Alexis Durand

François Maillard

Julie Foulon

Michel Chalot
email: michel.chalot@univ-fcomte.fr

Interactions between Hg and soil microbes: microbial diversity and mechanisms, with an emphasis on fungal processes

Keywords:

Mercury (Hg) is a highly toxic metal with no known biological function, and it can be highly bioavailable in terrestrial ecosystems. Although fungi are important contributors to a number of soil processes including plant nutrient uptake and decomposition, little is known about the effect of Hg on fungi. Fungi accumulate the largest amount of Hg and are the organisms capable of the highest bioaccumulation of Hg. While referring to detailed mechanisms in bacteria, this mini-review emphasizes the progress made recently on this topic and represent the first step towards a better understanding of the mechanisms underlying Hg tolerance and accumulation in fungal species and hence on the role of fungi within the Hg cycle at Hg contaminated sites.

Key points:

• The fungal communities are more resilient than bacterial communities to Hg exposure • The exposure to Hg is a threat to microbial soil functions involved in both C and nutrient cycles • Fungal (hyper)accumulation of Hg may be important for the Hg cycle in terrestrial environments • Understanding Hg tolerance and accumulation by fungi may lead to new remediation biotechnologies Keywords: mercury (Hg)

Introduction

History of Hg contamination

Long before modern science, the trace element (TE) mercury (Hg) was found to be a very special metal because of its particularities. Indeed, Hg found in cinnabar ore (HgS) has specific chromatic properties and can be used as a pigment to apply brilliant red colour (vermilion). The first traces of HgS use by humans were discovered during the Neolithic Age (ca. 4000-3000 BCE) as a preservative for human bones. Moreover, Chinese, Egyptian, Hindu, Roman or Greece civilizations used vermilion on skeletons, sculptures, ceremonial cups, buildings or paintings [START_REF] Parsons | A brief history of mercury and its environmental impact[END_REF]. Furthermore, Hg is liquid at ambient temperature, making it a special metal that Aristotle (384-322 BCE) referred to as "fluid silver" and "quicksilver" [START_REF] Wisniak | The history of mercury. From discovery to incommodity[END_REF]).Hg has one of the lowest boiling points of all metals, 357°C at atmospheric pressure [START_REF] Mukherjee | Mercury in waste in the European Union: Sources, disposal methods and risks[END_REF]); in addition, many other properties have been attached to Hg. For example, Hg was also used for religious ceremonies or as an ingredient in traditional medicines [START_REF] Pal | Bhasma : The ancient Indian nanomedicine[END_REF].

Hg is rare; it is the 16 th rarest element on Earth and is classified as one of the quantitatively most important pollutant groups of TE. Methyl-Hg (MeHg) poisoning due to seafood consumption was observed in Minamata Bay, where major environmental pollution was revealed in 1956. Studies on Minamata disease indicated impairment in intracellular calcium homeostasis, alteration of glutamate homeostasis and oxidative stress as important events in MeHg-induced neurotoxicity [START_REF] Farina | Mechanisms of methylmercury-induced neurotoxicity: evidence from experimental studies[END_REF]. The neurotoxic effects of MeHg is of concern for human health even at low concentrations [START_REF] Lebel | Evidence of early nervous system dysfunction in Amazonian populations exposed to low-levels of methylmercury[END_REF]. In 1956, the Minamata Bay seafood displayed high levels of Hg (5.61 to 35.7 ppm), while high levels of Hg (max. 705 ppm) were also detected in the hair of patients, their family and the inhabitants of the Shiranui Sea coastline [START_REF] Harada | Minamata disease: methylmercury poisoning in Japan caused by environmental pollution[END_REF]. Studies on the fate of Hg in the environment have revealed that aquatic ecosystems are particularly sensitive to MeHg exposure due to rapid biomagnification through the food webs of top aquatic predators, such as large fish or birds [START_REF] Campbell | Mercury biomagnification in the food web of Lake Tanganyika (Tanzania, East Africa)[END_REF]. MeHg is also incorporated into bird eggs during their formation [START_REF] Provencher | Mercury and marine birds in Arctic Canada: effects, current trends, and why we should be paying closer attention[END_REF] and can cross the mammalian placenta. The human population is primarily exposed to MeHg by seafood consumption [START_REF] Li | Assessing Sources of Human Methylmercury Exposure Using Stable Mercury Isotopes[END_REF], but other studies have indicated rice consumption as a possible source of Hg contamination [START_REF] Meng | Accumulation of total mercury and methylmercury in rice plants collected from different mining areas in China[END_REF].

Hg is a pollutant that affects all groups of organisms and ecosystems, including microbial communities, fishes, plants, birds and humans. Therefore, investigations of the Hg cycles and transfers within the biosphere have been widely studied to determine the fate of the Hg emitted into aquatic [START_REF] Lavoie | Biomagnification of mercury in aquatic food webs: A worldwide meta-analysis[END_REF]) and terrestrial [START_REF] Mahbub | Mercury toxicity to terrestrial biota[END_REF] environments to estimate and prevent the future development of Hg contamination. Nonetheless, the microbial processes involved in the cycle and the changes in microbial communities are still unclear. This review aims to expose the current understanding of Hg changes in microbial communities as well as Hg biotransformation and biomagnification in microorganisms, with a special emphasis on fungal organisms as known Hg bioaccumulators.

The fate of Hg released from natural and anthropogenic sources

Worldwide monitoring of Hg showed that this element has a ubiquitous distribution worldwide, occurring in all classes of igneous rocks, suggesting it was released in the atmosphere by volcanic activities in the early phase of Earth. The background level of Hg in all soils is approximately 5 µg/kg DM (Von [START_REF] Burg | Inorganic Mercury[END_REF][START_REF] Canil | Mercury in some arc crustal rocks and mantle peridotites and relevance to the moderately volatile element budget of the Earth[END_REF], but with significant local variations. Large Hg deposits have been found in China, Kyrgyzstan, Mexico, Peru and Russia. According to Merchant Research and Consulting Ltd , 85% of the world production came from China in 2016. Hg ores generally contain approximately one percent of Hg (1000 mg/kg), although the strata mined in Spain typically contain up to 9000 mg/kg [START_REF] Wang | Remediation of mercury contaminated sites -A review[END_REF]. Hg is also found in fossil fuels such as coal, gas and oil in nonnegligible amounts. For example, a study carried out in the USA showed that among the 900 million tons of coal burnt in 1999, there was 75 tons of Hg [START_REF] Pavlish | Status review of mercury control options for coal-fired power plants[END_REF]. The mining activity and fossil fuel consumption released Hg, which was contained as a trace element (TE) in geological stock (HgS), into the air, water and sediment. Hg is also released when used in products or in industrial and chemical processes such as small-scale gold mining in China or Brazil, silver and lead mining, silver amalgamation for dental applications or chlor-alkali industrial processes.

The global Hg consumption in 2005 was estimated to be approximately 3798 tons, and experts have determined that since the industrial revolution, the overall global Hg deposition in the environment has increased by a factor of 3 ± 1 [START_REF] Lindberg | A synthesis of progress and uncertainties in attributing the sources of mercury in deposition[END_REF]).The trends of global anthropogenic emissions shown a decrease in Europe and North America between 1990 and 2010 because of industrial efforts to decrease Hg emission, and worldwide emissions mostly came from East and Southeast Asia [START_REF] Pacyna | Global emission of mercury to the atmosphere from anthropogenic sources in 2005 and projections to 2020[END_REF]. Hg is also released in the environment from forest during potential wildfires [START_REF] Zhou | Mercury in terrestrial forested systems with highly elevated mercury deposition in southwestern China: The risk to insects and potential release from wildfires[END_REF]. The ratio between anthropogenic and natural source emissions was estimated in 2008, and anthropogenic sources accounted for 2320 tons of worldwide emitted Hg, mostly due to fossil fuel consumption (35%), while natural sources of Hg accounted for 5207 tons of Hg with a dominance of volatilization from the oceans (52%) [START_REF] Pirrone | Global mercury emissions to the atmosphere from anthropogenic and natural sources[END_REF].

Environmental Hg mostly exists in the gaseous elemental form (Hg 0 ), as inorganic divalent Hg(II), and as organo-Hg compounds, such as monomethyl Hg (MeHg) [START_REF] Hsu-Kim | Mechanisms regulating mercury bioavailability for methylating microorganisms in the aquatic environment: A critical review[END_REF]. Due to its chemical properties, Hg volatilization occurs at low temperature and pH (optimal at 30°C and at pH 2), and Hg is easily transferred into the atmosphere [START_REF] Takeuchi | Volatilization of mercury under acidic conditions from mercury-polluted soil by a mercury-resistant Acidithiobacillus ferrooxidans SUG 2-2[END_REF]. Atmospheric Hg is dominated by the extremely volatile form Hg 0 (generally > 95% of total airborne Hg), whereas only minor amounts of other species (mainly particulate-phase Hg (Hg(p)) have been detected [START_REF] Stratton | Use of a refluxing mist chamber for measurement of gasphase mercury(II) species in the atmosphere[END_REF]. Hg 0 can be oxidized into mercuric ion Hg 2+ in the Earth's atmosphere by photochemical reactions, and then it may precipitate by rainfall and dry deposition [START_REF] Bacci | Mapping mercury vapours in an abandoned cinnabar mining area by azalea (Azalea indica) leaf trapping[END_REF][START_REF] Wright | Overview of mercury dry deposition, litterfall, and throughfall studies[END_REF]. The oxidized Hg finally falls on terrestrial surfaces (60%) and into aquatic systems (40%) [START_REF] Mahbub | Mercury toxicity to terrestrial biota[END_REF]).

In the soil, an important factor is the complexation of Hg by dissolved organic matter (OM), whose increased with increasing pH [START_REF] Yin | Adsorption of mercury (II) by soil: effects of pH, chloride, and organic matter[END_REF]. Since the OM-bound Hg is less available, bioavailability of Hg in soil increases with low dissolved OM. Without any dissolved OM, a soil adsorbed more Hg(II) (or released less) under alkaline conditions than under acidic conditions [START_REF] Yin | Adsorption of mercury (II) by soil: effects of pH, chloride, and organic matter[END_REF]. A relevant indicator of the adsorption capacities of a soil is the organic carbon content (strong correlation R 2 =0.92) [START_REF] Yin | Adsorption/desorption isotherms of Hg (II) by soil[END_REF]. The processes of adsorption and desorption of Hg(II) in soils with larger organic matter content than 11.0 g C kg -1 show an S-type isotherm and not a L-type isotherm or C-type isotherm, suggesting that the interaction between OM and Hg affects adsorption rate. Together, these results revealed that the dissolved OM content of the soil is the major factor that restricts Hg diffusion.

Impact of Hg on soil fungi

A disturbance in the soil can imply i) the adaptation of the indigenous microorganisms, or ii) a change in the composition of the microbial community and a change or a conservation of the microbial-associated functions. Understanding the effects of Hg on soil fungal communities has progressed by the use of several approaches that are detailed below. It is important to consider whether the selected experimental designs reflect acute or chronic effects of Hg on the soil fungal community. Field experiments with long-term Hg exposure can be considered chronic Hg exposure, whereas experiments with artificial soil contamination in controlled conditions are considered acute Hg exposure [START_REF] Frossard | Long-and short-term effects of mercury pollution on the soil microbiome[END_REF].

Common approaches for the study of Hg toxicity towards fungi consist of measuring the growth rate of fungal strains on Hg-enriched synthetic growth media. The Hg chemical form most commonly used is HgCl2 (Table 1). Under axenic conditions, Hg fully inhibited the growth of 9 species of wood-rotting fungi at concentrations ranging between 10 and 100 µM of HgCl2 [START_REF] Baldrian | Influence of cadmium and mercury on activities of lignolytic enzimes and degradation of polycyclic aromatic hydrocarbons by Pleurotus ostreatus in soil[END_REF]. The growth of 21 fungi isolated from soil was inhibited in a range from 10 to 100 µM of HgCl2, [START_REF] Sanglimsuwan | Resistance to and uptake of heavy metals in mushrooms[END_REF]. Under controlled conditions, soil colonization by the white-rot fungus Pleurotus was strongly inhibited by 100 mg Hg/kg [START_REF] Baldrian | Influence of cadmium and mercury on activities of lignolytic enzimes and degradation of polycyclic aromatic hydrocarbons by Pleurotus ostreatus in soil[END_REF]. A negative effect of Hg was reported for 6 species of ectomycorrhizal (ECM) fungi, with a decrease in radial growth rate and biomass production in the presence of Hg [START_REF] Crane | Growth responses to and accumulation of mercury by ectomycorrhizal fungi[END_REF]. Corroborating those results, in a recent study we measured the Hg tolerance of 103 ECM and endophytic fungal isolates collected from TE-and Hg-polluted sites and 2 isolates from an unpolluted site by growing them on Hg-amended medium to determine their in vitro tolerance, which was measured by growth inhibition. For most fungi, the minimum inhibitory concentration (MIC) fell within the 10 to 200 µM Hg (HgCl2) range, testifying the high Hg toxicity for fungi under in vitro conditions (unpublished data). Three ectomycorrhizal and three endophytic isolates were further chosen to determine their response to Hg in more details, by measuring radial growth, biomass and Hg uptake (Fig. 1). These data highlight the wide range of Hg tolerance between fungal species. Literature converges toward a high Hg toxicity for the fungi under in vitro conditions compared to other TE. Indeed, when compared with Cd, Co, Cr, Cu, Mn, and Pb, Hg was the most toxic element tested [START_REF] Baldrian | Influence of cadmium and mercury on activities of lignolytic enzimes and degradation of polycyclic aromatic hydrocarbons by Pleurotus ostreatus in soil[END_REF].

In Ganoderma lucidum, the toxicity of TE on fungal growth in pure culture was classified in the following order: Hg>Cd>Cu>U>Pb>Mn=Zn [START_REF] Tham | Responses of Ganoderma lucidum to heavy metals[END_REF].

Hg toxicity was also assessed under natural conditions. In a field experiment, Jean-Philippe et al. ( 2011) investigated the effects of Hg on ECM fungi along a long-term Hgcontaminated gradient with up to 758 mg total Hg (THg)/kg in soil. Surprisingly, even though the maximal Hg concentration was relatively high in comparison with the Hg-enriched plate approach, they found no significant correlation between Hg and ECM fungal abundance. The lack of a negative impact of Hg on soil fungal abundance was confirmed by Hg acute exposure experiments [START_REF] Frossard | Tolerance of the forest soil microbiome to increasing mercury concentrations[END_REF]2018[START_REF] Tardy | Stability of soil microbial structure and activity depends on microbial diversity[END_REF] as well as Hg chronic exposure experiments [START_REF] Müller | The effect of long-term Hg pollution on the soil microbial community[END_REF][START_REF] Frossard | Long-and short-term effects of mercury pollution on the soil microbiome[END_REF]. Additionally, [START_REF] Frossard | Long-and short-term effects of mercury pollution on the soil microbiome[END_REF], in a field experiment, showed that the fungal growth rate was not affected by a high Hg soil concentration. Taken together, these results contradict the Hg toxicity assays conducted in vitro conditions [START_REF] Baldrian | Influence of cadmium and mercury on activities of lignolytic enzimes and degradation of polycyclic aromatic hydrocarbons by Pleurotus ostreatus in soil[END_REF][START_REF] Crane | Growth responses to and accumulation of mercury by ectomycorrhizal fungi[END_REF][START_REF] Sanglimsuwan | Resistance to and uptake of heavy metals in mushrooms[END_REF]. A possible explanation is that Hg generally presents a low biodisponibility in soil compared with the high Hg availability for in vitro toxicity assays. The ability to maintain a typical fungal abundance in Hg-contaminated soils could also be explained by the selection of Hg-resistant fungi potentially leading to a decrease in potential Hg-sensitive fungal taxa.

DNA-based experiments highlighted significant changes in the genetic structure of the soil fungal community in response to Hg contamination [START_REF] Frossard | Tolerance of the forest soil microbiome to increasing mercury concentrations[END_REF]2018;[START_REF] Tardy | Stability of soil microbial structure and activity depends on microbial diversity[END_REF]. [START_REF] Tardy | Stability of soil microbial structure and activity depends on microbial diversity[END_REF], using automated ribosomal intergenic spacer analysis (ARISA), explored the responses of the fungal community to Hg stress with a spike of 20 mg HgCl2/kg soil. After only one day of exposure, they found a change in the genetic structure of the fungal communities. In the study by [START_REF] Frossard | Tolerance of the forest soil microbiome to increasing mercury concentrations[END_REF], assessed the structure of the fungal communities of 7 soils enriched with Hg, following a gradient of HgCl2 (0.32, 3.2 and 32 mg/kg dry soil). Changes in the fungal community structure were observed at 32 mg/kg, while a lower Hg concentration had a limited impact [START_REF] Frossard | Tolerance of the forest soil microbiome to increasing mercury concentrations[END_REF]. [START_REF] Frossard | Long-and short-term effects of mercury pollution on the soil microbiome[END_REF] showed a substantial change in fungal community genetic structure along three sites presenting various Hg contamination levels (0.25, 3.02 and 36.10 mg THg/kg soil). In contrast, in the same study, the authors artificially contaminated soil microcosms with Hg and observed no change in the fungal community structure. The lack of change of the fungal community composition following Hg exposure was also supported by [START_REF] Rieder | Methyl-mercury affects microbial activity and biomass, bacterial community structure but rarely the fungal community structure[END_REF]. They demonstrated that the structure of the soil fungal community was not significantly affected (5 profiles on 66-90) by a spike in MeHg ranging from 5 µg/kg or by a gradient ranging from 5 and 90 µg/kg MeHg.

To explain the lack of consistency concerning the impact of Hg on the genetic structure of the soil fungal community, [START_REF] Frey | Response of forest soil bacterial communities to mercury chloride application[END_REF] suggested that Hg stress may be dependent on the soil properties and proposed a critical limit (environmental impacts) concentration for soluble Hg of 4 µg Hg/kg dry soil.

The assessment of the soil fungal diversity was generally conducted in parallel with the characterization of the genetic structure of the communities. At a Hg-contaminated chlor-alkali tailings dump with Hg concentrations ranging from 2.92 to 9.08 mg/kg soil, [START_REF] Durand | Environmental metabarcoding reveals contrasting belowground and aboveground fungal communities from poplar at a Hg phytomanagement site[END_REF] observed a negative correlation between the soil fungal diversity and soil Hg concentration. A decrease in fungal diversity was also measured in response to Hg addition in forest soil microcosms [START_REF] Frossard | Tolerance of the forest soil microbiome to increasing mercury concentrations[END_REF]. In contrast, Hg addition in pasture soil microcosms did not impact fungal diversity [START_REF] Frossard | Long-and short-term effects of mercury pollution on the soil microbiome[END_REF]. Surprisingly, in a field experiment, the soil fungal diversity increased with the soil Hg concentration [START_REF] Frossard | Long-and short-term effects of mercury pollution on the soil microbiome[END_REF]. Nevertheless, the factors explaining this phenomenon have not been yet clearly identified and could be due to the (1) the preadaptation of the soil fungal communities to Hg, (2) a change in fungal communities towards Hg-resistant taxa or (3) a lack of Hg toxicity due to low Hg bioavailability in soils.

Impact of Hg on soil bacteria

In this section, we analyse the impact of Hg on i) bacterial communities based on viable heterotrophic bacteria (VHB) counting and molecular biomass quantification and ii) bacterial community structure and diversity.

One of the most widely used techniques to study the effect of Hg exposure on bacterial communities relies on the assessment of viable heterotrophic bacteria (VHB) and the number of Hg-resistant (HgR) bacteria (Table 1). The percentage of HgR bacteria is usually evaluated by plating soil solutions on media enriched with HgCl2. In the literature, the effect of the number of VHB in response to acute Hg exposure remains contrasted. A study on the structure of bacterial communities after various types of perturbation showed that the addition of 100 mg HgCl2/kg leads to a decrease in the number of VHB [START_REF] Bressan | Response of soil bacterial community structure to successive perturbations of different types and intensities[END_REF]). However, unexpectedly, for other soil types, such as pasture, cultivated, forest and industrial, Hg spiking generally did not affect the VHB percentage [START_REF] Drønen | Effect of mercury addition on plasmid incidence and gene mobilising capacity in bulk soil[END_REF][START_REF] Harris-Hellal | Effects of Hg on soil microbial communities in tropical soils of French Guyana[END_REF][START_REF] Müller | The effect of long-term Hg pollution on the soil microbial community[END_REF][START_REF] Ranjard | Response of soil bacteria to Hg (II) in relation to soil characteristics and cell location[END_REF][START_REF] Boening | Ecological effects, transport, and fate of mercury: A general review[END_REF]. Additionnaly, a decrease in the number of VHB after Hg exposure has also been described in many studies [START_REF] Holtze | Mercury affects the distribution of culturable species of Pseudomonas in soil[END_REF][START_REF] Ruggiero | Hg bioavailability and impact on bacterial communities in a long-term polluted soil[END_REF].

More surprisingly, an increase in soil VHB could be measured in response to Hg exposure [START_REF] Rasmussen | Effects of mercury contamination on the culturable heterotrophic, functional and genetic diversity of the bacterial community in soil[END_REF][START_REF] Van Faassen | Effects of mercury compounds on soil microbes[END_REF]. In the case of acute Hg exposure of noninitially contaminated soils, an enrichment of HgR bacteria was often described [START_REF] Drønen | Effect of mercury addition on plasmid incidence and gene mobilising capacity in bulk soil[END_REF][START_REF] Holtze | Mercury affects the distribution of culturable species of Pseudomonas in soil[END_REF][START_REF] Müller | The effect of long-term Hg pollution on the soil microbial community[END_REF][START_REF] Ranjard | Response of soil bacteria to Hg (II) in relation to soil characteristics and cell location[END_REF][START_REF] Ranjard | A soil microscale study to reveal the heterogeneity of Hg (II) impact on indigenous bacteria by quantification of adapted phenotypes and analysis of community DNA fingerprints[END_REF][START_REF] Rasmussen | Effects of mercury contamination on the culturable heterotrophic, functional and genetic diversity of the bacterial community in soil[END_REF][START_REF] Ruggiero | Hg bioavailability and impact on bacterial communities in a long-term polluted soil[END_REF][START_REF] Van Faassen | Effects of mercury compounds on soil microbes[END_REF]. The HgR bacterial enrichment could explain the stable number of VHB before and after acute exposure by comparison with control soils. If the percentage of HgR bacteria generally increased after Hg exposure, a decrease in the percentage of HgR bacteria was also observed only a few days after Hg spiking [START_REF] Drønen | Effect of mercury addition on plasmid incidence and gene mobilising capacity in bulk soil[END_REF]. This phenomenon could be explained by volatilization or adsorption of Hg in soil, which could reduce the bioavailability of Hg, therefore decreasing the Hg selective pressure. Bacterial Hg resistance has been mainly attributed to the mer operon system and mostly the merA gene [START_REF] Barkay | Bacterial mercury resistance from atoms to ecosystems[END_REF]. The consistent trend of an increase in HgR bacteria in response to Hg contamination is strongly supported by the increase in the abundance of the merA gene across various soil types for both acute and chronic Hg exposure [START_REF] Frossard | Tolerance of the forest soil microbiome to increasing mercury concentrations[END_REF]2018). While the increase of HgR bacteria in response to Hg seems to be a consistent trend, the VHB response presents contrasted patterns.

Molecular biomass quantification of bacteria using a quantitative PCR approach after

Hg exposure presented the same trend as VHB counting, with a decrease [START_REF] Frossard | Tolerance of the forest soil microbiome to increasing mercury concentrations[END_REF][START_REF] Liu | Consistent responses of soil microbial taxonomic and functional attributes to mercury pollution across China[END_REF], no change [START_REF] Frossard | Long-and short-term effects of mercury pollution on the soil microbiome[END_REF][START_REF] Tardy | Stability of soil microbial structure and activity depends on microbial diversity[END_REF] or an increase [START_REF] Liu | Patterns of bacterial diversity along a long-term mercury-contaminated gradient in the paddy soils[END_REF] in the bacterial abundances. [START_REF] Liu | Patterns of bacterial diversity along a long-term mercury-contaminated gradient in the paddy soils[END_REF] followed the bacterial molecular biomass with 16S qPCR along a long-term Hg gradient and showed no correlation with THg.

Conversely, bacterial molecular biomass was remarkably positively correlated with MeHg under chronic Hg exposure [START_REF] Liu | Patterns of bacterial diversity along a long-term mercury-contaminated gradient in the paddy soils[END_REF], although MeHg is known to be one of the most toxic forms of Hg, especially for bacteria. However, MeHg can be very toxic to other soil ecological guilds, such as fungi or protozoa, which potentially interact with bacteria. This positive effect of MeHg on bacterial molecular biomass could be an indirect effect of changes in interactions between ecological guilds in soil, in favour of bacteria; for example, in changing the prey-predator dynamics between bacteria and protozoa [START_REF] Holtze | Prey-predator dynamics in communities of culturable soil bacteria and protozoa: differential effects of mercury[END_REF]. Direct quantification of bacterial biomass parallels the results obtained by VHB counting, thus indicating that bacterial abundance can not be used as a predictable trend showing the response of soil bacteria to Hg contamination. Bressan and collaborators (2008) followed the evolution of the soil bacterial community structure after various minor perturbations (CuCl2, 50 mg/kg DM; herbicide atrazine, 1.5 mg/kg DM; or heat shock, 35°C for 48 h) and a more severe perturbation with 100 HgCl2/kg dry soil. They found that HgCl2 addition resulted in a severe perturbation of the bacterial community structure. Besides, the study showed that Hg spikes have less important impact on the bacterial community structure on soils previously exposed to perturbations. As this effect was observed for the three minor perturbations, the authors hypothesized that a generic response occurred at the community level and that no metal tolerance selection was particularly involved at a community level. Few studies have attempted to compare bacterial communities under chronic Hg exposure in the environment.

A study on Hg-enriched perturbed soil of a chlor-alkali tailings dump revealed a clear dominance of the genus Pseudomonas of Gammaproteobacteria [START_REF] Zappelini | Diversity and complexity of microbial communities from a chlor-alkali tailings dump[END_REF]. A second study at the same chlor-alkali site revealed that bacteria from the Chloroflexi phylum dominated the soil community [START_REF] Durand | Bacterial diversity associated with poplar trees grown on a Hgcontaminated site: Community characterization and isolation of Hg-resistant plant growth-promoting bacteria[END_REF]. Other studies showed significant changes in the genetic structure of the soil bacterial community after acute Hg exposure [START_REF] Frossard | Tolerance of the forest soil microbiome to increasing mercury concentrations[END_REF][START_REF] Harris-Hellal | Effects of Hg on soil microbial communities in tropical soils of French Guyana[END_REF]Mahbub et al. 2016a and b;[START_REF] Ranjard | A soil microscale study to reveal the heterogeneity of Hg (II) impact on indigenous bacteria by quantification of adapted phenotypes and analysis of community DNA fingerprints[END_REF][START_REF] Tardy | Stability of soil microbial structure and activity depends on microbial diversity[END_REF]. In contrast, chronic Hg exposure seemed to not impact the genetic structure of the bacterial community [START_REF] Frossard | Long-and short-term effects of mercury pollution on the soil microbiome[END_REF]). These results may indicate that bacterial communities are more sensitive to acute Hg exposure than chronic Hg exposure. A low concentration of soil Hg (between 4.4 and 11.1 mgTHg/kg DM) leads to a statistically significant decrease in the bacterial diversity of 20% (Mahbub et al. 2016b). The decrease in bacterial diversity was correlated with the Hg concentration in neutral and alkaline low organic carbon-containing soil but not in rich organic carbon-containing acidic soil, which contains less watersoluble Hg. In addition, the water-soluble Hg obtained with deionized water extraction (Mahbub et al. 2016a) led to a change in the bacterial diversity at lower concentrations ranging from 3.8 to 22.8 μg Hg 2+ /L and could be an informative index for the bioavailability of Hg in soils. These negative impacts of Hg on soil bacterial diversity were observed in other studies, mostly with acute exposure [START_REF] Frossard | Tolerance of the forest soil microbiome to increasing mercury concentrations[END_REF][START_REF] Liu | Consistent responses of soil microbial taxonomic and functional attributes to mercury pollution across China[END_REF][START_REF] Müller | The effect of long-term Hg pollution on the soil microbial community[END_REF][START_REF] Ruggiero | Hg bioavailability and impact on bacterial communities in a long-term polluted soil[END_REF]. Surprisingly, chronic Hg exposure appeared to lead to an increase in bacterial diversity [START_REF] Frossard | Tolerance of the forest soil microbiome to increasing mercury concentrations[END_REF][START_REF] Liu | Patterns of bacterial diversity along a long-term mercury-contaminated gradient in the paddy soils[END_REF]2018). As for the bacterial community genetic structure, the impact of Hg on soil bacterial diversity seems to be linked with the Hg exposure type.

Impact of Hg on soil microbial functions

Microbes play a crucial role in soil C (carbon) and nutrient cycles, directly contributing to soil C stocks as well as plant productivity [START_REF] Van Der Heijden | The unseen majority: soil microbes as drivers of plant diversity and productivity in terrestrial ecosystems[END_REF]. By impacting the microbial communities, Hg may alter the associated microbial functions. [START_REF] Baldrian | Influence of cadmium and mercury on activities of lignolytic enzimes and degradation of polycyclic aromatic hydrocarbons by Pleurotus ostreatus in soil[END_REF] demonstrated in pure culture that Hg negatively impacts the production of two fungal enzymes (laccase and manganese peroxidase) involved in organic matter degradation. These in vitro results are supported by other studies demonstrating a decrease in C mineralization and heterotrophic respiration in response to Hg exposure [START_REF] Bringmark | Effects on mor layer respiration by small experimental additions of mercury and lead[END_REF][START_REF] Landa | The retention of metallic mercury vapor by soils[END_REF][START_REF] Harris-Hellal | Effects of Hg on soil microbial communities in tropical soils of French Guyana[END_REF][START_REF] Tardy | Stability of soil microbial structure and activity depends on microbial diversity[END_REF]. All these studies concerned acute Hg exposure, and chronic Hg exposure seemed to not affect microbial respiration [START_REF] Frossard | Long-and short-term effects of mercury pollution on the soil microbiome[END_REF][START_REF] Holtze | Prey-predator dynamics in communities of culturable soil bacteria and protozoa: differential effects of mercury[END_REF][START_REF] Müller | The diversity and function of soil microbial communities exposed to different disturbances[END_REF]. [START_REF] Casucci | Effects of mercury on microbial biomass and enzyme activities in soil[END_REF] showed that Hg decreased the activity of two enzymes implicated in phosphorus and sulfur cycles in soil. Hg also negatively impacted the soil nitrogen mineralization rate and nitrification rate [START_REF] Van Faassen | Effects of mercury compounds on soil microbes[END_REF][START_REF] Liu | Effects of mercury on the activity and community composition of soil ammonia oxidizers[END_REF].

To determine the effect of Hg on ammonia-oxidizing bacteria (AOB), real-time PCR assays of the amoA genes were used [START_REF] Li | Indole degrading of ammonia oxidizing bacteria isolated from swine wastewater treatment system[END_REF]). The AOB community was dominated by Nitrosopira-like sequences, and this result appeared to be a consistent trend; the Nitrosopiralike sequences decreased as the concentration of Hg increased. Moreover, Hg led to a major disturbance that had a negative impact on ammonium oxidizers and nitrifiers (Mahbub et al., 2016b). Taken together, these results suggest that Hg could be a threat to microbial soil functions involved in both C and nutrient cycles.

Microbial mechanisms related to Hg exposure, what about fungi?

Microorganisms are present in all types of environments and are essential for the life of higher-level organisms [START_REF] Nair | Impact of endophytic microorganisms on plants, environment and humans[END_REF]. Therefore, ubiquitous organisms have been exposed to all types of pollutants, and selective pressures from a metal-containing environment have led to the development of resistance systems for virtually all toxic metals in bacteria [START_REF] Bruins | Microbial resistance to metals in the environment[END_REF] or fungi [START_REF] Bellion | Extracellular and cellular mechanisms sustaining metal tolerance in ectomycorrhizal fungi[END_REF]). Autonomous cells have developed a wide variety of mechanisms to maintain homeostasis in a highly concentrated metal environment; for example, extracellular chelation by excreted ligands, cell-wall binding, enhanced efflux, intracellular chelation or subcellular compartmentation. The effects of the inorganic form of Hg on many cellular processes (dehydrogenase activity, urease production, nitrification, ATP production, total microbial population, total nitrogen fixers, microbial biomass carbon, alkaline phosphatase, methane oxidation and respiration) have been previously reviewed [START_REF] Mahbub | Mercury toxicity to terrestrial biota[END_REF].

Microbial resistance mechanisms to Hg are not fully understood, although some mechanisms have been proposed: (i) cell wall binding of Hg limits Hg uptake [START_REF] Kinoshita | Biosorption of heavy metals by lactic acid bacteria and identification of mercury binding protein[END_REF]), (ii) production of thiols (such as glutathione) to chelate Hg in sulfhydryl groups (SH) limit the bioavailability of Hg [START_REF] Wu | Intracellular speciation and transformation of inorganic mercury in marine phytoplankton[END_REF], (iii) similarly production of selenium-rich proteins may chelate Hg [START_REF] Kavčič | Localization, ligand environment, bioavailability and toxicity of mercury in Boletus spp. and Scutiger pes-caprae mushrooms[END_REF]) and (iv) biovolatilization systems involving mercuric reduction and methylation (Urík et al. 2014;[START_REF] Lu | Anaerobic mercury methylation and demethylation by Geobacter bemidjiensis Bem[END_REF]. After a brief update on bacterial mechanisms, this review will focus on fungal mechanisms.

Lessons learned from research on bacteria

Research on Hg and bacteria has produced three times more publications than on Hg and fungi (Fig. 2). The Hg resistance biovolatilization systems of bacteria have attracted many researchers. The most studied example of biovolatilization in the environment takes place during the polar spring in the arctic region. The process is known as atmospheric Hg depletion events. Within a year, Hg(II) wet and dry deposition occurs, and in the summer, after the snow and ice have melted, water carries Hg(II) into lakes where it could be re-emitted by Hg-resistant bacteria or volatilized to Hg 0 as a resistance system [START_REF] Steffen | A synthesis of atmospheric mercury depletion event chemistry in the atmosphere and snow[END_REF][START_REF] Lescord | Factors affecting biotic mercury concentrations and biomagnification through lake food webs in the Canadian high Arctic[END_REF]. The best understood biochemical mechanism of Hg bacterial biovolatilization involves the reduction of Hg(II) to volatile Hg(0) by inducible mercuric ion reductase coded by the mer operon [START_REF] Dash | Bioremediation of mercury and the importance of bacterial mer genes[END_REF]. The mer operon contains genes that encode the functional proteins for regulation (merR), transport (merT, merP and/or merC, merF) and reduction (merA) [START_REF] Nascimento | Operon mer: Bacterial resistance to mercury and potential for bioremediation of contaminated environments[END_REF]. The additional merB gene (coding for organomercurial lyase) is required to confer resistance to many organomercurials, such as methylHg and phenylHg, by hydrolysing the R-Hg bond before the reduction of Hg 2+ . In general, additional merB genes are found downstream of the merA gene in the mer operon [START_REF] Osborn | Distribution, diversity and evolution of the bacterial mercury[END_REF]). The merA gene has been mostly detected in subsurface [START_REF] Oregaard | High diversity of bacterial mercuric reductase genes from surface and sub-surface floodplain soil (Oak Ridge, USA)[END_REF] and water [START_REF] Moller | Mercuric reductase genes (merA) and mercury resistance plasmids in High Arctic snow, freshwater and sea-ice brine[END_REF] environments. The importance of microbial processes for Hg volatilization in comparison to abiotic transformation in the Hg global cycle has been questioned. In aquatic systems, experiments have shown that the biotic-lightdependent transformation of ionic Hg (Hg 2+ ) in Hg 0 accounted for an average of 30% of the total natural transformation, while photochemistry contributed to 30% and low-light production accounted for 40% [START_REF] Kuss | Mercury emission by the Baltic Sea: A consequence of cyanobacterial activity, photochemistry, and low-light mercury transformation[END_REF]. The experimental conditions used allowed phytoplankton genera to carry out photosynthesis, but not the high-light adapted cyanobacteria. These results suggested that the cyanobacterial genera Synechococcus and Aphanizomenon were the main transformers of ionic Hg (Hg 2+ ) into Hg 0 .

Studies are also exploring other bacterial adaptation mechanisms to Hg exposure through chemical transformation. Indeed, microbial Hg methylation transforms inorganic Hg into the highly volatile and bioaccumulated neurotoxin methylHg with a rapid transfer into the food web. Moreover, a balance exists between anaerobic methylation and demethylation of inorganic Hg by microorganisms, influencing net MeHg production in anoxic water and sediments [START_REF] Lu | Anaerobic mercury methylation and demethylation by Geobacter bemidjiensis Bem[END_REF], hence the interest in methylation and demethylation in microorganisms is growing. In soils, the best-known contributors to methylation were found to be sulfate-reducing bacteria (SRB) [START_REF] Holloway | Spatial and seasonal variations in mercury methylation and microbial community structure in a historic mercury mining area, Yolo County, California[END_REF], while iron-reducing bacteria (IRB) coexisting with SRB were involved in the methylation process in the freshwater environment [START_REF] Yu | Contribution of coexisting sulfate and iron reducing bacteria to methylmercury production in freshwater river sediments[END_REF]. Although some bacterial taxa are known to be capable of methylation and demethylation under anaerobic conditions [START_REF] Pak | Mercury methylation and demethylation in anoxic lake sediments and by strictly anaerobic bacteria mercury methylation and demethylation in anoxic lake sediments and by strictly anaerobic bacteria[END_REF], the lack of a genetic marker for microbial MeHg production has prevented non-culture-dependent studies of the distribution of Hg-methylating organisms in nature. By directly measuring MeHg production in several bacterial and archaeal strains encoding hgcAB, it was confirmed that a specific gene cluster (hgcAB) [START_REF] Parks | The genetic basis for bacterial mercury methylation[END_REF]) was linked to Hg methylation [START_REF] Gilmour | Mercury methylation by novel microorganisms from new environments[END_REF]). Since then, many species have been defined as MeHg producers, including sulfate and iron-reducing bacteria but also methanogens, syntrophic, and acetogenic bacteria, or fermentative Firmicutes [START_REF] Gilmour | Mercury methylation by novel microorganisms from new environments[END_REF][START_REF] Figueiredo | Aerobic mercuryresistant bacteria alter mercury speciation and retention in the Tagus estuary (Portugal)[END_REF][START_REF] Lu | Anaerobic mercury methylation and demethylation by Geobacter bemidjiensis Bem[END_REF]. Recently, [START_REF] Lu | Anaerobic mercury methylation and demethylation by Geobacter bemidjiensis Bem[END_REF] isolated the strain G. bemidjiensis Bem, which has the capacity to mediate the three main mechanisms related to Hg: Hg(II) reduction, Hg(0) oxidation, and MeHg production/degradation under anoxic conditions.

Are fungi terrestrial Hg hyperaccumulators?

A long history of the ecological survey of Hg concentrations in edible and nonedible mushrooms revealed that many fungi bioaccumulate large amounts of Hg in the environment (Supplemental Table S1), and some fungi could be considered hyperaccumulators of Hg referring to the metal hyperaccumulation defined for plants. In figure 3, we compared the maximum Hg concentration and maximum Hg bioconcentration factor (BCF) of microfungi.

The BCF is defined as the ratio between the THg concentration in fungi and the THg concentration in soil. The BCFs of Boletus pinophilus or Boletus aereus were found to be 491 and 425, respectively [START_REF] Melgar | Mercury in edible mushrooms and underlying soil: Bioconcentration factors and toxicological risk[END_REF]) (Fig. 3). Boletus edulis from the Indira mine region in Slovenia showed 69.0 ± 14.8 mg/kg DM (in cap, mature) and in young cap was 103 ± 7.60 mg/kg DM [START_REF] Kavčič | Localization, ligand environment, bioavailability and toxicity of mercury in Boletus spp. and Scutiger pes-caprae mushrooms[END_REF]. The Hg uptake capacity by fungi of the Boletaceae family is even more obvious in control environment. For example, Hg concentrations in Boletus species from background (unpolluted) areas can reach 9.9 ± 2.7 mg/kg DM with a bioconcentration factor (BCF) of 250 considering the low concentration of Hg in the soil of the Borecka Forest in Poland (0.005 to 0.096 mg/kg DM) [START_REF] Falandysz | Total mercury in mushrooms and underlying soil substrate from the Borecka Forest, Northeastern Poland[END_REF]. High Hg BCF values were also found for the Agaricus genus; for example, a value of 365 was reached for Agaricus macrosporus (Fig. 3). Other genera, such as Lepista, Lycoperdon and Macrolepiota, showed Hg accumulation from their environment (Fig. 3).

Consequently, some fungal species have been proposed to be bioindicators for Hg pollution, including Agaricus arvensis, Agaricus campestris, Agaricus edulis, Agaricus haemorrhoidarius, Agaricus xanthodermus, Amanita rubescens, Amanita strobiliformis, Coprinus comatus, Lycoperdon perlatum, Marasmius oreades and Mycena pura [START_REF] Mejstrik | Applicability of fungi to the monitoring of environmental pollution by heavy metals[END_REF][START_REF] Wondratschek | Monitoring of heavy metals in soils by higher fungi. Plants as biomonitors. Indicators for heavy metals in the terrestrial environment[END_REF]. We could suggest that several Boletaceae, such as Boletus edulis, Boletus pinophilus and Boletus aereus, could be added to the list. Obviously, the THg content in the soil should influence the BCF of Hg in fungi. A recent study proposed to survey the BCF of 15 saprotrophic and wood-rotting fungal strains grown in semi-natural medium enriched with an Hg gradient from 0 to 20 ppm to measure the translocation rate. The results showed that the BCF decreased with the increase in Hg in the substrate, whereas the THg in the biomass rose with the Hg content in the substrate [START_REF] Gabriel | Translocation of mercury from substrate to fruit bodies of Panellus stipticus, Psilocybe cubensis, Schizophyllum commune and Stropharia rugosoannulata on oat flakes[END_REF].

Fungi accumulate significant amounts of Hg, particularly at sites enriched in Hg (such as the mercuriferous belt), but also accumulate significant amounts of Hg at unpolluted sites. It is a peculiarity of macromycetes to accumulate Hg in a soil with a low Hg content that contrast with vascular plants, essentially accumulating Hg from Hg-enriched soils [START_REF] Falandysz | Macro and trace mineral constituents and radionuclides in mushrooms: Health benefits and risks[END_REF]. In the terrestrial environment, fungi are the best bioaccumulating organisms for Hg, and the total Hg distribution in the vegetal cover of a forest area showed variations of accumulation among the various components of the ecosystem as follows: peas, rosehips < tree leaves (early summer) < leaves (flowering plant) < apple blossoms < berries < twigs < shrub leaves < sedge < needles (average) < ferns < aquatic vegetation < grasses < bark < liverwort < mosses < tree fungi < lichens < saprophytic fungi < ectomycorrhizal fungi [START_REF] Nasr | Biomonitoring and assessing total mercury concentrations and pools in forested areas[END_REF].

Several factors have been studied for a better understanding of Hg bioaccumulation in fungi. The Hg content of higher fungi appeared to be species-dependent and varied as a function of the fungal tissues; globally, the cap contained more Hg than the stalk. The Hg concentration is also affected by the developmental stage of the fruiting bodies, which decrease during senescence in some fungi (emergence > maturity > senescence) [START_REF] Nasr | Quantifying Hg within ectomycorrhizal fruiting bodies, from emergence to senescence[END_REF]. This was for instance confirmed in the work of [START_REF] Kavčič | Localization, ligand environment, bioavailability and toxicity of mercury in Boletus spp. and Scutiger pes-caprae mushrooms[END_REF] that measured 103 ± 7.60 mg THg /kg DM in younger caps of B. edulis and 92.6 ± 18.4 mg THg /kg DM in fruiting bodies of Scutiger pes-caprae both from Slovenia, while they only measured 69.0 ± 14.8 mg THg /kg DM in mature caps of B. edulis and 41.9 ± 2.25 mg THg /kg DM in fruiting bodies of S. pes-caprae.

In those fungi the hymenium contained more than 50% of the accumulated Hg. Nonetheless some macromycetes species may not follow this trend (Falandysz et al. 2020a). The morphology of the hymenium and/or the mode of development of the hymenium could be a key aspect of the accumulation of Hg in macromycetes fruiting bodies.

The total Hg content in fungi is positively correlated with the Hg bioavailability in myceliated soil and substrate type (wood, forest soil and mineral soil) [START_REF] Nasr | Hg concentrations and accumulations in fungal fruiting bodies, as influenced by forest soil substrates and moss carpets[END_REF]. It was previously highlighted that metal bioaccumulation in fungi is primarily influenced by species differences and not by soil factors [START_REF] Leyval | Effect of heavy metal pollution on mycorrhizal colonization and function: Physiological, ecological and applied aspects[END_REF]); however, environmental variables such as pH, temperature, and geological properties affect Hg uptake by organisms [START_REF] Boening | Ecological effects, transport, and fate of mercury: A general review[END_REF]. Moreover, it was shown that the THg content in fungi was negatively correlated with the total sulfur concentration in myceliated soil and positively correlated with the total sulfur content in fungi [START_REF] Nasr | Biomonitoring and assessing total mercury concentrations and pools in forested areas[END_REF]. These results implied that Hg bound to organic matter mitigates Hg accumulation in fungi. Nevertheless, readers should keep in mind that Hg deposition after atmospheric transit can also be a source of fungal contamination, at least under certain climates. Indeed, Hg accumulation was found in fungi isolated from Gongga Mountain in China. The absence of industrial activities, urbanization and Hg ore deposits in this area suggested deposition as the major source of elevated Hg observed; for instance, 3,285 mg THg /kg DM was measured in the cap of Gymnopus erythropus. Gongga Mountain has its own microclimate, a 'circumpolar' climate, and hence is sensitive to contamination by persistent pollutants that are trapped by cold temperature and wet precipitation [START_REF] Falandysz | Mercury in the fairy-ring of Gymnopus erythropus (Pers.) and Marasmius dryophilus (Bull.) P. Karst. mushrooms from the Gongga Mountain, Eastern Tibetan Plateau[END_REF]. Moreover, determination of the bioaccumulation factor (BAF), defined as the ratio between THg concentration in the fruiting body of the fungi and the bioavailable Hg concentration in the soil, could be a better indicator for Hg uptake by fungi but requires bioavailable Hg to be determined. In conclusion, results from the literature suggest that fungal species may be the main factor influencing the accumulation of Hg in fungal tissues, and some fungal species behave like Hg sponges in the terrestrial environment. Despite the potential threat of edible mushrooms hyperaccumulating Hg, even in soils with low Hg concentrations, the specific mechanisms leading to Hg accumulation and resistance of fungi remain fairly undiscovered. Nonetheless, understanding the fungal mechanisms of uptake and transformations of Hg is arousing growing interest in the scientific community (Falandysz 2019a(Falandysz , 2019b(Falandysz , 2020b)).

Hg tolerance mechanisms in fungi

The fungi had high Hg bioaccumulation factors despite low Hg concentrations or low bioavailability in the soil, which raises the question of how fungal cells manage to cope with Hg toxicity. Fungi show resistance to Hg at ranges far superior to those measured for bacteria [START_REF] Rajapaksha | Metal toxicity affects fungal and bacterial activities in soil differently[END_REF]). In the fungal kingdom, it has been assumed that the accumulation and transport of Hg could depend on the production of proteins rich in cysteine and their sulfhydryl (SH) groups. These proteins, also named thiols [START_REF] Stijve | Mercury, cadmium, lead and selenium content of mushroom species belonging to the genus Agaricus[END_REF], are produced in ECM fungi and include metallothioneins and glutathione chelators [START_REF] Khullar | Ectomycorrhizal fungi and its role in metal homeostasis through metallothionein and glutathione mechanisms[END_REF]. It has been suggested that there is a strong correlation between the amount of SH groups in fungal proteins and Hg content in fungi (Agaricus), while almost no correlation for other TE was found [START_REF] Kojo | Cadmium and mercury in macrofungi -mechanisms of transport and accumulation[END_REF]. Induction of metallothioneins by HgCl2 has not been demonstrated for fungi but exists in rats [START_REF] Piotrowski | Mercury binding in the kidney and liver of a rats repeatedly exposed to mercuirc chloride: induction of metallothionein by mercury and cadmium[END_REF]. A recent study showed that higher Hg contents in the edible mushrooms B. edulis, B. aereus and Scutiger pes-caprae resulted in greated Hg tetra-thiolate complexation, but also HgSe complexation [START_REF] Kavčič | Localization, ligand environment, bioavailability and toxicity of mercury in Boletus spp. and Scutiger pes-caprae mushrooms[END_REF]). The study revealed that Hg mainly coordinated to di-thiols (43-82%), followed by di-selenols (13-35%) and tetra-thiols (12-20%). In addition to selenium-rich proteins that may limit Hg bioavailability in mushrooms, they found evidence of di-amino Hg complex with Hg(N-R)2 or Hg complex with two N in heterocyclic amines.

The toxicity of Hg compounds is dependent on the ability to release Hg 2+ , which has a high affinity for other cellular ligands. Compounds providing two or three thiolate ligands activate organomercurials for cleavage, indicating that the number of thiolates ligated to Hg is influenced by binding to carbon [START_REF] Miller | Cleaving C-Hg bonds: two thiolates are better than one[END_REF]. A study showed that arbuscular mycorrhizal fungi (AMF) enable the uptake of Hg-tetra-thiolate complexes in plants grown on Hg-enriched soils (50 ppm), while Hg forms di-thiolate and di-amino complexes in non-AMF plants.

Moreover, this process is AMF density-dependent, showing that AMF can modify the Hg binding environment in plant roots [START_REF] Kodre | Arbuscular mycorrhizal fungi alter Hg root uptake and ligand environment as studied by X-ray absorption fine structure[END_REF]. In soils where Hg can have extremely stable bonds with the carbon of organic matter, thiolate production by fungi may greatly influence the bioavailability of Hg from organomercurial forms. Consequently, the potential toxicity of Hg in soil may be affected by fungal communities. Moreover, the Hg resistance of some fungi could depend on the ability of those organisms to modify the Hg binding environment by SH groups to immobilize Hg in a nontoxic form.

In Hg-contaminated soil, fungi could have the potential to detoxify organomercurials by volatilization. Some fungi, such as Yarrowia strains, have been proven to be able to convert organomercurials to Hg(II) by fungal organoHg lysis and then reduced Hg(II) to Hg 0 [START_REF] Gadd | Geomycology: biogeochemical transformations of rocks, minerals, metals and radionuclides by fungi, bioweathering and bioremediation[END_REF][START_REF] Oyetibo | Mercury bioremoval by Yarrowia strains isolated from sediments of mercury-polluted estuarine water[END_REF]. A study proposed an experiment with several soil fungi, and Hg was measured in the fungal biomass (Urík et al. 2014). The authors concluded that bioimmobilization of Hg was negligible compared to volatilization, and they demonstrated the biovolatilization potential of Aspergillus niger and Cladosporium cladosporioides, which reached 80% of 32.7 mg HgCl2/l. In such experiments, we proposed that a survey of Hg volatile compounds such as MeHg should be performed to exclude the bioimmobilization hypothesis.

Hg methylation has been less studied in fungi than in bacteria, however some fungi may produce Hg volatile compounds such as MeHg [START_REF] Vonk | Studies on the methylation of mercuric chloride by pure cultures of bacteria and fungi[END_REF]. A study monitored the production of MeHg in bacteria and fungi and revealed that A. niger, Scopulariopsis brevicaulis and Saccharomyces cerevisiae were able to produce MeHg from sublethal HgCl2 concentrations [START_REF] Vonk | Studies on the methylation of mercuric chloride by pure cultures of bacteria and fungi[END_REF]. MeHg was essentially found in the mycelium in contrast to bacteria, where the MeHg produced was mostly found in the media. For A. niger, studies agreed on the media detoxification capacity of the fungus but not on the fate of the Hg (biosorption or volatilization). A study on the microbial consortium of periphyton showed that Hg methylation were the most involved, however inhibition of fungal and algae also reduced methylation rates. Although, sulfate reducing bacteria and methanogens are the most involved in MeHg formation [START_REF] Correira | Mercury methylation and the microbial consortium in periphyton of tropical macrophytes: effect of different inhibitors[END_REF], fungi may produced MeHg in the soil, which has been recognized as the most toxic Hg compound [START_REF] Clarkson | The toxicology of mercury and its chemical compounds[END_REF].

The mechanisms involved in volatilization, bioaccumulation or bioimmobilization of Hg compounds by fungi need to be clarified. The fate of Hg in fungal cells has been poorly studied in the past when compared to other TE.A study evaluated the factors affecting the biosorption of methylHg and Hg(II) by S. cerevisiae [START_REF] Madrid | Speciation of methylmercury and Hg(II) using baker's yeast biomass (Saccharomyces Cerevisiae). Determination by continuous flow mercury cold vapor generation atomic absorption spectrometry[END_REF]. The results showed that MeHg bound easily to the yeast cell at a wide range of pH values and temperatures compared to Hg(II), which was less effectively bound to the cells. The slow Hg(II) uptake followed Michaelis-Menten kinetics for 24 h, implying saturation of binding sites or transport.

The explanation proposed here for the rapid binding of MeHg was that S. cerevisiae was able to rapidly reduce CH3Hg + to other more volatile Hg species, such as Hg(I) or Hg 0 . In addition, among various ions, chloride ions at high concentrations strongly increased the binding of Hg(II). When the concentration of metals is higher in the extracellular compartment than in the cell, metals can accumulate inside the cell through diffusion or through other transport channels.

To decipher the blurred mechanisms of metal uptake by S. cerevisiae, researchers have compared the biosorption capacity of S. cerevisiae in different forms, such as living cells, dead cells, inactivated cells, and free cells, and they revealed that the vacuole was involved in detoxifying Zn, Mn, Co, and Ni but not Cu or Cd [START_REF] Wang | Biosorption of heavy metals by Saccharomyces cerevisiae: A review[END_REF]. In the literature, information on the uptake or subcellular compartmentation of Hg in fungal cells is scarce.

However, recent work found a putative transport system for Hg (HgCl2) in S. cerevisiae and explored subcellular compartmentation in the higher fungus Podospora anserina (Durand et al. unpublished data).

We proposed a schematic representation of cellular mechanisms potentially involved in Hg tolerance in ectomycorrhizal fungi (Fig. 4). Mechanisms that may be involved in the tolerance of fungi to excess metals like mercury in their environments include a reduction of metal uptake into the cytosol by extracellular chelation through extruded ligands (e.g. hydrogen sulfide (H2S) gas which co-precipitates Hg [START_REF] Amin | Isolation and characterization of H2S producing yeast to detoxify mercury containing compounds[END_REF]) and binding to cell wall components (e.g. carboxyl groups of cell walls such as chitin and chitosan, and hydrophobins [START_REF] Karunasagar | Biosorption of inorganic and methyl mercury by a biosorbent from Aspergillus niger[END_REF][START_REF] Puglisi | Identification of differentially expressed genes in response to mercury I and II stress in Trichoderma harzianum[END_REF]). Hg uptake could occur through specific transporters for the acquisition of essential metals that have not yet been described .

Intracellular chelation of Hg in the cytosol by a range of ligands (glutathione, metallothioneins and phytochelatins [START_REF] Bolchi | Genome-wide inventory of metal homeostasis-related gene products including a functional phytochelatin synthase in the hypogeous mycorrhizal fungus Tuber melanosporum[END_REF][START_REF] Khullar | Ectomycorrhizal fungi and its role in metal homeostasis through metallothionein and glutathione mechanisms[END_REF]), and increased efflux from the cytosol to the exterior of the cell (by methylations and by volatilization [START_REF] Gadd | Geomycology: biogeochemical transformations of rocks, minerals, metals and radionuclides by fungi, bioweathering and bioremediation[END_REF][START_REF] Oyetibo | Mercury bioremoval by Yarrowia strains isolated from sediments of mercury-polluted estuarine water[END_REF]Urík et al., 2014)) or into sequestering compartments (storage in vacuoles by i.e., the ycf1 transporter [START_REF] Gueldry | Ycf1pdependent Hg (II) detoxification in Saccharomyces cerevisiae[END_REF]) are also key mechanisms that confer metal tolerance. The free-radical scavenging capacity provided by the activity of catalase [START_REF] Kojo | Cadmium and mercury in macrofungi -mechanisms of transport and accumulation[END_REF] or the binding of Hg to Se and SH groups [START_REF] Baldrian | Interactions of heavy metals with white-rot fungi[END_REF][START_REF] Collin-Hansen | First report of phytochelatins in a mushroom: induction of phytochelatins by metal exposure in Boletus edulis[END_REF][START_REF] Kavčič | Localization, ligand environment, bioavailability and toxicity of mercury in Boletus spp. and Scutiger pes-caprae mushrooms[END_REF] comprise another line of defence against the toxic effects of mercury .

Molecular methods could reveal new insights into cell mechanisms induced by Hg

stress. Transcriptomic approaches were carried out on Elodea nuttallii (aquatic plant) exposed to various acute Hg exposure, and the upregulated genes were determined [START_REF] Regier | Analysis of the Elodea nuttallii transcriptome in response to mercury and cadmium pollution: Development of sensitive tools for rapid ecotoxicological testing[END_REF].

The downregulated genes were related to 'homeostasis of metal ions', 'heavy metal ion transport' and, more surprisingly, genes related to detoxification, although not related to metallic detoxification. Another significant work used the suppression subtractive hybridization (SSH) technique to identify the differentially expressed genes in the strikingly Hg-resistant fungus Trichoderma harzianum IMI 393899 grown in the presence of HgCl2 or Cd [START_REF] Puglisi | Identification of differentially expressed genes in response to mercury I and II stress in Trichoderma harzianum[END_REF]. All 8 genes identified as upregulated after HgCl2 exposure were also upregulated by Hg2Cl2, and among them, hydrophobin was predicted as having a possible role in the tolerance mechanism. Hydrophobins are a group of small cysteine-rich proteins only expressed by filamentous fungi and have the ability to dissolve hydrophobic molecules in aqueous media.

Hydrophobins could be involved in the retention of Hg at the cell surface, leading to improved Hg resistance. Similarly, [START_REF] Müller | Gene expression profiling of a Zn-tolerant and a Zn-sensitive Suillus luteus isolate exposed to increased external zinc concentrations[END_REF] suggested that hydrophobin plays a role in Zn resistance in Suillus luteus, whereas other studies showed that this taxon could accumulate Hg from the environment [START_REF] Minagawa | Accumulation route and chemical form of mercury in mushroom species[END_REF][START_REF] Falandysz | Total mercury in wild mushrooms and underlying soil substrate from the city of Umeå and its surroundings, Sweden[END_REF][START_REF] Saba | Mercury bioaccumulation by Suillus bovinus mushroom and probable dietary intake with the mushroom meal[END_REF]. We suggest that such molecular approaches may unravel fungal mechanisms involved in acute or chronic exposure to Hg.

Conclusions

Environmental contamination by Hg is a serious problem worldwide, and an understanding of its transfer in the biosphere is needed. Microorganisms such as bacteria and fungi are believed to have many interactions with Hg; however, little is known about their impact on the cycle of Hg accumulation in the biosphere. Concerning fungi, even fewer data are available in the scientific literature compared to bacteria. Fungi accumulate the largest amount of Hg in forest ecosystem and are the organisms capable of the highest bioaccumulation of Hg. Their role in soil organic matter recycling seems to be correlated with their role in the Hg cycle. Indeed,

Figure 1 .

 1 Figure 1. The effects of HgCl2 on the radial growth of 6 selected fungal isolates. P. involutus

Figure 2 .

 2 Figure 2. Peer reviewed publications on mercury in the bacterial or fungal kingdom and

Figure 3 .

 3 Figure 3. Comparison between the maximum Hg concentration and maximum Hg

Figure 4 .

 4 Figure 4. Schematic representation of the cellular mechanisms potentially involved in Hg

  Figure 2

Table 1 .

 1 Overview of tolerance of microbial communities towards Hg. The main effects are provided for the listed microbial communities, together with Hg concentrations in soils and references.

	Reference	Community	Type of	Soil type	Initial	Hg form	Exposure	[Hg]	Effect Feature	Method
		type	study		contamination		type
	Akerblom et	total	controlled	forest : mor	yes/no	HgCl2	acute	2.010	-	heterotrophic respiration for	respirometry
	al. 2010		conditions	layer				mg/kg		the F layer
								0.54 to	o	heterotrophic respiration for
								1.93		L and H layer
								mg/kg
								0.88 to	-	heterotrophic respiration for
								1.29		the milled L-layer
								mg/kg
	Baldrian et	fungi	axenic	na	na	HgCl2	na	13.6 mg/l	-	growth of 9 wood-rotting	na
	al. 2000		culture					to 136		basidiomycetes
								mg/l
	Baldrian et	fungi	controlled	cultivated,	no	HgCl2	chronic	50 to 100	-	soil colonization rate by the	visual observation
	al. 2000		conditions	acidic				mg/kg		fungi
				cambizol					-	laccase and manganese	enzymatic assay
										peroxidase maximal activity
									o/+	no or positive effect of	Liquid
										degradation of PAH by the	chromatography
										fungi
	Bressan et	bacteria	controlled	pristine	no	HgCl2	acute	100	change bacterial community	ARISA
	al. 2008		conditions	calcareous				mg/kg		structure
				silty-clay soil
	Bringmark	total	controlled	forest soil	no	HgCl2	acute	5 mg/kg	-	respiration rate	respirometry
	et al. 1998		conditions
	Campos et	total	field	mining soil	yes	total	chronic	4068	o	dehydrogenase activity	enzymatic assay
	al. 2018		experiment					mg/kg
	Casucci et	total	controlled	Typic	no	HgCl2	acute	0.136 to	-	microbial biomass C,	fumigation ,
	al. 2003		conditions	Ustocreps,				2.72		alkaline phosphatase	enzymatic assay
				Haplustol				mg/kg		activity, arylsulfatase
				Fluventic,						activity , o-DPO
				Mollic					-/+	FDA hydrolysis increase or
				Psammaquent						decrease in function of the
										type of soil
	Crane et al.	fungi	axenic	na	na	HgCl2	na	1.36 mg/l	-	growth rate, increase lag	na
	2010		culture					to 10.88		phase of 6 ectomycorrhizal
								mg/l		fungi
	Crane et al.	fungi	controlled	forest podzol no	HgCl2	acute	88 mg/kg -	ectomycorrhizal	morphological
	2012		conditions							colonization, richness and	identification and
										diversity of root tips	counting

Funding: This work was supported by the French ANR Research Program (ANR BIOFILTREE 2010-INTB-1703-01 to M.C.), by the ADEME (grant n o 1172C0053, by the Région Franche-Comté (project "Environnement-Homme-Territoire") and by Pays de Montbéliard Agglomération, and by the French national programme EC2CO-MicrobiEen FREIDI-Hg.

Author contributions

MC and AD conceived the content of the review. AD, FM and JF investigate the literature on the topic. AD and FM wrote the manuscript. MC supervised the final arrangement of the manuscript. All authors discussed the content of the review and approved the final manuscript.

Compliance with Ethical Statement

Conflict of Interest: Alexis Durand, François Maillard, Julie Foulon and Michel Chalot declare that they have no conflict of interest.

Ethical approval: This article does not contain any studies with human participants or animals performed by any of the authors.