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The Open Biotechnology Journal, 2016, 10, (Suppl-2, M10) 379-389
379
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DOI: 10.2174/1874070701610010379
REVIEW ARTICLE
The Geomicrobiology of Chromium (VI) Pollution: Microbial
Diversity and its Bioremediation Potential
Huda Al-Battashia, Sanket J. Joshib, Bernhard Pracejusc,* and Aliya Al-Ansaria
a
Department of Biology, College of Science, Sultan Qaboos University, Oman
Central Analytical and Applied Research Unit, College of Science, Sultan Qaboos University, Oman
c
Department of Earth Science, College of Science, Sultan Qaboos University, Oman
b
Received: April 26, 2016
Revised: August 02, 2016
Accepted: September 27, 2016
Abstract: The role and significance of microorganisms in environmental recycling activities marks geomicrobiology one of the
essential branches within the environmental biotechnology field. Naturally occurring microbes also play geo-active roles in rocks,
leading to biomineralization or biomobilization of minerals and metals. Heavy metals, such as chromium (Cr), are essential
micronutrients at very low concentrations, but are very toxic at higher concentrations. Generally, heavy metals are leached to the
environment through natural processes or anthropogenic activities such as industrial processes, leading to pollution with serious
consequences. The presence of potentially toxic heavy metals, including Cr, in soils does not necessarily result in toxicity because
not all forms of metals are toxic. Microbial interaction with Cr by different mechanisms leads to its oxidation or reduction, where its
toxicity could be increased or decreased. Chromite contains both Cr(III) and Fe(II) and microbial utilization of Fe(II)- Fe(III)
conversion or Cr (III) - Cr (VI) could lead to the break-down of this mineral. Therefore, the extraction of chromium from its mineral
as Cr (III) form increases the possibility of its oxidation and conversion to the more toxic form (Cr (VI)), either biologically or
geochemically. Cr (VI) is quite toxic to plants, animals and microbes, thus its levels in the environment need to be studied and
controlled properly. Several bacterial and fungal isolates showed high tolerance and resistance to toxic Cr species and they also
demonstrated transformation to less toxic form Cr (III), and precipitation. The current review highlights toxicity issues associated
with Cr species and environmental friendly bioremediation mediated by microorganisms.
Keywords: Chromite, Chromium (VI) toxicity, Chromium (III), Microbial bioremediation.
INTRODUCTION
Microorganisms play critical geo-active roles in the environment. They are involved in biogeochemical element
recycling and bio-transformation of metals and minerals, bioweathering and bioleaching in soils and sediments. The
microbial biochemical and metabolic activities have an effect on metal speciation, solubility, mobility, bioavailability,
and toxicity [1]. Some of these mechanisms are essential and considered as a part of biogeochemical cycles for metalsrecycling, where the metals are involved in microbial metabolism, development, and cell-differentiation [2, 3].
Microbial resistance to toxic metals is common, ranging from limited in uncontaminated environment up to 100% in
highly contaminated environment [3, 4]. The survival of these microbes depends on protective mechanisms: redox biotransformations, expression of metal binding stress proteins, precipitation, transport, efflux and intracellular
compartmentalization. Significant metal binding abilities of cell walls and other cell structural components lead to
different mobility rates [3, 4]. Chromium belongs to the heavy metals, which are considered as human hazards and thus
microorganisms are required to control its concentrations. A slight elevation in the level of Cr (VI) elicits environmental
and health problems because of its high toxicity, mutagenicity and carcinogenicity, while the reduced trivalent form (Cr
* Address correspondence to this author at the Sultan Qaboos University, Department of Earth Science, College of Science, Sultanate of Oman; Tel:
+968-24146838; Fax: +968-24146834; E-mail: pracejus@squ.edu.om
1874-0707/16
2016 Bentham Open
380 The Open Biotechnology Journal, 2016, Volume 10
Al-Battashi et al.
(III)) is less toxic and an essential nutrient for living organisms [5].
CHROMITE
Chromite (FeCr2O4) is a brownish black, weekly magnetic cubic mineral belonging to the spinel group [6]. It mainly
forms in ultramafic igneous rocks (peridotite) where it is accumulated during the early stages of magma crystallization
[7]. It is also found in serpentinites, which develop through hydrothermal alteration of peridotites. Since its discovery in
1798, chromite is still considered as an economic source for chromium extraction. Theoretically, it is mainly composed
of around 32.0% FeO and 68.0% Cr2O3 and, also contains a number of additional elements, such as Al, Ti, Mg and V,
which either substitute for Cr or Fe, respectively [6]. Generally, chromite is chemically inert and insoluble in water. The
annual production of chromite ore was 23,700-24,000 x103 metric tons, in year 2010-2011. The estimated world reserve
is projected at >480 x 106 metric tons of shipping-grade chromite ore with approximately 45% Cr2O3. The foremost
resources are situated in Kazakhstan (220 x106 t), southern Africa (200 x106 t), India (54 x106 t) and United States (0.62
x106 t) [8]. Chromium is extensively used in metallurgical and chemical industries for the production of ferrochrome
and chemicals, such as sodium dichromate. Chromate production from chromite ore usually leads to high environmental
pollution with a low extraction ratio. Usually, the processing of chromite is done by roasting with sodium carbonate at
1200 °C, in a rotary kiln with the addition of limestone and dolomite, ending up with >70% yield of Cr [9].
CHROMIUM CYCLE
Soil composition, texture, physical conditions in the soil, and the flora are the major aspects influencing the Cr
mobility [10]. The oxidation states of Cr in aqueous environments are +2, +3 and +6; although +3 and +6 are the most
common. Compared to Cr (III), aqueous hexavalent chromium (Cr (VI)) is the most oxidized, mobile, reactive, and
toxic form of Cr with no sorption in most sediment at pH> 7 [11]. The hexavalent chromium speciation at different pH
values are shown in Table 1. The common Cr (III) species include Cr(OH)3 as aqueous and solid form, while Cr(OH)4[pH > 9], CrOH2+, Cr(OH)2+, and Cr3(OH)45+ occur in solution.
Table 1. The Cr (VI) speciation at different pH value.
Cr (VI) species
pH
-
Monochromate anion (HCrO4 )
< 6.5
Chromate anion (CrO42-)
> 6.5
Bichromate anion (HCr2O7-)
<1
2
Dichromate anion (Cr2O7 )
1< pH <7.5
Chromate anion (CrO42-)
> 7.5
Under alkaline to slightly acidic conditions, chromate (CrO42-), bichromate (HCrO4-), and dichromate (Cr2O72-) are
weakly attached to loams leading to a high mobility in the subsurface [12]. On the other hand, Cr (III) is much less
mobile and precipitates readily as Cr(OH)3 or FexCr1-x(OH)3 at pH values >6.0, in soil [12]. Reduction of Cr (VI) to Cr
(III) is an effective means of immobilization and can be induced by inorganic or biological agents. The principal
chemical reactions leading to chromium cycling are hydrolysis, oxidation-reduction, and precipitation [10]. In partial
equilibrium with oxygen - soils and sediments contain Mn-oxides and carbon, which also play an important role in
redox reactions with Cr, such reactions are thermodynamically spontaneous [13]. Mn-oxides have high inner surfaces
(e.g., tunnels in structure) and possess a high cation exchange capacity, thus acting as strong scavengers for heavy
metals under neutral pH condition. Cr (III) oxidation (Eq. 1) in soil is directly proportional to Mn(IV) oxides in the soil
[14]. Organic matter, such as hydroquinone has been observed to reduce Cr (VI) in soils as well (Eq. 2) [13].
Cr3+ + 1.5MnO2 + H2O → HCrO4− + 1.5Mn2+ + H+
C6H6O2 + CrO42− + 2H2O → 0.5Cr2O3 + 1.5C6H4O2 + 2.5H2O + 2OH−
(1)
(2)
In general the oxidation of Cr (III) to Cr (VI) in environment is quite difficult as compared to the reduction of Cr
(VI) to Cr (III). In general, under normal environmental conditions, a higher pH value favors the oxidation while a
lower pH value favors reduction [13].
The Geomicrobiology of Chromium (VI) Pollution
The Open Biotechnology Journal, 2016, Volume 10 381
MICROBIAL DIVERSITY IN CHROMITE
Microbial communities residing in igneous rocks remain inadequately characterized. These rocks constitute
approximately 95% of the Earth’s crust. Investigating such microbiological habitat is essential to understand which
organisms are involved in the initial weathering of rocks and minerals and the mobilization of heavy metals, such as Cr
[15]. Generally, rocks are considered an extreme environment for microbial communities because they are unvegetated
and nutrient-limited. Characterizing microbial communities within chromite could yield new insights as found in other
extreme endolithic communities [16]. Some researchers reported different culturable and unculturable microbiological
methods (molecular biology tools) to characterize the diversity and abundance of microbes occupying in weathered,
nutrient-limited terrestrial chromite rocks. Chromium tolerant and reducing bacteria and fungi have been isolated
mostly from chromium-contaminated soil, wastewater, and industrial effluents [17]. Kourtev et al. [18], reported the
effect of hexavalent chromium concentration on microbial communities using denaturing gradient gel electrophoresis
(DGGE), where the addition of chromium led to enrichment and emergence of different microbial populations. Dos
Santos et al. [19], tested the microbial diversity using soil microcosms contaminated with crude oil with or without Cr
and Cu by performing DGGE of PCR amplified 16S rDNA. The oil contaminated soil showed normal growth with a
genetic diversity while the oil and metal contaminated samples had low microbial growth in comparison with the
control group/experiments.
CHROMIUM POLLUTION
The CrO42- and HCrO2- ions are two highly mobile forms of Cr in soils, which can be readily absorbed by plants and
easily be leached out from decomposing organic matter into different soil layers, thus leading to surface and ground
water contamination [20]. In aqueous solution, Cr exists both in trivalent (Cr (III)) and hexavalent (Cr (VI)) forms [21].
Those two forms are commonly dispersed in the environs as a result of different anthropogenic events [22, 23] as well
as geochemical mobilization [17]. The industrial processes from which Cr is released include electroplating, petroleum
refining, leather tanning, wood preservation, photography, metal finishing, pulp processing, and dye and textile
industries [9, 24, 25]. Cr can also naturally be elevated in soils derived from the weathering of Cr-bearing ultramafic
rocks and serpentinites which cover approximately one percent of the global land surface [26]. Moreover, mining
operations in chromite-sediments often create enormous dumps surrounding the mining area. These waste piles contain
waste rocks, unwanted minerals, low-grade chromite ores, and soils. Natural leaching and weathering process occurring
in these materials frequently cause hexavalent chromium production and percolation, causing environmental
contamination and pollution [8, 27].
CHROMIUM TOXICITY
Chromium (III) is an essential micro-nutrient for plants, animals and humans, which is less toxic than hexavalent
chromium [28, 29]. It is reported to play a crucial role in sugar, protein, and lipid metabolism of mammals [8], lowering
blood glucose levels to control diabetes, and to reduce blood cholesterol levels. Generally Cr is present in a variety of
foods: broccoli, Brewer’s yeast, liver, cheese, whole grain breads, and cereals. Some claims have been made that Cr
also facilitates muscle growth [7]. Hexavalent chromium-mediated toxicity is mainly due to its easy uptake across the
cell-membrane, which leads to the development of oxidative stress, DNA damage, carcinogenicity, mutagenicity and
altered gene expression [30]. Furthermore, Cr (VI) is quite soluble in aqueous environments, in the wide pH range,
whereas, Cr (III) tends to be adsorbed on soil surfaces or precipitate at pH values >6.0. Thus, Cr (VI) poses the highest
threat as an environmental contaminant, particularly in surface and ground water systems [31]. Cr (VI) is identified as
one amongst seventeen highly hazardous chemicals to humans by the United States Environmental Protection Agency
(USEPA) [32]. The maximum allowable limit recommended by the World Health Organization for Cr into inland
surface water is 0.1 mg/l, and into drinking water is 0.05 mg/l. If ingested in large doses, elevated levels of Cr (VI) may
lead to loss of life. The LD50 for oral toxicity in rats ranges from 50 to 100 mg/kg for Cr (VI) and 1900- 3000 mg/ kg for
Cr (III) [33]. Higher Cr in soils can potentially cause toxicity to plant life [34, 35]. Due to Cr stress, the reactive oxygen
species like H2O2, OH-, and O2 – are produced, which leads to reduction in CO2 fixation, inhibition of electron transport,
inactivation of enzymes, and chloroplast disorganization [36 - 38]. It is somewhat difficult to assess the Cr-toxicity to
soil microorganisms, as the environments studied were also frequently contaminated with diverse organic pollutants and
heavy metals [39]. It is reported that prokaryotes show higher resistance to Cr (VI) than eukaryotes [40]. In general
Gram-positive bacteria are reported to be more chromate tolerant than Gram-negative bacteria [41]. It was also reported
that Cr negatively affects the microbial activity in soil, and lead to the accumulation of organic carbon [42]. Speir et al..
[43], also reported that short-term Cr (VI) exposures leads to inhibition of enzymes such as phosphatase and sulphatase,
382 The Open Biotechnology Journal, 2016, Volume 10
Al-Battashi et al.
and also decreases the microbial population in soil.
CHEMICAL AND BIOLOGICAL TREATMENT OF CHROMIUM (VI)
Hazardous hexavalent chromium wastes pose significant environmental pollution risks [9]. Due to several industrial
and manufacturing activities, annually more than 170,000 t of Cr waste are released into the environment [40, 25].
Therefore, several technologies to remove Cr (VI) from aqueous solutions have been developed [24]. The industrial
waste and soil are treated by various physico-chemical methods: electrochemical reduction [44], electrocoagulation
[45], precipitation, adsorption [46], ion exchange [47] and membrane separation [48]. However, the initial costs to set
up the necessary tools and to manage those methods are quite high for treatments on a bigger scale. In addition, most of
these approaches are quite incompetent due to incomplete contaminant removal, higher chemical usage and energy
requisites, while still polluting the water tables due to the production of secondary hazardous wastes. The major
limitations of some of those methods are - cost-effectiveness only at high or reasonable contaminant-concentrations and
not at lower concentrations (1 to 100 mg/l) [49].
Microbe-based technologies can be applied as a cost-effective technique for contaminant-metal removal, especially
from sites containing lower contaminant-concentrations [50]. Bioremediation offers huge advantages for the
development of detoxification technologies for Cr (VI)-contaminated soils [27, 51]. Microbial transformation of toxic
Cr (VI) provides the tools for green technologies which are more cost-effective. Biotechnological approaches used to
limit the toxicity of metal pollution are achieved by selectively enriching those naturally occurring microorganisms to
treat particular toxic wastes. The processes by which microorganisms influence removal and recovery of the toxic
metals are: biosorption (passive uptake of metal without reduction), bioaccumulation (active uptake without reduction),
and biotransformation by enzymatic reduction [52, 53]. Chromium (III) is approximately one thousand-times less toxic
than chromium (VI), because the cell membrane is almost impermeable to Cr (III) [54]. For this reason, the
biotransformation of chromium (VI) to chromium (III) and precipitation of Cr (III) has been considered as a promising
way of treatment by various authors [55, 56]. The first reported bacterial strain with the ability to reduce Cr (VI) was a
Pseudomonas sp., isolated from industrial wastewater in 1977 [57]. Several researchers have reported different
microbes resistant to Cr (VI) isolated from different sites (Table 2). There are different mechanisms that microbes use to
overcome the chromium toxicity in their environment, including oxidation-reduction (redox) reactions, sorptiondesorption, and precipitation-dissolution [58].
Table 2. The Cr (VI) tolerant/resistant microorganisms from different sources.
Microorganism
Source of Isolation
Chromium tolerance/resistance
Reference
Brevundimonas sp.,
Sphingomonas sp. and,
Azospirillum sp.
Magnetite mine drainage from Hebei
China
Chromate-resistant and reducing bacteria
Lu et al.. [71],
Bacillus sphaericus
Serpentine soils of Andaman, India
Chromate-resistant and reducing bacteria
Pal & Paul [72],
Bacillus sp. JDM-2-1 and,
Staphylococcus capitis
Wastewater samples, Sheikhupura,
Pakistan
Reduced 81-85% of the Cr (VI) to Cr (III) at
pH 7, 37 °C after 96 h
Zahoor & Rehman [73],
Sediments of Lanzhou Reach of the
Yellow River, China
Aerobically reduce 94.5% of 0.4 mM Cr (VI)
to Cr (III) in 120 h
Zhang et al. [74],
Bacillus sp.
Chromate contaminated soil, India
Cr resistant and reducing bacteria (enzyme chromate reductase)
Elangovan et al. [75],
Desulfovibrio vulgaris
Hildenborough strain (DSM 644)
Accumulated precipitates of Cr (III) on their
cell surfaces
Goulhen et al. [76],
Cellulomonas sp.
Cr(VI) impacted core from a borehole in
a dichromate plume on the US
Department of Energy’s Hanford facility
in southeastern Washington State
Reduction of Cr (VI) to Cr (III)
Viamajala et al. [77],
Arthrobacter sp., and a
Bacillus sp.
Long-term tannery waste contaminated
soil, Mount Barker, South Australia
Cr (VI)-reducing ability and resistance to
Cr(VI)
Megharaj et al. [22],
Pseudomonas sp. strain
RNP4
Long-term tannery waste contaminated
soil, Chennai, India
Cr (VI) reduction
Rajkumar et al. [78],
Staphylococcus aureus
Serratia marcescens
Acinetobacter and
Ochrobactrum
Local tannery effluent, Concepción, Chile Cr resistant and reducing bacteria (enzyme chromate reductase)
Activated sludge of a wastewater
treatment plant, central Portugal
Cr (VI)-reducing ability and resistance to
Cr(VI)
Campos et al. [79],
Francisco et al. [80],
The Geomicrobiology of Chromium (VI) Pollution
The Open Biotechnology Journal, 2016, Volume 10 383
(Table ) contd.....
Microorganism
Source of Isolation
Chromium tolerance/resistance
Reference
Enterococcus casseliflavus
Tannery effluent, Vellore district, India
Reduce the Cr (VI) through adsorption
process
Saranraj et al. [81],
Enterococcus gallinarum
Tannery waste-contaminated soil, Fez,
Morocco
Reduce chromate to 100% at a concentration
of 200 mg/ l, in aerobic conditions
Sayel et al. [82],
Pseudomonas sp.,
Industrial wastewater
Reduce Cr (VI)
Romanenko & Korenkov
[57],
Ochrobactrum intermedium
BCR400
Cr contaminated soil collected from
Vadodara, Gujarat, India
Reduced 100 mg Cr(VI)/L completely in 52
h with initial Cr(VI) reduction rate of 1.98
mg/L/h.
Kavita & Keharia [83],
Bacillus sp. BT1
Cr polluted soil collected from Tannery
industry, Tamilnadu, India
In the presence of anthraquinone-2-sulfonic
acid (AQS), it reduced a total of 400 mg
Cr(VI)/L
Kavita & Keharia [84],
Aspergillus sp.
Cr contaminated sites
Cr(VI)-reducing ability and resistance to
Cr(VI)
Coreño-Alonso et al. [85];
Fukuda et al. [86]; Shugaba et
al. [34]; Srivastava & Thakur
[34],
A. flavus, A. niger,
Fusarium solani, and
Penicillium chrysogenum
Contaminated peri-urban agricultural
soils of Faisalabad, Pakistan
Cr tolerant capability
Iram et al. [87],
Trichoderma species
Cr contaminated sites
Chromium tolerant and resistant capability
Debpali et al. [88]; El-Kassas
et al. [89]; Shriram et al. [90],
Paecilomyces sp.
Isolated from Mexico
Complete disappearance of Cr (VI), with the
concomitant production of Cr (III)
Cárdenas-González &
Acosta-Rodríguez [68],
Glomus intraradices
(mycorrhizal fungus)
Chromium contaminated agricultural soil,
Salt Lake City, Utah
Cr tolerance and hyperaccumulation
Davies et al. [91],
A. niger and A. flavus
Contaminated soil and water, Guimaras
Province, Philippines
Cr tolerance (600 ppm)
Bennett et al. [92],
Aspergillus niger and T.
viride
Chromium contaminated soil, Tamilnadu,
India
30.8 - 83.4% reduction
Sunitha & Rajkishore [93],
T. harzianum
Provided by Center for advance studies in
botany (CAS), University of Madras,
India
Removal by biosorption (90.2%)
Sarkar et al. [94],
Uptake capacity of 2.55 mg/g
Kumar et al. [95],
Cr reduction efficiency of 91.15%
Sukumar [96]
Aspegillus nidulans,
Samples of sewage, sludge and industrial
Rhizopus arrhizus, T. viride
effluents, Haryana, India
Rhizopus oryzae
Procured from IMTECH, Chandigarh,
India
Bacterial Reduction of Chromium (VI)
Even though heavy metals are quite toxic to majority of microorganisms, some bacteria developed the resistance
mechanism. Several habitats exposed to anthropogenic or natural metal contamination over an extended period of time
showed that long-term exposure to those metals acted as a selective-enrichment which favored the growth of metaltolerant microbes [59]. The transformation of Cr (VI) to Cr (III) in the microbial cells leads to the formation of
intermediate 'O' radicals and unstable oxidation states of chromium respectively (such as Cr(V) and Cr(IV)), which are
more toxic than Cr (III) [60]. Despite these highly reactive compounds such unique microorganisms have alternative
mechanisms to overcome these problems: chromate resistant plasmids, iron efflux systems, and variations in the
reduction mechanisms [22, 60, 61]. Generally, chromium reduction mechanisms can be enzymatic or chemical, where
the microbes follow either single or combined process. Aerobic bacteria such as Bacillus, Pseudomonas, Streptomyces,
and Leucobacter spp. are associated with soluble chromate reductases (using NADH or NADPH as cofactors to reduce
the Cr), whereas the anaerobic ones including Shewanella, Enterobacter, and Sulfate reducing bacteria (SRB) have
membrane-bound reductase such as flavin reductase, cytochrome, and hydrogenase, which are part of the electron
transport system. They use Cr (VI) as a terminal electron acceptor instead of NO3- or SO42-, as has been reported for
Desulfotomaculum reducens [62]. These two Cr (VI)-reducing activities were found in Desulfovibrio vulgaris where
Cytochrome c3 was reported to catalyze Cr (VI) and uranium (VI) reduction. Here, the cytochrome may function as
both U (VI) and Cr (VI) reductase [55]. However, microbial reduction of Cr (VI) may also take place through chemical
reactions associated with intra/extra cellular compounds, such as amino acids, vitamins (vitamin C, in particular),
nucleotides, sugars, organic acids or glutathione. In addition, some microbes have their own plasmid resistant
mechanism, which is considered to be a powerful detoxification process of Cr (VI) [63].
384 The Open Biotechnology Journal, 2016, Volume 10
Al-Battashi et al.
Bacterial Oxidation of Chromium (III)
It is well-known that chromite has very low water solubility and high chemical resistance to dissolution. The
oxidation of Cr (III) could occur via O2-, H2O2, and Mn-oxides. In subsurface environments, the direct oxidation of Cr
by oxygen is limited because of the slow kinetics [64]. However, the oxidation of Cr by H2O2 is probably insignificant
due to its limited subsurface production. Mn(IV) oxides are the only known naturally occurring oxidants for Cr (III).
Karen et al. [65], reported Mn(II) oxidizing Bacillus sp. strain SG-1, which accelerated the Cr (III) oxidation 7-times
faster than without bacteria, in the presence of Mn. Mn(II) oxidizing bacteria accelerate the production of reactive
Mn(IV) oxides, which are required in the oxidation of Cr (III). Another study conducted with soil containing Mn oxides
(Mn3+, Mn4+) showed that the oxidation of Cr (III) took place under aerobic system, in presence of Mn oxidizing
Pseudomonas putida species [66].
Fungal Chromium Resistance
Chromium resistance has been reported in both yeast and filamentous fungi isolated from Cr contaminated
environments (Table 2). Cr resistance in the yeasts strains including Candida and Rhodosporidium was related to their
ability to reduce ion uptake, rather than to their biological reduction of Cr (VI) to Cr (III). Species like zygomycetes
fungus Mucor rouxii and the yeasts Candida albicans, Yarrowia lipolytica, and S. cervesiae were very sensitive to
chromium without any change in chromium concentration in the media [55]. In contrast, strain RR1 of Candida sp.
showed a high capacity to resist Cr, being highly efficient in reducing Cr (VI) to the less toxic form via contact cells and
crude cell-free extracts [67]. Moreover, Paecilomyces sp. showed its capacity to reduce 50 mg/l Cr (VI), with the
associated production of Cr (III) in the growth medium after 7 days of incubation [68]. Congeevaram et al. [69],
reported that Aspergillus sp. isolated from contaminated site showed a high resistance to chromium of up to 10,000 mg/l
via biosorption processes. Viti et al. [70] reported in-depth comparative analysis of molecular mechanisms for Cr(VI)
resistance in bacteria and fungi compared with classical microbiological approaches: Both bacteria and fungi respond to
presence of Cr(VI) by combining cellular networks systems at several levels, such as the reducing power generated by
iron and sulfur metabolism, protein oxidative stress protection, DNA repair, Cr-efflux pumps, and detoxification
enzymes - such as Cr(VI) reductases. Chromate-resistance determinants (CRDs) have been reported from different
microbes, which consist of genes belonging to the chromate ion transport (CHR) superfamily. Generally, CRDs include
the chrA gene, which encodes a putative chromate efflux protein. However it has been reported that chrA genes provide
Cr(VI) protection only in very low concentrations range [70]. They concluded that the currently available knowledge is
largely theoretical and still a molecular understanding of many aspects of Cr(VI) resistance in microorganisms are
poorly understood.
CONCLUSION
Microorganisms exist in different environments and many of those environments have been investigated. Although
considerable knowledge about microbial diversity has been amassed, most of these organisms remain uncharacterized,
due to several reasons including habitats that have not been investigated, organisms that are difficult to culture in
laboratory conditions, and inaccurate identification of compiled samples. Many microbes isolated from the
contaminated area were capable of reducing the chromium toxicity and the search continues to find more tolerant
microbes. The presence of high chromium-tolerant or/and resistant microbes shows that they have evolved different
mechanisms to reduce the detrimental effects on their cells. This brief review based on published literature summarizes
the importance of employing indigenous microorganisms to reduce chromium toxicity as a cost effective and
environmentally sustainable technology.
CONFLICT OF INTEREST
The authors confirm that this article content has no conflict of interest.
ACKNOWLEDGEMENTS
Declared none.
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