Screening for rock phosphate solubilizing Actinomycetes from Moroccan phosphate mines

Three Moroccan phosphate mines, an original biotope rich in insoluble rock phosphate (RP), were explored for the presence of RP solubilizing Actinobacteria. Three hundred actinomycete isolates originating from these mines were tested for their ability to grow on a synthetic minimum medium (SMM) containing insoluble RP as unique phosphate source. Only 55 isolates (18.3%) were able to weather RP in SMM medium. Eight isolates showed the most active growth and solubilization capability. These isolates were shown to be able to solubilize RP in liquid cultures. The study of mechanisms involved in these weathering processes indicated that the isolates produce siderophores but not organic acids. Seven of these strains were shown to belong to the genus Streptomyces and one, to the genus Micromonospora. This study is expected to lead to the formulation of novel bio-phosphate fertilizers constituted by the association of pulverized RP and spores of the ad hoc actinomycete strains.