Isolation and characterization of halotolerant phosphate-solubilizing microorganisms from saline soils

Halotolerant phosphate-solubilizing microorganisms (PSMs) capable of producing plant-growth-promoting traits were grown on salt medium containing Ca₃(PO₄)₂ or egg yolk. The number of colonies on plates with Ca₃(PO₄)₂ was higher than that on plates with egg yolk. Further, a total of 42 PSM isolates were purified. The majority were Bacillus spp., while one Providencia rettgeri strain was confirmed, for the first time, as a PSM. All PSMs had a phosphate-solubilizing index (PSI) between 1.1 and 2.58 and a strong capacity for dissolving calcium phosphate between 2.25 and 442 mg·L^-1. In contrast, these PSMs were less effective when dissolving aluminum phosphate, ferric phosphate and lecithin. Isolates were also tested for growth-promoting substances. The results showed that all isolates were able to secrete indole-3-acetic acid in amounts ranging from 2.7 to 31.8 mg·L^-1 and exopolysaccharide within the range 74.3 and 225.7 mg·L^-1. Only 12 siderophore-producing strains with siderophore units of 1.9-42.1% were detected. Among them, ten isolates with solubilization rates greater than 200 mg·L^-1 and relatively high NaCl tolerance (1.5 M) were classified as candidate PSMs. Eight different organic acids with different contents were detected in the culture filtrates, and propionic and oxalic acids have been proposed as the main mechanisms for solubilization. The ten isolates have the potential for use as bioinoculants to protect plants in saline environments.