# Potential of Halophytes-Associated Microbes for the Phytoremediation of Metal-Polluted Saline Soils
## 4 Designing an Appropriate Inoculum for Phytoremediation or Phytostabilization  
Until recently, the design of an inoculum was most of the time based on the selection of one microbial strain with various PGP traits, and only a few studies tried to elaborate a mixed inoculum (more representative of the initial microbiome) [162]. However, both methods (single strain or combination of several strains) can sometimes lead to \"a depletion of metabolic diversity of the inoculum, whereas, in nature, cooperation within the microbial communities leads them to perform complex tasks\" [162]. [PERSON] et al. [163] emphasized the difficulties of designing such an optimal inoculant because techniques are based on the artificial combination of strains that are most of the time selected based on an initial in vitro characterization. Even if these principles should enable us to precisely target potential microbial candidates, direct applications of the selected strains pose risks because their behavior can change in the field, as well as their effective PGP features. Moreover, screening microorganisms for their potential in microbial-assisted remediation leads to genetic trade-offs that could impair their natural metabolism or their capacity of roots colonization, for example [163]. Nevertheless, [PERSON] et al. [84] encouraged the combination of several microorganisms as inoculant, as it can increase soil microbial diversity and the phytoremediation yield through synergic mechanisms. Indeed, the study of [PERSON] et al. [95] showed an increase of beneficial effects of co-inoculation compared to single inoculation on the growth of _Salicornia_ sp. inoculated with an endophytic _Staphylococcus_ sp. strain and a rhizosphere one and subjected to diverse NaCl concentrations. The success of phytoremediation greatly depends on rhizosphere microbiome functions and the way the plant interacts with its microbiome [164]. Furthermore, high amount and diversity of root exudates will favor competitive PGP microbes [164]. The soil microbiome can compete with the microbial inoculant for the same niche and alter the inoculant more or less rapidly [163]. When inoculants are successfully established, their spread into the environment must be carefully monitored because possible downstream impacts can occur, namely non-suitable changes in microbial diversity and/or biological invasion [163; 165]. For example, the inoculation of _Alliaria petiolate_ with two strains of _Gomus intraradics_ (not native from the oldfield meadow soil used in the experiment), reduced the AMF local community diversity [166]. Similar effects have been found with a commonly used commercial inoculum strain of _Rhizophagus irregularis_ in the study of [PERSON] et al. [167]. Indeed, introducing this foreign AMF strain significantly reduced the abundance of native AMF species and the amount of extraradical mycelium. However, in a few cases, inoculation can increase the diversity of microbial communities [168]. Thus, the main challenge in the design of an inoculum is probably to find an optimal composition in order to have the least possible impact on the original plant microbiome while suitably contributing to host growth. In addition, bioremediation strategies have to be designed according to the specificity of each polluted site, taking into account the metals' nature, soil characteristics, microbes, and plant communities [7], with many unpredictable parameters that evidently cannot be totally tested in the laboratory. This could be particularly the case in saline coastal areas where large fluctuations of salinity, humidity, and temperature can occur [122]. The availability of nutrients, the moisture content, the pH, the permeability, and the temperature of the soil matrix have to be characterized before the application of any bioremediation [169]. Moreover, vegetal cover and plant species diversity also modify soil parameters and shape microbial communities. In a field experiment of salt phytoremediation in a coastal area, [PERSON] et al.