Bacterial Resistance to Mercury: A Mini-Review

Abstract

With the increase in mercury pollution around the world, several bacteria have been identified that are capable of resisting mercury toxicity. With this in mind, the aim of this review was to determine which genes are involved in mercury resistance, which bacterial genera exhibit this resistance, and which bacterial isolation sources have been most reported. To answer these questions, the PICO method (population, intervention, comparison, and outcome) was used, three databases were searched, and 17 relevant articles were included. As a result, resistance is due to a set of mer genes that transcribe mer proteins. The most important genes identified were merA and merR, and their proteins confer resistance by reducing Hg to Hg⁺² or Hg⁰. Among the bacteria studied, those of the genera Pseudomonas, Escherichia, and the phylum Cyanobacteria stand out, the most important being Escherichia coli and Synechocystis sp., which are highly efficient and fast at reducing Hg. Based on the results, Escherichia coli and Synechocystis sp. are promising candidates for reducing environmental Hg, especially in aquatic environments. However, there is a lack of studies on the mechanism of bioremediation carried out by cyanobacteria and the influence of abiotic factors on the presence and/or expression of mer genes.

1. Introduction

Mercury (Hg) and its by-products are some of the most hazardous environmental contaminants with bioaccumulative, neurotoxic, carcinogenic, and genotoxic effects that pose risks to environmental and human health [1,2]. Due to its high use worldwide, the level of Hg pollution is constantly increasing. According to the United Nations Environment Program, global mercury emissions from human activities reached 2200 tons in the atmosphere in 2015 [3].

Mercury is classified as a toxic element (TE), and its presence in the environment can be due to natural causes such as volcano emanations, weathering, mercuric evasion from the aquatic environment and continental degassing, along with anthropogenic activities such as mining, paint manufacturing, metallurgical activities, charcoal burning, dental waste and the production of chlorine and caustic soda, among others [4,5,6,7,8,9,10,11]. Mercury occurs in three different states: elemental mercury Hg⁰, which presents oxidation 0; inorganic Hg, such as mercurous or monovalent Hg (Hg⁺), which presents oxidation I, or divalent Hg (Hg⁺²), which presents oxidation II; and organic Hg, such as ethyl (C₂H₅Hg⁺) and methyl Hg [(CH₃Hg)⁺] [12].

Human activities have led to the increased contact of organisms with these harmful substances, leading them to adapt in an attempt to survive under extreme conditions. One group of these organisms is represented by bacteria, which have selected a set of genes providing them with resistance to Hg [13]. The operon involved is the mer operon, which is a set of genes that make up the Hg resistance system. It can be found in several positions in the genome and can occur in genomic DNA, plasmids, integrons or Tn21 transposon components [14]. The mer operon presents several functional genes responsible for producing proteins that will promote the sequestration and volatilization of Hg, promoting resistance to it.

The main opening reading frames (ORFs), merR, merA, and merP, are responsible for encoding the transcriptional regulator (MerR), mercuric reductase (MerA), and a periplasmic Hg binding protein to organomercurial lyase (MerB). MerB and MerA are key enzymes in the volatilization of Hg, reducing Hg⁺² to Hg⁰ and releasing it into the environment. MerR, on the other hand, is responsible for encoding an activator–repressor protein that will act by regulating the transcription of genes involved in resistance in the presence of Hg. In some bacterial species, additional genes such as merD, an additional transcriptional regulator, as well as merE and merF, auxiliary transporters, may be present. Among other genes, merT, merC, merG, and merH are responsible for encoding internal transmembrane proteins [15].

Resistance to Hg makes bacteria a highly important tool for reducing the level of Hg pollution as they have a high removal efficiency in addition to being able to be used as a low-cost tool that safely restores polluted environments [16]. Such pollutant removal through bioremediation can be a process of eliminating Potentially Toxic Elements (EPTs) from contaminated sites through micro-organisms that will transform the complex and more toxic contaminants into simpler and less toxic substances [17,18]. Some studies used the metagenomic approach, which, in addition to functional analysis, identified numerous different bacterial genera that could be used in the biodegradation of toxic elements. Among them, there were bacteria belonging to the genera Bacillus, Pseudomonas, Acinetobacter and Staphylococcus [19]. In another study carried out by [20], a total of 92 bioremediation bacterial species from 45 genera were identified. Among these, Pseudomonas citronellolis, Flavobacterium aquatile, Methylibium petroleiphilum, Pantoea agglomerans, Bordetella petrii, Flavobacterium aquatile, and Methylibium petroleiphilum were found in high abundance.

Concerning this, it is known that a wide variety of micro-organisms, such as Shigella flexneri, Pseudomonas aeruginosa, Serratia marcescens, Xanthomonas sp., Staphylococcus aureus, and some Cyanobacteria, possess mer genes. However, when mer genes were compared, their numbers and operon organization differed among several bacterial genera. Against this background, the aim of this work was to identify which bacterial genera presented such genes and to determine which genes were most expressed and in which environment these bacteria were most found in the last decade. We also provide relevant information regarding the control of Hg contamination, which is an issue to be solved urgently.

2. Materials and Methods

After three sequential steps, two authors (D.G.M.D. and R.G.F.) independently preliminarily selected the abstracts and titles of the identified articles. The search was limited to manuscripts written in English and published between January 2013 and May 2023. Editorials, letters, and doctoral theses were excluded. The results were reported according to the preferred reporting items for systematic review and meta-analyses (PRISMA).

2.1. Focus Questions

The questions were developed according to the PICO (population, intervention, comparison and outcome) method. The following questions were asked: Which bacterial genera exhibit such resistance? What genes are involved in Hg resistance? What sources of isolation have been reported?

2.2. Information Sources

A bibliographic search was performed using terms in the Medical Subject Headings (MeSH) in the databases PubMed, Embase, and Web of Science. The screening was conducted from May 2023 to August 2023. Search Component 1 (SC1) included the population: bacteria* OR micro-organism OR microorganisms OR “bacterial genes” OR “resistance genes” OR microbiota OR microbiome. Search Component 2 (SC2) included the intervention mercury OR Hg. After retrieving the search component results, the Boolean operator “AND” was used to combine SC1 and SC2.

2.3. Risk of Bias Assessment

Possible sources of bias include study inclusion/exclusion criteria, the chosen database, date, language, number of articles, and article type selected for this study. Another important bias assessment refers to the analytical methodologies used by the different researchers in the detection of Hg-resistant bacteria and the genes involved as well as bacteria capable of methylating and demethylating Hg.

3. Results

3.1. Literature Search

A total of 5820 articles were identified: 1591 in PubMed, 1531 in Embase, and 2698 in Web of Science (WOS). Of these, 4721 were duplicates or triplicates and were excluded, leaving a total of 1099 articles. After reading the titles and abstracts, only 118 articles were selected because they dealt with subjects appropriate to the aim of the study with 981 articles being excluded. Finally, 17 articles out of the 118 were selected and used in the study, since they focused on identifying Hg-resistant bacteria as well as detecting the genes responsible for this resistance and identifying the environment from which they were collected (Figure 1).

3.2. Bacteria and Mercury Concentration

From the selected articles, the presence of 23 bacterial genera that showed resistance to Hg was verified with the genera Escherichia and Pseudomonas being the most studied ones (Table 1).

One of the explanations for this predominance of the genus Escherichia is that it presents individuals considered model organisms for scientific research. The species of this genus are easy to manipulate and cultivate and multiply rapidly. In addition, they can easily survive in environments with altered oxygen contents, low nutrient availability, and fluctuating temperatures [21]. Escherichia, as a well-studied genus, is a promising candidate for the bioremediation of Hg-contaminated sites since it can easily be manipulated and genetically modified, facilitating its production. This also favors the development of other mechanisms, such as silica encapsulation, which provides protection for the biocatalyst and avoids the dispersion of bacteria and their general mechanical structures through effluents [22]. These bacteria, depending on the genetic material used, can remove Hg at a range of different concentrations: for example, 5 μM Hg⁺² [22], 50 μg mL⁻¹ of Hg [23], or 1 mg L⁻¹ of methylmercury chloride, which is 1000 times higher than the levels normally observed in contaminated environments [24,25,26].

The genus Pseudomonas contains bacteria that are prevalent in several environments, mainly in water and soil, because of their flagellar mobility and biofilm production. These, although not tested at high Hg concentrations by the authors, are efficient in performing Hg bioremediation and can, e.g., withstand concentrations of 280 μM of HgCl₂ [27] or 7 mg L⁻¹ of Hg without losing their activity, achieving an efficiency of up to 99.2% in the removal of Hg from the medium [28].

Regarding the Hg concentration, the highest concentrations used in the tests were 500 µM and 500 ppm. As shown in Table 1, the bacteria able to multiply at these concentrations were Synechocystis sp., Enterobacter cloacae, Enterobacter ludwigii, and Klebsiella pneumoniae, with growth at 500 µM, and Bacillus cereus, Bacillus sp. and Brevundimonas diminuta, grown at a Hg concentration of 500 ppm. Therefore, these studies demonstrate the efficiency of these bacteria in terms of surviving and removing Hg from the environment, being potential candidates in the remediation of Hg-contaminated sites. However, more studies are needed, as many of these bacteria have pathogenic potential, such as Bacillus cereus, Enterobacter, and Klebsiella pneumoniae.

In addition, a critical point is that some of the bacteria used in these studies were also resistant to antibiotics. Such resistance may have been caused by the presence of antibiotics at the collection sites, since many of the bacteria studied were obtained from heavily contaminated places but may also be associated with resistance to heavy metals also found in these bacteria. There may be a correlation between antibiotic resistance and resistance to heavy metals. This correlation may be due to co-resistance, in which the genes involved in both heavy metal resistance and antibiotic resistance are found in the same genetic element. If this element is mobile, such as a plasmid or transposon, it may result in the transmission of both resistances to other bacteria, since they are highly transmissible elements. An example of this is the gene that confers resistance to Hg, which was located in the pU302L plasmid, found in Salmonella, conferring resistance to various classes of antibiotics [29]. Another form of this correlation is through cross-resistance, which results from the crossing of mechanisms used to provide resistance. Generally, this correlation is associated with a non-specific membrane transport system in relation to its substrates, giving rise to other resistances [29]. An example of this is the multidrug efflux pump MdrL found in Listeria monocytogenes, which functions as an antibiotic resistance mechanism, reducing the efficacy of drugs such as cefotaxime, erythromycin or josamycin; at the same time, this transport system functions for heavy metals, reducing the levels of Zn, Co, and Cr [30]. Finally, this resistance can occur through co-regulation, in which case exposure to heavy metals will alter the expression of coding genes that confer antibiotic resistance [29]. As an example, the CscRS regulatory system encoded by the cscRs gene, which can be found in Pseudomonas aeruginosa, reduces the expression of a porin called OprD, through which the carbapenem antibiotic called imipenem enters the bacteria. This system, in the presence of zinc, influences the transcription of the czcCBA operon, which encodes an RND-type efflux pump, conferring resistance to zinc, cadmium, and cobalt [31].

In addition to the genera mentioned above, some studies have pointed to Cyanobacteria as strong bioremediators that can be used in the removal of Hg from aqueous ecosystems. As Cyanobacteria can perform oxygenic photosynthesis and fix atmospheric N₂ in most species, they readily adapt to extreme environments, making them suitable candidates for this bioremediation [32,33,34]. Furthermore, they have a high affinity to toxic metals and can bind with multiple contaminants [35,36] in addition to being easily genetically manipulated [37,38]. Thanks to these advantages, Cyanobacteria are highly efficient removers of Hg and its derivatives in the aquatic environment.

However, several genera of Cyanobacteria are capable of producing cyanotoxins, and cyanotoxin-producing cyanobacterial blooms are generally referred to as harmful algal blooms (HABs) [39,40]. The presence of these toxic HABs can become a worrying factor to consider, as their presence in drinking water can lead to health risks for animals and humans [41]. In addition, some Cyanobacteria are capable of producing cyanotoxins with such high toxicities that they can be used as chemical weapons [42,43].

It should be emphasized that Cyanobacteria present a certain sensitivity to antibiotics, as reported by [44], who identified isolated strains of Cyanobacteria sensitive to penicillin G and streptomycin. These authors also observed that such bacteria produced, in bioremediation, in addition to Hg⁰, a β-HgS molecule. This molecule is one of the two different structural forms of HgS (mercuric sulfide) found in the environment [45] and a semiconductor that can potentially be used in infrared sensors or optoelectronic devices [46].

One of the Cyanobacteria studied by these authors is Synechocystis sp., which is a species highly amenable to genetic modification. It presents a well-known genome and is available in several genetic systems. In addition, this species has a model species called Synechocystis 6803 [47]. Based on the studies included in this review, Synechocystis sp. is a potential candidate for the remediation of Hg-polluted sites.

Table 1. Number of isolates studied, genes searched, the method used to find the genes, as well as the test concentration and maximum demonstrated growth and the site of sample collection, carried out respectively by each paper studied.

Source	Bacteria	Genes	Experimental Design	[Hg] Tested	[Hg] Max 1	References
Water	Staphylococcus epidermidis	merA, merT, merC and merR	PCR	40–100 µM	100 µM	[29]
Laboratory	Synechocystis sp.	merA and merR	PCR	0–500 µM	500 µM	[34]
Wellwater	Achromobacter xylosoxidans	merR, merT, merP, merC, merA, merD, merE and merR	WGS	-	-	[48]
Laboratory	Escherichia coli	merR	Laboratory manipulation	5 nmol L−1	5 nmol L−1	[49]
Soil	Enterobacter cloacae, Enterobacter ludwigii and Klebsiella pneumoniae	mer operon (merA)	Laboratory manipulation	10–250 µM and 25–500 µM	200 µM and 500 µM	[50]
Laboratory	Salmonella enterica I 4,[5],12:i:-	merR and merT	In silico	-	-	[51]
Laboratory	anammox bacteria of the genus Candidatus Kuenenia	merA, merB, merD and merR	PCR	0–50 mg L−1	50 mg L−1	[52]
Slaughterhouses	Escherichia coli	merA and merC	PCR	25 μg/mL and 50 μg/mL	25 μg/mL and 50 μg/mL	[24]
River water	Klebsiella sp., Escherichia coli, Serratia marcescens, Proteus sp., Citrobacter sp., Pseudomonas sp., Acinetobacter sp. and Enterobacter sp.	merA, merD, merR, merP, merT and merB	Isolation	10 mg L−1	10 mg L−1	[53]
Water of aquaculture system	Aeromonas sp., Salmonella sp., Shewanella sp., Pseudomonas aeruginosa, Myroides odoratus, Serratia liquefaciens, Vibrio fluvialis and Chryseobacterium sp.	merA	PCR	0.005–2.5 mM	2 mM	[54]
Laboratory	Escherichia coli K12	merR		-	-	[55]
Laboratory	Escherichia coli	merR	Laboratory manipulation	${10}^{-5}$ mmol L−1	10 μmol L−1	[56]
Laboratory	Bacillus cereus, Bacillus sp. and Brevundimonas diminuta	merA, merP, merT and merB	Isolation	50–500 ppm	500 ppm	[57]
Wastewater	Rheinheimera tangshanensis	merT, merR, merC, and merA	PCR	0–120 mg L−1	solid medium: 120 mg L−1 and liquid medium: 60 mg L−1	[58]
Seawater	Pseudomonas, Bacillus and Pseudoalteromonas	merA	PCR	25–100 mg L−1	100 mg L−1	[59]
Wastewater	Pseudomonas	merR, merA and merT	WGS	-	60 ppm	[60]
Soil	Pseudoxanthomonas sp.	merA	PCR	1–6 mg L−1 and 5–80 mg L−1	3 mg L−1 and 40 mg L−1	[61]

¹ maximum concentration at which growth occurred.

Table 1. Number of isolates studied, genes searched, the method used to find the genes, as well as the test concentration and maximum demonstrated growth and the site of sample collection, carried out respectively by each paper studied.

Source	Bacteria	Genes	Experimental Design	[Hg] Tested	[Hg] Max 1	References
Water	Staphylococcus epidermidis	merA, merT, merC and merR	PCR	40–100 µM	100 µM	[29]
Laboratory	Synechocystis sp.	merA and merR	PCR	0–500 µM	500 µM	[34]
Wellwater	Achromobacter xylosoxidans	merR, merT, merP, merC, merA, merD, merE and merR	WGS	-	-	[48]
Laboratory	Escherichia coli	merR	Laboratory manipulation	5 nmol L−1	5 nmol L−1	[49]
Soil	Enterobacter cloacae, Enterobacter ludwigii and Klebsiella pneumoniae	mer operon (merA)	Laboratory manipulation	10–250 µM and 25–500 µM	200 µM and 500 µM	[50]
Laboratory	Salmonella enterica I 4,[5],12:i:-	merR and merT	In silico	-	-	[51]
Laboratory	anammox bacteria of the genus Candidatus Kuenenia	merA, merB, merD and merR	PCR	0–50 mg L−1	50 mg L−1	[52]
Slaughterhouses	Escherichia coli	merA and merC	PCR	25 μg/mL and 50 μg/mL	25 μg/mL and 50 μg/mL	[24]
River water	Klebsiella sp., Escherichia coli, Serratia marcescens, Proteus sp., Citrobacter sp., Pseudomonas sp., Acinetobacter sp. and Enterobacter sp.	merA, merD, merR, merP, merT and merB	Isolation	10 mg L−1	10 mg L−1	[53]
Water of aquaculture system	Aeromonas sp., Salmonella sp., Shewanella sp., Pseudomonas aeruginosa, Myroides odoratus, Serratia liquefaciens, Vibrio fluvialis and Chryseobacterium sp.	merA	PCR	0.005–2.5 mM	2 mM	[54]
Laboratory	Escherichia coli K12	merR		-	-	[55]
Laboratory	Escherichia coli	merR	Laboratory manipulation	${10}^{-5}$ mmol L−1	10 μmol L−1	[56]
Laboratory	Bacillus cereus, Bacillus sp. and Brevundimonas diminuta	merA, merP, merT and merB	Isolation	50–500 ppm	500 ppm	[57]
Wastewater	Rheinheimera tangshanensis	merT, merR, merC, and merA	PCR	0–120 mg L−1	solid medium: 120 mg L−1 and liquid medium: 60 mg L−1	[58]
Seawater	Pseudomonas, Bacillus and Pseudoalteromonas	merA	PCR	25–100 mg L−1	100 mg L−1	[59]
Wastewater	Pseudomonas	merR, merA and merT	WGS	-	60 ppm	[60]
Soil	Pseudoxanthomonas sp.	merA	PCR	1–6 mg L−1 and 5–80 mg L−1	3 mg L−1 and 40 mg L−1	[61]

¹ maximum concentration at which growth occurred.

3.3. Genes Involved

Regarding the genes involved in Hg removal, several techniques have been used, such as PCR (polymerase chain reaction), genome sequencing, or laboratory manipulation. Of these, the most used technique was PCR.

Among the most researched genes using PCR, merA stands out. Most likely, this is because it is responsible for transcribing MerA, an enzyme considered key to the Hg reduction system, which is capable of reducing Hg²⁺ in Hg⁰; thus, it can be determined whether the bacteria can bioremediate Hg. This protein, during this transformation process, is dependent on a reduced pyridine nucleotide (NADPH) and an exogenous thiol; the nucleotide donates electrons and is converted into NADP+. Another important and much-researched gene is merR, which is responsible for the transcription of MerR. This is an enzyme that regulates the transcription of other genes in the presence of Hg, repressing transcription in its absence or stimulating transcription in its presence, which makes it a fundamental gene for the bioremediation process [15] The functions of the other genes and proteins, as well as the merR and merA genes and their proteins (MerR and MerA), are shown in Figure 2.

Regarding Cyanobacteria, such as Synechocystis sp., of all the studied mer genes, only merA and merR have been reported in their genome so far; they are located distant from each other in the genome, forming a separate clade. In these bacteria, these genes are responsible for producing proteins homologous to MerR and MerA, proteins Slr0701 and Slr1849, respectively. Additionally, in Cyanobacteria, the mer genes are not organized into operons, unlike in other bacterial genera such as Pseudomonas and Escherichia [62]. Another important point is that the proteins produced by these genes have been identified mainly in Cyanobacteria from freshwater lakes, although they have also been detected in soil crusts [63]. However, even though this difference has been proven, the organization of the mer genes in Cyanobacteria is poorly understood, and more studies are needed to better elucidate the reason for this difference and their evolutionary importance.

Ref. [62] reported that upon contact with mercury (II) chloride [HgCl₂], the proteins produced by merA and merR were altered, showing a gradual decrease in negative ellipticity, causing a relaxation in their secondary structures upon binding to HgCl₂. Initially, this decline may be attributed to this binding, whereas a steeper decline in this negative ellipticity may ultimately suggest a denaturation of the protein. In addition, the presence of these mer genes resulted in a lower effect of Hg damage on the photosynthetic performance of Cyanobacteria. Such mer genes allow Synechocystis sp. to survive and grow even in the presence of high Hg concentrations (up to 500 µM).

3.4. Environments in Which the Gene Was Found

Nine different environments were identified (water, laboratories, wells, soil, slaughterhouses, river water, water of aquaculture systems, wastewater, seawater, and fish) with a total of 18 locations (Table 1). Of these, the aquatic environment was the most reported one. Water facilitates the transport and transfer of genes linked to resistance [64,65] and is, therefore, an ideal medium for the study of Hg-resistant bacteria. Soil, on the other hand, is generally poor in nutrients, with lower temperature control and less fluidity, impeding bacterial growth as well as gene transfer and transport. However, more bacteria can be found in warmer, nutrient-rich sites such as the rhizosphere, the phyllosphere, and decaying animals and plants [66]. Their presence in slaughterhouses is explained by the same mechanism as that working in the soil, in addition to the presence of more fluid media such as blood, favoring growth and gene transfer. Notably, the studies evaluated did not mention whether abiotic factors such as pH, temperature, and salinity influenced gene expression.

The presence of Hg-contaminated water and soil contributes to the selection and expression of these genes due to the constant contact of bacteria with Hg. This contamination occurs due to several factors with anthropogenic activity being the most important one [67]. Through increased deforestation, logging, no-till farming, and, consequently, increased soil erosion, the transport of this TE from contaminated soil to water systems facilitates surface runoff [68]. Similarly, untreated sewage pollutes river waters, and the Hg concentrations of rivers can be up to 15 times higher than those in springs receiving treated sewage.

Another problem is mining, mainly for gold, Hg, lead, and silver. In Brazil, the amalgamation method, used in gold mines, results in a significant release of atmospheric Hg vapors. These vapors contaminate the water used, which, when discarded, further contaminates the environment [69,70]. In addition, the vapors released reach the soil through the absorption of Hg from the atmosphere by plants, contaminating the soil and aquatic systems [71].

Regarding combustion and contamination by industries, according to [72], Asia alone is responsible for approximately 50% of the total anthropogenic Hg emissions, which is mainly due to the combustion of coal for power generation. The industry also plays an important role in Hg emissions to the environment, such as in the manufacturing of cements, fluorescent lamps, thermometers, and electrical appliances [73,74,75].

In general, atmospheric Hg emission currently remains one of the main and most important forms of Hg contamination in the world. To date, there is no model that presents an absolute value of the total global atmospheric emission of Hg, and the models proposed so far differ in their estimates between 4000 and 9230 mg year⁻¹, with 45% to 66% related to natural emissions and re-emissions. This difference is due to the fact that the assessment of the global emission levels is dependent on numerous factors, such as the various methodologies that can be used or dependence on many assessment processes to analyze the various transformations of Hg. Another factor that may contribute to this difference is the occurrence of unusual phenomena in the environment, such as Hg depletion events (MDEs), which consist of sudden drops in the total concentration of gaseous Hg in the air of the Arctic or Antarctic. These factors also contribute to a lack of precision in assessing the ratio between the amount of Hg emissions from anthropogenic and natural sources. In relation to global Hg emissions from natural sources on land, a total emission range of 1600 to 2500 mg year⁻¹ from re-emissions has been estimated with a range of 790 to 2000 mg year⁻¹ [76].

4. Conclusions

In the context of this review, it is assumed that the bacterial genera Pseudomonas and Escherichia were the most frequently identified and studied in the last decade in relation to Hg-resistant genes. However, of the species studied, Escherichia coli and Synechocystis sp. are those that have most frequently been characterized as species suitable for the bioremediation of Hg-polluted sites. Synechocystis sp., a highly efficient cyanobacterium that can survive at high Hg concentrations, is one of the most efficient bacteria in highly contaminated aquatic environments and can tolerate Hg concentrations of up to 500 µM. Another relevant fact in the use of this bacterium is its ability to perform photosynthesis, which means that there is no need for additional methods to maintain appropriate nutrient levels. In contrast, Escherichia coli is a bacterium with a mapped genome, which facilitates its manipulation toward a species that can grow under adverse environmental conditions. In addition, most strains of E. coli are commensals, unlike Cyanobacteria, which mostly contain cyanotoxins.

Not all species have the mer operon or mer genes. For example, in Cyanobacteria, so far, only merA and merR have been reported, and they are not organized in an operon. This leads to the conclusion that these two genes are the most important and fundamental ones to confer Hg resistance. The aquatic environment was the most studied one and the most important source of Hg-resistant bacteria. In this environment, Hg-resistant bacteria were also most efficient in terms of bioremediation. To date, there are no reports regarding the impacts of abiotic factors such as pH, temperature, and salinity on the presence and/or expression of Hg resistance genes.