Biomethylation, also sometimes known as biovolatilization, is defined as the biological conversion of metals and metalloids to both volataile and nonvolatile methylated metabolites
[52][69]. First observed in fungi, biomethylation plays an important role in microbial arsenic detoxification as well as in the biogeocycling of arsenic. The ability of microorganisms to both biomethylate and resist arsenic is conferred through the activation of the arsM gene
[53][70]. Comparing orthologs of the arsM gene in different microorganisms revealed its prokaryotic origin, explaining its similarity to the ars operon genes used by bacteria
[54][71].
45. Recently Isolated Arsenic Tolerant Microorganisms and Approaches Used in the Microbial Bioremediation of Arsenic
4.1. Bacteria
5.1. Bacteria
Numerous bacterial species are known to not only tolerate high concentrations of arsenic but also transform it from one ionic species to another. Since the ionic species of arsenic decides its toxicity as well as its mobility in the environment, bioremediation approaches usually rely on the biotransformation capability of microorganisms.
The oxidation of As(III) is of particular interest to bioremediation studies due to its potential application in large scale pre-treatment of arsenic-contaminated groundwater. In typical water treatment processes, As(V) can be easily removed through conventional physico-chemical techniques such as adsorption or ion exchange
[55][81]. However, treating As(III) is considered more challenging due to its solubility and its low affinity for absorbents
[56][82]. To remedy this, most operations rely on converting As(III) to As(V) through chemical pre-oxidation, which, by itself, is inherently inefficient due to the slow reaction rate and its tendency to form potentially toxic oxidation byproducts. Recent technologies are now shifting towards utilizing the inherent ability of some microorganisms to oxidize As(III) and coupling it with other arsenic removal strategies.
Some of the most effective microorganisms used in arsenic bioremediation, both in batch reactor experiments and in situ applications, are sulfate-reducing bacteria (SRB)
[57][84]. Investigating the effect of inoculating
Desulfovibrio vulgaris in waters to simulate arsenic-contaminated aquifers, researchers were able to observe the successful reduction of As(V) with and without sulfate amendment
[58][85]. To probe the biotransformation of arsenic compounds, researchers screened for native arsenic-tolerant microorganisms in dam tailings from a gold mine from Iran
[59][86].
Bacterial species belonging to the genus
Bacillus are among the most robust microbes used in the treatment of arsenic in soils. In a study which screened for potential arsenic hyper-tolerant microorganisms from a gold mine in Brazil, a strain of
Bacillus cereus was isolated and found to be resistant to up to 3000 ppm of As(III) in lab conditions
[60][79]. Taking advantage of the arsenic resistance of the bacterial strain, its ability to bioaccumulate and oxidize As(III) in vitro was also assessed.
Perhaps one of the most remarkably arsenic-tolerant bacterial strains isolated so far is a strain of
Bacillus firmus isolated from soil near the Lonar lake in India
[61][78]. The strain, characterized as
Bacillus firmus L-148, showed exceptional tolerance to exceedingly high concentrations of As, capable of thriving in concentrations of up to 247,241 ppm of As(III) and 299,686 ppm of As(V). The hyper-tolerant strain was also able to oxidize As(III) in the presence of other heavy metals and in alkaline conditions. While the latter is expected due to the fact that the lake where it is indigenous is naturally basic, the bacteria were capable of oxidizing As(III) even in buffered conditions.
4.2. Fungi
5.2. Fungi
Compared to bacteria, fungi have the advantage of being deemed as the dominant living biomass present in soils
[62][90]. The intimate association between fungi and soil is due to the low degree of shear strain experienced by soils, allowing the development of the fungal hyphal network
[63][91]. Despite this, the potential of fungi in bioremediating arsenic in soils remains restricted. Nevertheless, recent studies have gradually started exploiting the various metabolic capabilities of these organisms against arsenic contamination.
Explorations into filamentous fungi such as Penicillum
[64][65][92,95], Fusarium
[66][93], Trichoderma
[67][94], Humicola
[68][96], and Aspergillus
[65][66][93,95] showed notable resistance to high arsenic concentrations. Screening of As(V)-contaminated agricultural soil in India revealed that a strain of Penicillium coffeae can tolerate As(V) concentrations of up to 37,461 ppm in vitro
[64][92]. The fungus was also able to tolerate the same concentration of As(V) under basic conditions, whether living, dead, or as treated biomass. However, the exact mechanism the fungus utilizes in tolerating As(V) remains unknown.
Among the numerous filamentous fungi used for arsenic bioremediation, the most used and studied is
Aspergillus. Past research
[69][70][71][97,98,99] has already attested to the efficacy of using indigenous
Aspergillus sp. present in soils to mitigate arsenic contamination.
4.3. Microbial Consortium
5.3. Microbial Consortium
A relatively unexplored approach to the bioremediation of arsenic is the identification and application of arsenic-resistant mixed microbial consortia. When using pure cultures, bioremediation efficiency may be limited by the difficulty of maintaining pure cultures, especially in in situ applications. Another important consideration is the ability of pure cultures to bioremediate complex contamination scenarios. Efficient bioremediation using mixed microbial cultures has consistently been observed for a myriad of pollutants, which has been attributed to the symbiotic and co-metabolic action between the different species in a specific consortium
[72][103].