# Potential of Halophytes-Associated Microbes for the Phytoremediation of Metal-Polluted Saline Soils
## 3 Selection of Microbes for the Phytoremediation of Metal Contaminated Saline Soils
### Metal Tolerance and Metal Inactivation Capacities  
#### 3.2.1 Metal Tolerance  
To protect themselves against metals or salt stress, microorganisms deploy various mechanisms that could be beneficial to plants. In bacteria, the most commonly encountered metal-resistance mechanisms are metal sequestration by biopolymers [57; 91; 128], metal efflux via specific transporters, enzymatic detoxification [51; 129], metal ion reduction, and intracellular storage [91; 130].  
Some bacteria can actively extract and accumulate metals. [PERSON] et al. [93] have evaluated the resistance to chromate, arsenate, tellurite, selenite, selenate, and biselenite of _Bacillus_ strains isolated from Iranian saline soils. They showed that increasing NaCl concentrations from 5 to 15% incremented the metalloids' resistance of most of the strains. Similar results were obtained on moderate halophilic bacteria from the genus _Salinicoccus_ isolated from saline soils in Iran: it was able to reduce tellurite in tellurium, extracting 75% of the total tellurite content in the medium containing 0.5 mM potassium tellurite and 10% NaCl [131]. They evaluated the influence of pH, temperature, and KCl and Na\({}_{2}\)SO\({}_{4}\) concentrations on metal-extraction efficiency. The best results were reached with pH 7.5 at 35 \({}^{\circ}\)C and 1.5 M NaCl, 1 M KCl, and 0.5 M Na\({}_{2}\)SO\({}_{4}\) were the optimal saline concentrations to remove potassium tellurite. [PERSON] et al. [89] screened halophilic members of _Alcaligenes_, _Vibrio_, _Kurthia_, and _Staphylococcus_ for their Cd- and Pb-resistance, via the evaluation of their minimum inhibitory concentrations (MICs) under saline conditions (5, 10, and 15% NaCl). The in vitro experiment showed that most selected isolates optimally removed Cd and Pb from the solution in presence of 10% NaCl. [PERSON] et al. [92] showed that _Kocuria flava_ and _Bacillus vietnamensis_ strains from the mangrove rhizosphere tolerate high concentrations of As (35 mM of arsenite for _Kocuria flava_ and 20 mM for _Bacillus vietnamensis_).  
As previously mentioned, salinity and metal stress lead to the production of reactive oxygen species (ROS), whose concentrations can be regulated by the activities of certain antioxidant enzymes. In this view, antioxidant activities are also commonly explored when screening microbes for their PGP features. Enzymes such as the superoxide dismutase (SOD), which transforms the radical \(\bullet\)O\({}_{2}\)- into H\({}_{2}\)O\({}_{2}\); the catalase (CAT), which transforms H\({}_{2}\)O\({}_{2}\) into H\({}_{2}\)O and O\({}_{2}\); or the glutathione oxidase (GSH), which converts the glutathione to glutathione disulfide, are involved in the detoxification of ROS and protect microbial cells against oxidative damages. Hence, measuring the activity of these enzymes helps in evaluating the stress levels and the level of tolerance to metal toxicity. Salt stress can additionally increase the activity of some antioxidant enzymes in halophilic bacteria, also inducing a higher metal tolerance, relating to similar mechanisms [62]. Among several PGP traits of a halophilic _Enterobacter_ strain, it has been demonstrated that extreme salinity and alkaline conditions also increased the activities of the SOD, the CAT, the ACCD and the glutathione oxidase (GSH) [117].