## 4 Discussions
Phosphate-solubilizing bacteria can improve the uptake of P by the roots through the production of organic acids [83], and EPS have a synergistic effect by holding free P in the medium [84]. Siderophores can also enhance P solubilization by chelating substances that form stable complexes with P adsorbents [85]. This interaction may explain the frequent co-occurrence of EPS and siderophore production and P-solubilization capacities in the collection of isolates. In turn, P and Fe are important for N-fixation because P is required for energy transfer processes and Fe is a constituent of nitrogenase and leghaemoglobin [86; 87]. However, in haplotype endophytichetic bacteria this dependence may not be strict. Bacilli represented the majority of the isolates that combined siderophore production with P-solubilization capacity, but N-fixation capacity was not detected in any of them. Only one endosphere isolate (_Kushneria phyllosphaerae_) accumulated the capacities of phosphate solubilization, siderophore production and nitrogen fixation. N-fixation was detected in a halophilic _Kushmeria marisflavi_ strain, isolated from the rhizosphere of _Salicornia_ sp. in hypersaline soils of Tunisia, but that isolate lacked phosphate solubilization capacity [88]. N-fixation was overall less frequent than either P-solubilization or siderophore production and was not represented in one of the subsets of isolates (Al). Nitrogen fixation is energetically costly for bacteria [30] and negatively affected by soil salinity [89; 90], which can explain the low number of isolates exhibiting this feature. In Salicornia-dominated areas, cyanobacteria have a very important role in Nitrogen fixation, especially in warmer months and low plant density areas [91]. N-fixing cyanobacteria have been detected in the epiphytic communities associated with halophytes [92; 93]. However, our strategy of cultivation and isolation did not allow the detection of these prokaryotes. It also possible that the method we used to screen nitrogen fixation was not fully adequate for our isolates. In addition to these factors, there is the possibility of loss of the ability to express N-fixation genes during successive cultivations in TSA medium. All N-fixing isolates came from the endosphere, which represents a more protected environment in comparison to soil or the rhizosphere [64]. Actinobacteria have been found to dominate N-fixing endophytic PGP bacteria in _S. europaea_[30]. In this study, only one out of five N-fixing endophytic isolates associated with _S. rannossisima_ belonged to Actinobacteria (EB3, genus _Brevbacterium_). The majority of the plant-associated N-fixing isolates found in this study were \(\gamma\)-Proteobacteria (genera _Halomonas_, _Kuslmeria_, _Psychrobacter_ and _Stentorophomonas_). The predominance of Gram-negative taxa among N-fixing isolates from saline environments, namely the rhizosphere of _Salicornia_, has been reported [88]. IAA production and ACC-deaminase are two of the most valued plant-growth promoting traits and are considered as requirements for endophytic and rhizosphere competence [94]. One common effect is the increase in root length, surface area, and root hair density, improving the access to soil nutrients under stress conditions [64, 81]. IAA production was detected in 54 isolates (45%), and six of them produced moderate to high amounts of IAA, ranging from 40 \(\mu\)L mL\({}^{-1}\) to over 72 \(\mu\)L mL\({}^{-1}\). In previous studies, 3.6% of isolated bacteria from _Halimione portulacoides_ produced high amounts if IAA (over 100 \(\mu\)L mL\({}^{-1}\)), and bacteria isolated from Mediterranean halophytes produced IAA in the range of 17.92-112.5 \(\mu\)g mL\({}^{-1}\)[95, 96]. ACC-deaminase activity was slightly less frequent. Some isolates failed to grow and could not be tested, which hindered an accurate estimate of the frequency of this PGP trait. However, approximately 41% of the isolates were able to cleave ACC, and 25 isolates expressed both ACC-deaminase activity and IAA production.