification mechanism for Pb2+ can also be achieved by sequestration. In several bacterial species and via the use of intra- and extracellular binding of Pb2+, they can avoid toxicity as in
Nine candidate core biomarker genes might be tightly correlated with the response or transport of heavy metals. These genes, namely,
The mechanism of lead resistance in
4. Cadmium bioremediation
Cadmium (Cd2+), the heavy metal, is toxic in its ionized form to microbes and humans. It is found in the biosphere and often associated with zinc ores at concentrations approaching 0.01–1.8 ppm. It can enter the bacterial cell normally by essential divalent cations via transport systems. Cadmium toxicity has effect by inhibiting respiration via binding to essential proteins’ sulfhydryl groups and can also cause single-strand breakage of DNA in
The full resistance to Cd2+ required the interplay of a P-type ATPase that exported cytoplasmic ions to periplasm and a CBA transporter that further exported periplasmic ions to the outside. Furthermore, membrane transport pumps export metal ions from the cell and binding factors involved in creating tolerance to heavy metal ions through detoxify metals by sequestration (i.e., cell wall components (exopolysaccharides) and intracellular binding proteins (like metallothioneins and metallochaperones)) [34]. As cytoplasmic metal cation-binding proteins, metallothioneins can lower the concentrations of free ion in the cytoplasm. SmtA from
In
The
The identified gene which is
5. Selenite reduction
Selenium, in the form of selenocysteine or selenomethionine, is found in several stress proteins including glutathione peroxidase, alkyl hydroperoxidase, and multiple disulfide reductases. The deprotonated electrons of selenium cofactors make the selenoproteins’ reduction–oxidation reactive, explaining why many identified selenoproteins are involved in thiol and oxidative stress resistance. Since selenite generates these stresses in the cell, the stress-related selenoproteins may function doubly in detoxification and removal of free selenite ions from the cytoplasm. About 20% of sequenced bacteria contain selenoproteins [50].
The detoxification mechanism of selenite reduction in aerobic condition by microorganisms is not yet fully elucidated. Previously, it has been reported that selenite reduction may be catalyzed by a periplasmic nitrate reductase as in a selenate reductase, a periplasmic nitrate reductase in
However, all of the
Many bacteria including
In summary, selenite resistance may result from oxidation of selenite to the less toxic selenate using SorA.
When examined under scanning electron microscope, selenite-resistant
The elemental composition analysis of these nanospheres was investigated by the use of SEM-EDAX. As predicted, these nanospheres exhibited an elevation. Three absorption peaks in EDAX analysis at 1.37 keV (peak SeLα), 11.22 keV (peak SeKα), and 12.49 keV (peak SeKß) can be produced from selenium absorption. The first peak is related to 1.37 keV (peak SeLα) (keV = kilo electron Volt), whereas the second peak is met with 11.22 keV (peak SeKα) [61, 64].
6. Conclusion
Heavy metals are harmful to human health via interference with the function of vital cellular components. Lead (Pb), cadmium (Cd), copper (Cu), and selenite (SeO32−) are metals and metalloids that are widespread in the environment. P-type ATPase system that exported cytoplasmic ions to the periplasm and a CBA transporter that further exported periplasmic ions to the outside are general mechanisms in resistance Co, Pb, and Cd. Furthermore, in metals detoxification by sequestration, binding factors will be involved in creating tolerance to heavy metal ions.
Acknowledgments
We thank Teal Furnholm and Robert Mooney for their help with the photography and Nancy Chemin for her help with the electron microscopy.
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