Relative numbers and approximate biomass of the soil microbiota in a fertile soil [8].
1. Introduction
Metals are essential components of the ecosystem, whose biologically available concentrations depend mainly on geological and biological processes [1]. There are several definitions of heavy metals, and some of them are based on the mass density of these elements. Authors of numerous publications use different limits to define the threshold density for a “heavy metal”, ranging from 3.5 to 7 g×cm-3, however, the majority of authors suggests that the mass density of heavy metals should be greater than 4.5 g×cm-3 [2]. Within the group of heavy metals one can distinguish both elements that are essential for living organisms (microelements) and the elements whose physiological role is unknown and thus they are “inactive” towards organisms. The metals that serve as microelements in living organisms usually occur in trace amounts, precisely defined for each species and both their deficiency and excess badly affect living organisms [3]. The term “heavy metal” is linked in many people’s minds to metals that are toxic. However, this is not always the truth. The effect of any substance on a living system is always dependent on its available concentration to cells. Also, several heavy metal ions are crucial in metabolic processes at low concentrations but are toxic at high concentrations [2]. Nevertheless, locally elevated levels of these elements can create significant environmental and health problems when the release of metals through various biological, geological and anthropogenic processes far exceeds its natural content resulting from processes of metal cycling. Heavy metal pollution of terrestrial environments is of great concern, due to the persistence of metals in the ecosystem and their threat to all living organisms [4].
Given the importance of the subject of soil heavy metal pollution and its effect on soil microorganisms, this chapter gives an overview of the severity of the problem when it comes to the reaction of soil microbial community to the environmental pollution. The first part of this chapter deals with the abundance of microorganisms in soils and their role in this environment. The next part concerns major sources of heavy metals in soils with particular emphasis on the most important source of soil pollution, i.e. human activity (and more precisely – industry and mining). The following part discusses the effects that toxic levels of heavy metals may have on the microbial population in soils. The last two parts of this chapter describe the ways of dealing with heavy metal pollution – one introduces the term of phytoremediation (soil remediation with the use of plants) and the other one focuses on the use of microorganisms resistant to heavy metals in the process of soil remediation.
2. The complexity of microbial community in soils
Except for occasional insects or earthworms, once visible traces of plant biomass are removed, soil appears as a lifeless mass, that is composed of mineral particles and organic residues. However, even desert soils are abundant source of living microorganisms. This seemingly lifeless matter contains complex microbial community, including bacteria, fungi, protozoa and viruses. The integrity of the aboveground and belowground ecosystems depends on the stability, resilience and function of the soil microbial community [5].
Soil is an interesting medium for growing microorganisms, as it contains various nutrients that the microbes need for their metabolism. Unfortunately, nutrients are not always readily available [6]. However, it is one of the richest reservoirs of microorganisms, i.e. 1 gram of agricultural soil may contain even several billion colony forming units (CFUs) of microorganisms belonging to thousands of different species [7], and even though microorganisms constitute less than 0.5% of the soil mass, they have a major impact on soil properties and processes [5]. Table 1 presents the average numbers of soil microorganisms in a “typical” temperate soil. Destruction of the soil microbiota through mismanagement or environmental pollution causes decline or even death of the aboveground plant and animal populations.
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Bacteria | 1013-1014 | 108-109 | 300-3000 |
Actinomycetes | 1012-1013 | 107-108 | 300-3000 |
Fungi | 1010-1011 | 105-106 | 500-5000 |
Microalgae | 109-1010 | 103-106 | 10-1500 |
The most characteristic feature of microbial habitats is the great micro-spatial variability of environmental parameters, like temperature or nutrient availability. Many basic requirements of heterogeneous microorganisms are satisfied by various soil microhabitats. This is the reason why, in ecological terms, a number of varying microbial niches can be described. Therefore, the microbial community is composed of diverse taxa with different nutritional demands within small microenvironments [9]. Analysis of the spatial distribution of bacteria at microhabitat levels showed that in soils subjected to different fertilization treatments, the majority of bacteria were located in micropores of stable soil micro-aggregates (2 – 20 µm), as they contained over 80% of cells [10]. Such microhabitats offer the most favorable conditions for microbial growth in terms of water and substrate availability, gas diffusion and protection against predation. The microhabitat-adapted groups of microorganisms form so-called consortia which are held together by mutually facilitating metabolic processes. The consortia are characterized by more or less sharp boundaries, and variable level of interaction with each other and with other parts of the soil biota. Numerous investigations emphasize the impact of soil structure and spatial isolation on microbial diversity and community structure [11]. Some studies indicate that the soil particle size affects the diversity of microorganisms and community structure to a greater extent than other factors such as bulk pH and the type or amount of available organic compounds [12]. Other investigations show that the type and amount of available organic substrates strongly affect the abundance of microbial groups and their functional diversity in soils [13]. Fierer and Jackson [14] claim that the structure of soil bacterial communities is not random also at continental scale and that the diversity and composition of soil bacterial communities at large spatial scales can be predicted to a large extent by a single variable, that is soil pH. The diversity of soil microorganisms comprises different levels of biological organization. It includes genetic variability among taxa (species), number (richness), relative abundance (evenness) of taxa and functional groups within communities [11]. The overall biodiversity of soil microflora comprises bacteria, fungi, actinomycetes and photosynthetic microorganisms [6].
Bacteria constitute the most numerous group of soil microbes – a teaspoon of productive soil contains between 100 million and 1 billion bacterial cells. As soil environment changes rather drastically, spore-forming bacteria tend to be the most common. When environmental conditions become too difficult for normal growth, the bacteria form spores and remain dormant until the environment returns to proper conditions [6]. They facilitate various processes in soils, e.g. those related to water dynamics, nutrient cycling or disease suppression [15]. Soil-dwelling bacteria may be divided into different groups based on:
Shapes: rods (also called bacilli), sphere (also called cocci) and spiral (also called spirilla)
Their reaction to oxygen: aerobic (bacteria that need oxygen for their survival) and anaerobic (the ones that do not require oxygen and in most cases cannot bear oxygen that is deadly for them)
Result of Gram staining: Gram negative (stain pink and have thinner cell walls, they are the smallest ones and tend to be more sensitive to water stress) or Gram positive (stain violet, have thicker cell walls, are larger in size and tend to resist water stress)
Source of carbon they use: autotrophs (obtain carbon from carbon dioxide – some autotrophic bacteria directly use sunlight in order to produce sugar from carbon dioxide, while others depend on various chemical reactions) or heterotrophs (they obtain carbohydrates from their environment)
Classification based on phyla: based on morphology, barcode DNA sequences, physiological requirements and biochemical characteristics, bacteria have been classified into 12 phyla. Each phylum corresponds to a number of bacterial species and genera [15].
Tate [5] lists the most commonly encountered soil bacterial genera as:
Bacteria facilitate a number of physical and biochemical alterations or reactions in soils and thereby directly or indirectly support the development of higher plants. Their performance is vital for a variety of processes that include: decomposition of cellulose or other carbohydrates (e.g.
On the other hand, soil fungi form three functional groups: decomposers, mutualists and pathogens. Fungi, along with bacteria, are important decomposers of hard to digest organic matter and they increase nutrient uptake of phosphorus. Mycorrhizal fungi support plants by promoting root branching and increasing nitrogen, phosphorus and water uptake. They improve plant resilience to pests, diseases or drought and improve soil structure, as fungal hyphae binds soil particles together to create water-stable aggregates. They in turn create the pore spaces in the soil that enhance water retention and drainage [17]. The most common fungi found in soil belong to the
Actinomycetes are a large group of microorganisms, systematically identified as bacteria, that grow as hyphae. They decompose a wide range of substances, but they are particularly important in degrading recalcitrant (difficult to degrade) compounds such as chitin, lignin, keratin and cellulose. Moreover, they produce a number of secondary metabolites such as antibiotics i.e. streptomycin [18] or geosmine which is responsible for “earthy” smell after soil plowing [15]. Actinomycetes are important in forming stable humus, which enhances soil structure, improves soil nutrient storage and increases water retention in soils. According to Tate [5], the most commonly encountered soil actinomycetes belong to
Algae are the most common among photosynthetic microorganisms found in soil. They are found only near soil surface, where light is readily available [6]. The most common genera of green algae found in soil are:
Although biomass of all microorganisms living in soil constitutes only several percent of organic matter content, they play an important role in the functioning of entire ecosystems [20]. They take part in soil formation, mineralize organic substances, provide plants with bioavailable compounds, cooperate with plants or may be used as a source of insecticidal substances [21]. One of the most important and most widely studied microbial groups in terms of beneficial effects to soil and plants is the group of Plant Growth Promoting Rhizobacteria (PGPR) [22]. This group includes bacterial species from genera such as
Despite beneficial effects of numerous soil microbes on plant growth or development, soil structure and functioning, some soil-dwelling microorganisms may cause plant, animal and human diseases. Similarly to the beneficial soil microflora, soil pathogens include bacteria, fungi and viruses. One of the example of the most important or best known plant pathogens include
Undoubtedly, soil is an inexhaustible reservoir of microorganisms, both beneficial and pathogenic ones. Causing the imbalance between groups of soil macro- and microorganisms may be irreversible and result in a variety of effects, sometimes unpredictable. Such imbalance may be caused by soil pollution resulting from developing industry, therefore understanding the sources and effects of industrial soil pollution is an important element in preventing the environmental degradation.
3. Sources of soil heavy metal pollution
Chemical compounds, entering the ecosystem as a result of different human activities, may accumulate in soil and water environments. Therefore, soil may be regarded as a long-term reservoir of pollutants, from which these compounds may be introduced to food chains or groundwater [36]. Inappropriate and careless disposal of industrial waste often results in environmental pollution. The pollution includes point sources such as emission, effluents and solid discharge from industry, vehicle exhaustion and metal smelting or mining, as well as nonpoint sources (e.g. the use of pesticides or excessive use of fertilizers) [37]. Each of the sources have their own damaging effects on plant, animal and human health, but those that add heavy metals to soils are of serious concern due to the persistence of these elements in the environment. They cannot be destroyed, but are only transformed from one state to another [38].
Soil pollution may be defined as presence of xenobiotics (e.g. chemical compounds, radioactive elements) that alters the soil properties – both chemical, physical and biological. Soil pollution, including heavy metals, may be of natural origin, like volcanic eruptions, animal excrements or ore leaching. Nevertheless, human activity and mostly chemical industry, mining and metallurgy, as well as municipal management and traffic emissions are the main source of environmental pollution. Some authors also mention that waste disposal, waste incineration, fertilizer application and long-term application of wastewater in agricultural lands may result in heavy metal pollution of soils [39].
Heavy metals occur naturally in soils due to pedogenetic processes of weathering parent materials, however concentrations of these metals are regarded as trace (<1000 mg×kg-1) and rarely toxic [40]. Due to the disturbance and acceleration of the natural slow geochemical cycles of metals by man, most soils of rural and urban environments accumulate one or more heavy metals above the defined background levels, high enough to cause risks to ecosystems [41]. Nevertheless, heavy metals occurring in soils from anthropogenic sources tend to be more mobile, therefore more bioavailable than pedogenic or lithogenic ones [42].
Communication routes, such as roads, railways etc., are an important source of soil pollution, especially in the case of lead and zinc. Despite restricted use of leaded gasoline adopted in most countries, lead remains one of the most serious automotive-originating metal pollutant. The areas located nearby roads, particularly in urban sites, are the most vulnerable to automotive pollution. Apart from lead and zinc, chromium, cadmium, nickel and platinum are among the pollutants emitted by combustion engine-powered vehicles [43]. Heavy metals enter the environment as a result of tire wear and damage of vehicle parts. Moreover, grease used in vehicles may also be the source of cadmium pollution along roads [44]. Nickel emission results from this metal being added in gasoline and atmospheric abrasion of nickel-containing parts of automobiles [45]. The changes in the concentrations of lead, nickel, cadmium, copper and zinc in roadside soils are frequently attributed to traffic density [46].
Standard agricultural practices are also a significant source of heavy metals in soils, as application of fertilizers and pesticides has contributed to a continuous accumulation of these elements. Heavy metals can accumulate in soils due to the application of liquid and solid manure, as well as inorganic fertilizers [47]. The application of numerous biosolids, such as livestock manures, composts and municipal sewage sludge on agricultural soils leads to the accumulation of various heavy metals, such as, Cd, Cr, Cu, Hg, Mo, Ni, and Zn [48]. Lime and superphosphate fertilizers contain not only major elements necessary for plant nutrition and growth but also trace metal impurities such as cadmium. The presence of high concentrations of Cd in some fertilizers (particularly in phosphatic fertilizers) is of most concern due to the toxicity of this metal and its ability to accumulate in soils as well as due to its bioaccumulation in plant and consequently in animal tissues [49, 50]. Additionally, copper-containing compounds have been widely used in agricultural practice as pesticides. Copper oxychloride is annually applied on vineyards as a fungicide to control a significant number of plant diseases. Inevitably, this Cu ends up in the agricultural soil and adjacent pristine natural vegetation [51]. Lead arsenate was used in fruit orchards for many years to control some of the parasitic insects. Arsenic-containing compounds were also extensively used to control pests in banana plantations in new Zealand and Australia [52]. High fertilizer applications and acid atmospheric deposition, combined with insufficient liming, may also cause a decrease in pH and thus increase heavy metal bioavailability, aggravating the problem of deteriorating food quality, metal leaching and impact on soil organisms [53]. The application of municipal wastewater or industrial waste as fertilizers and liming agents in agriculture is a separate issue. Application of this type of waste requires constant monitoring of the amount and proportion of harmful factors, including heavy metals. The high risk of soil pollution with Cd, Zn, Ni and Pb as a result of industrial waste application as fertilizers was also evidenced [50].
Airborne sources of heavy metals include stack emissions or fugitive emissions such as dust from storage areas or waste heaps. Stack emissions can be distributed over a wide area by natural air currents, while fugitive emissions are often distributed over much smaller areas. In general, concentrations of pollutants are much lower in fugitive emissions compared to stack emissions. The type and concentration of metals emitted from both types of sources depend on site-specific conditions. All solid particles in smoke from fires and other emissions from factory chimneys are deposited on land or sea. Most forms of fossil fuels contain some heavy metals and this form of environmental pollution has been increasing since the industrial revolution began. For instance, very high concentrations of Cd, Pb and Zn have been found in plants and soils adjacent to smelting plants. Another major source of soil pollution is the aerial emission of lead from combustion of petrol containing tetraethyl lead; this contributes substantially to the content of Pb in soils in urban areas and in those adjacent to major roads [52].
Another, and one of the most significant sources of heavy metal pollution of soils, includes heavy industry, e.g. mining and metallurgy. Industrial airborne heavy metal contamination of the nonferrous smelters surrounding landscapes is a well-known and widely occurring phenomenon. Emissions of metallurgical dust are spread according to the wind direction and particle size while soil is the main receiver of heavy metals in dry land. Dust emissions from smelters using sulfide copper-nickel ores are similar, regardless of their location, owing to the fact that the same raw materials are used in metallurgical processes. The following major metal-containing compounds are deposited onto the landscape in the form of dust emissions from smelters: pentlandite (Ni,Fe)9S8, pyrrotite Fe7Sg(Nix), chalcopyrite CuFeS2, chalcosite Cu2S, covellite CuS, cuprite Cu20, tenorite CuO, and metal copper and nickel [54]. Surface soil layers in the mining or metallurgy areas are often heavily polluted with copper. In the vicinities of steel plants the concentration of this element exceeds several thousand ppm and the pollution remains for a long time, even after the operation of mines or steel plants had been stopped [50]. The fine fractions of dust are enriched with lead, arsenic, and zinc. The quantity and composition of dust derived from different sources (metallurgical processes) varies according to the raw materials and the condition of the gas cleaning systems [54]. The cause for the frequently widely dispersed metal pollution in habitats of mining areas was found in the formation of acid mine drainage (AMD). The runoff from mining heaps of active and abandoned mines can be extremely acidic, with pH values reaching as low as pH 2 [9]. Chemical and biological oxidation of the abundant mineral pyrite (FeS2) occurs after the unearthing of pyrite-containing rock formations and results in an acidification of the dump material [55]. Under acidic conditions, the majority of heavy metals is leached from the waste dump and they are transported as AMD in streamwaters [9]. Galvanization industry may cause soil pollution with silver as well as other industrial facilities that use silver salts. Additionally, the increased amount of silver may by introduced to soils with municipal sewage. Municipal sewage contains also large amounts of highly soluble forms of zinc, which may then easily contaminate soil environment [50]. Zinc is also extensively used in metallurgical industry, as an anti-corrosion agent in alloys and in galvanization. It is frequently used in paint industry [50]. The concentration of cadmium highly increases in soils polluted with emissions from nonferrous metal plants, which constitute over 60% of all anthropogenic sources of this element in soils. Municipal sewage contains on average 10 – 40 ppm of cadmium, while industrial sewage may contain over 1000 ppm. This is also a case of large amounts of lead that may be introduced into soils from municipal sewage and waste, as they contain mobile forms of this element. This may result in large increase in the concentration of lead in soils that may exceed several times the admissible limits. Additionally, dust emissions from landfills of nonferrous metal plants may become dangerous sources of lead in soils [50]. Table 2 shortly summarizes the major sources of different heavy metals in soil.
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As | Semiconductors, petroleum refining, wood preservatives, animal feed additives, coal power plants, herbicides, volcanoes, mining and smelting |
Cu | Electroplating industry, smelting and refining, mining, biosolids |
Cd | Geogenic sources, anthropogenic activities, metal smelting and refining, fossil fuel burning, application of phosphate fertilizers, sewage sludge |
Cr | Electroplating industry, sludge, solid waste, tanneries |
Pb | Mining and smelting of metalliferous ores, burning of leaded gasoline, municipal sewage, industrial wastes enriched in Pb, paints |
Hg | Volcano eruptions, forest fire, emissions from industries producing caustic soda, coal, peat and wood burning |
Se | Coal mining, oil refining, combustion of fossil fuels, glass manufacturing industry, chemical synthesis (e.g., varnish, pigment formulation) |
Ni | Volcanic eruptions, land fill, forest fire, bubble bursting and gas exchange in ocean, weathering of soils and geological materials |
Zn | Electroplating industry, smelting and refining, mining, biosolids |
4. The effects of heavy metals on soil microorganisms
Metals without biological function are generally tolerated only in minute concentrations, whereas essential metals with biological functions, are usually tolerated in higher concentrations [9]. They have either metabolic functions as constituents of enzymes or meet structural demands, e.g. by supporting the cell envelope. Frequently the concentration and the speciation of metal determine whether it is useful or harmful to microbial cells [9].
Microorganisms are the first biota that undergoes direct and indirect impacts of heavy metals. Some metals (e.g. Fe, Zn, Cu, Ni, Co) are of vital importance for many microbial activities when occur at low concentrations. These metals are often involved in the metabolism and redox processes. Metals facilitate secondary metabolism in bacteria, actinomycetes and fungi [9; 57]. E.g. chromium is known to have stimulatory effect on both actinorhodin production and growth yield of the model actinomycete
The toxic concentration of heavy metals may cause enzyme damage and consequently their inactivation, as the enzymes-associated metals can be displaced by toxic metals with similar structure [59]. Moreover, heavy metals alter the conformational structures of nucleic acids and proteins, and consequently form complexes with protein molecules which render them inactive. Those effects result in disruption of microbial cell membrane integrity or destruction of entire cell [62]. Heavy metals also form precipitates or chelates with essential metabolites [63].
Various metals may affect different microbial populations and the resulting impact may vary depending on the metal whose limit concentrations in soils were exceeded. For instance, the pollution of soils with copper affects microorganisms that take part in nitrification and mineralization of protein compounds [50]. Silver is one of the most toxic metals to heterotrophic bacteria. This effect is used for the production of antiseptic preparations. However, there are some silver-resistant bacteria, both in clinical and natural conditions. Some strains of
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Total number of bacteria | 17×105 | 58×104 |
Total number of fungi | 26×103 | 42×102 |
Actinomycetes | 43×103 | 18×101 |
23×103 | 17×101 | |
21×103 | 16×102 |
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Total number of mesophilic bacteria | 22.50×102 – 10.44×106 | 0 – 13.15×105 |
Total number of fungi | 84.00×101 – 21.03×103 | 0 – 57.90×103 |
Actinomycetes | 62 – 99.50×103 | 0 – 20.26×103 |
0 – 28.90×102 | 0 – 57.00×101 |
However, one of the reasons of decreasing biodiversity of microorganisms in heavy metal polluted soils is the selection for tolerant species or strains. Metal exposure may lead to the establishment of tolerant microbial populations, that are often represented by several Gram-positive genera such as
5. General outline of soil remediation strategies
The overall objective of any soil remediation approach is to create a final solution that is protective both for human health and the environment [74]. For heavy metal-polluted soils, the physical and chemical form of the heavy metal contaminant in soil strongly influences the selection of the appropriate remediation treatment approach. Details on the physical characteristics of polluted soils, type and level of the pollution at the site must be known to enable accurate assessment of the problem severity and adjustment of remedial measures [52].
Remediation of heavy metal-polluted sites is very expensive and difficult, therefore the best method to protect the environment from contamination is to prevent it. Nevertheless, it is not always possible and once metals are introduced and pollute the soil, they will remain there. Unlike carbon-based organic pollutants, heavy metals cannot be degraded or eliminated completely, therefore the traditional treatments for heavy metal pollution of soils are complicated and cost-intensive.
There are several technologies for remediation of heavy metal-polluted soils. One of the classifications divides the methods into
6. The use of plants for biological remediation of heavy metal polluted soils
Phytoremediation is one of the best techniques for treatment of heavy metal-polluted sites. It is an
Phytoextraction uses hyperaccumulating plants to remove metals from soil by absorption into the roots and shoots of the plant. The aboveground shoots can be then harvested to remove metals from the site and subsequently stored as hazardous waste or employed for the recovery of metals. The ideal plant for phytoextraction should grow rapidly, produce high amount of biomass and be able to tolerate and accumulate high metal concentrations in shoots [80]. Hyperaccumulating plants belong to the families of
Phytostabilization is based on the use of plants to limit the mobility and bioavailability of metals in soil. Plants used in this method are characterized by high tolerance of metals in surrounding soils together with their low accumulation. Phytostabilization can be carried out through the process of sorption, precipitation, complexation, or metal valence reduction. This technique is useful for the removal of Pb, As, Cd, Cr, Cu, and Zn [82]. This process is advantageous because in this case disposal of hazardous material/biomass is not required, and it is very effective when rapid immobilization is needed to preserve soils or ground and surface waters [82].
Rhizofiltration (or phytofiltration) removes metals from contaminated soil via absorption, concentration and precipitation by plant roots. This technique is used to remove pollutants from groundwater and aqueous-waste streams rather than for the remediation of polluted soils [76]. Apart from the above described phytoremediation methods, some authors [83] include also phytovolatization and phytodegradation.
Phytovolatization involves the use of plants to volatilize pollutants from their foliage such as Se and Hg, while phytodegradation uses plants and associated microorganisms to degrade organic pollutants. Even though phytoremediation strategies are inexpensive, effective, environmentally friendly and can be implemented
7. Application of microorganisms to remediate heavy metal-polluted soils
Another approach for biological remediation of heavy metal-polluted soils includes the use of microorganisms to detoxify metals by valence transformation, extracellular chemical precipitation or volatilization etc. [56]. Bioleaching is the method that uses microorganisms to solubilize heavy metal pollutants either by direct bacterial processes, or as a result of interactions with metabolic products, or both [76]. It can be used
Another solution for soil bioremediation using microorganisms is to apply microbially-mediated biochemical processes, such as oxidation/reduction or methylation reactions [87]. Often, biostimulation and bioaugmentation are the components of bioremediation strategies. Biostimulation is a form of
Mechanisms involved in biochemical interactions between bacteria and metal ions involve specific enzymes that catalyze the oxidation, reduction, methylation, dealkylation and precipitation reactions. Microorganisms transform a substantial number of metals and metalloids by reducing or oxidizing them directly to a lower or higher redox state. Additionally, indirect oxidation or reduction is an alternative for immobilization of toxic metals in the environment. Methylation is an important process involved in geochemical cycling of metals and the removal of metal pollutants from soils. Methylation processes derive the methyl group from methylocarbolamine (CH3B12) which is implicated in the methylation of multiple metals and metalloids, such as Pb, Sn, Pd, Pt, Au, Ti, As, Se and Te [93]. Methylation of Hg, Sn and Pb can be mediated by a range of microbes, including
Microorganisms play an important role in the environmental biogeochemical cycle of metals and their properties are of significant interest in the remediation of contaminated sites. The microbial ability to absorb and transform metals is a promising aspect in respect of solving the pollution problems [4]. The potential of numerous microbial metal transformations in treatment of environmental pollution may be employed and some processes are already in commercial operation. However, many processes are still at the laboratory scale and yet to be tested in a rigorous applied and/or commercial context [94]. Another interesting aspect of the microbial community is their ability to multiply even under undesirable environmental conditions. These microorganisms sometimes affect soil environment more quickly than abiotic processes can. Therefore, the structure of soil microbial populations may be useful as a highly sensitive bioindicator of soil disturbance and progress of remediation [95].
Facing the increasing heavy metal pollution severity accompanied by rising land prices the communities around the world need to struggle for available investment grounds. This is mostly the problem of big cities, especially those with limited opportunities for development due to geographical barriers such as seashores, mountain ranges or desert areas. In such situations the polluted industrial areas cannot be left unused for long time to recover naturally. This creates a need for the development of various remedial procedures adjusted to changing contamination level, environmental conditions, available time and funding. Thus, remedial measures need to be almost always modified in order to meet those criteria. This makes that the continuous effort should be made to increase the effectiveness, flexibility and decrease the cost and side effects of the procedures available today. Although a number of measures was developed to remove the even toxic level of contamination, there are many degenerated areas that still cannot be successfully treated now. Those cases involve sites where remediation would be too expensive, time consuming or even technically disputable with currently available treatment procedures.
8. Conclusion
Heavy metals pose a significant threat towards the soil environment and the rapid industrialization will result in increasing problems of environmental pollution. Therefore, it is necessary to carry out the continuous monitoring of both industrial areas and their vicinities for possible transgressions of the limits given by the authorities. When necessary, the remedial measures should be applied as soon as possible by all available means. On the other hand, research should be promoted to understand the mechanisms of microbial response to heavy metal pollution and to enable screening for possible resistant microorganisms that could be used for both remediation and restoration of soil environment fertility.
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