Tolerance and uptake of heavy metals by Pseudomonads
Introduction
Metals play an integral role in the life processes of living organisms. Some metals (e.g. Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni and Zn) are essential, serve as micronutrients and are used for redox-processes, to stabilize molecules through electrostatic interactions; as components of various enzymes; and regulation of osmotic pressure [1]. Many other metals have no biological role (e.g. Ag, Al, Cd, Au, Pb, and Hg), and are nonessential [1] and potentially toxic to living organisms, specially microorganisms. Toxicity of nonessential metals occurs through the displacement of essential metals from their native binding sites or through ligand interactions [1], [2]. In addition, at high levels, both essential and nonessential metals can damage cell membranes, alter enzyme specificity, disrupt cellular functions, and damage the structure of DNA [1].
Heavy metals are metals with densities higher than 5 g/cm3 [2]. Heavy metals in wastewater come from industries and municipal sewage, and they are one of the main causes of water and soil pollution. Accumulation of these metals in wastewater depends on many local factors such as type of industries in the region, way of life and awareness of the impacts done to the environment by careless disposal of wastes [3], [4]. Therefore the presence of heavy metals in wastewater is not only of great environmental concern, but also strongly reduces microbial activity and as a result adversely affects biological wastewater treatment processes [5], [6], [7], [8], [9], [10]. Moreover the toxicity of heavy metals in wastewater was shown to be dependant on factors like metal species and concentration, pH, wastewater pollution load [11], [12] and solubility of the metal ions [13].
The impact of heavy metals on the environment and their accretion through the food chain have promoted research aimed at developing alternative, efficient and low cost wastewater purification systems [14]. Conventional methods for removing dissolved heavy metals include chemical precipitation and sludge separation, chemical oxidation or reduction, ion exchange, reverse osmosis, filtration, adsorption using activated charcoal, electrochemical treatment and evaporative recovery [15], [16]. However, these techniques can be expensive, as they may not always be feasible and their metal-binding properties are non-specific [17]. These are reasons why alternative processing methods, such as those using microbial biomass are now being considered more seriously [16], [18], [19].
Bioremediation of industrial wastes containing heavy metals has been demonstrated by several biotechnology companies employing bioaccumulation [20], [21], [22], [23]. Biosorption, biopreciptation, and uptake by purified biopolymers derived from microbial cells provide alternative and/or additive processes for conventional physical and chemical methods [24], [25], [26]. Intact microbial cells, live or dead, and their products can be highly efficient bioaccumulators of both soluble and particulate forms of metals [27], [28], [29], [30], [31]. The cell surfaces of all microorganisms are negatively charged owing to the presence of various anionic structures. This gives bacteria the ability to bind metal cations. Various microbial species, mainly Pseudomonas, have been shown to be relatively efficient in bioaccumulation of the different heavy metals from polluted effluents [32], [33]. Accordingly the aim of this work was to study the tolerance and the uptake of different heavy metals by a group of Pseudomonas sp. isolated from the effluent of wastewater treatment plant in western Alexandria, Egypt and to investigate the influence of the binary metals and the combination of metal ions on the accumulation of metals by this group of microorganisms.
Section snippets
Microorganisms, media and growth conditions
Strains of Pseudomonas sp. showing a good ability for metal tolerance and bioaccumulation were isolated from sewage samples collected from western Alexandria sewage treatment plant, Alexandria, Egypt, by an enrichment culture technique and was employed in this study. Isolation was carried out using enrichment medium supplemented with different concentrations of the four heavy metals under investigation to obtain the most resistant strains [34]. Four Pseudomonas strains, exhibiting an ability to
Results and discussion
Cr(VI) is the most toxic and mutagenic metal ion in biological systems. Results presented in Fig. 1 revealed that amongst all metals used, Cr(VI) has the greatest inhibition effect on cell growth, since the Cr(VI)-resistant strain P. fluorscens bacterial growth was directly inhibited when Cr(VI) concentration in the growth medium reached 3 mmol/L. The presence of binary metal ions in the reactive mixture was found to decrease the amount of Cr accumulation. This can be explained in terms of the
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