# Potential of Halophytes-Associated Microbes for the Phytoremediation of Metal-Polluted Saline Soils
## 1 Introduction
### Potential of Halotolerant and Halophilic Microbes in the M mediation of Metal Stress in Plants  
Microbes in the rhizosphere can drastically influence soil parameters in soils [21]. According to [PERSON] et al. [27], microbial activities can limit the bioavailability of metals for the ecosystem and thus control the absorption of metals by accumulating plants. Moreover, microbial interactions with plants include a communication system made of an exchange of several secondary metabolites that will thus lead to changes in plant responses to stress [53; 81]. Hence, the microbiome must also be considered when engineering phytoremediation techniques. According to [PERSON] et al. [59], \"any community of root-associated microorganisms would be dominated by plant-growth-promoting rhizobacteria (PGPR) and endophytic bacteria\". The number of publications describing the mechanisms related to PGP microbes in double stressful conditions, namely with high metal and salt concentrations, is relatively low. Indeed, when using the items \"halophyte\" AND \"PGP\" AND \"metals\" OR \"metal\" on Scholar Google ([[https://scholar.google.com/](https://scholar.google.com/)]([https://scholar.google.com/](https://scholar.google.com/)), accessed on 12 March 2023), we found 684 results. In plants, salinity exerts a selective pressure that could induce resistance to other stresses, such as metal toxicity [82], and the same process could have happened for microbial communities. Moreover, [PERSON] et al. [83] suggested that most of the bacteria isolated from saline environments have to cope with multiple fluctuating abiotic parameters, such as temperature, pH, and salinity, and are consequently likely to tolerate other stresses, an idea which is also supported by [PERSON] et al. [84]. Dealing with salinity implies adaptations related to proteins' stability [85], lipid composition and membrane fluidity [15; 86], ion homeostasis, and the transport system to regulate Na concentrations in cells, the production of compatible solutes [15]. Moreover, microbes have to face a fluctuant salinity in tidal environments throughout the day and the year. [PERSON] et al. [87] emphasized that the good adaptability of some non-extreme halophilic and halotolerant microbes to several stressors could be linked to a repeated change of the salinity level, namely occurring in coastal areas, leading to an adaptive process on their proteins that make them stable under diverse salinities. [PERSON] et al. [51] asserted that candidates for metal bioremediation can be found in both halophilic and halotolerant bacteria and archaea. Several publications showed a potential in metal bioremediation with halotolerant [45; 88; 89; 46] and halophilic microbes [89; 90; 91; 92; 93]. According to [PERSON] et al. [84], the halophyte microbiome could be enriched in genes involved in plant salt resistance and PGP capacities. These authors showed that the microbiome of _Suueda salsa_ was enriched in bacteria with genes related to salt stress acclimatization (ABC transporters), nutrient acquisition (phosphatase, pyrroloquinoline-quinone synthase, nitrogen fixation protein, nitronate monooxygenase, formamidase, and nitrite reductase), and competitive root colonization (site-specific recombinase/integrase and NADH dehydrogenase). Positive effects of halophytes-associated microbes on plant growth and salt resistance have also been shown, especially for some plant genera: _Salicornia_[94; 95; 30; 96], _Limonium_[97], _Arthrocenum_[45], _Spartina_[98; 46], and _Sulla_[99]. In view of the particularly fluctuant environmental conditions of the seashore areas, the salt adaptations of halophytes-associated microbes are linked to multiple PGP properties that are relevant in the bioaugmentation of metal phytoremediation applied to such sites (Figure 1). Figure 1: Factors influencing the phytoextraction and the phytostabilization by halophytes and their associated microbes in estuarine metal-contaminated saline soils.