Phosphate solubilization activities and action mechanisms of two phosphate-solubilizing bacteria

Phosphate-solubilizing bacteria play an important role in increasing soluble phosphorus content in soils and utilization efficiency of phosphatic fertilizers. Halo zone on agar plate was used to screen solubilizing-phosphate bacteria and the molybdenum antimony resistance colorimetric and disodium phenyl phosphate methods were used to analyze solubilizingphosphate activities and phosphatase activities of the bacteria. The organic acids produced by solubilizing-phosphate bacteria were analyzed using thin layer chromatography (TLC). The taxonomic status of the solubilizing-phosphate bacteria was determined on the basis of physiological and biochemical characteristics and phylogenetic analysis of 16S rRNA gene sequences. The aim of the study was to select bacteria strains solubilizing both inorganic and organic phosphate, and to explore their phosphate solubilization activities and mechanisms. The results showed that two bacteria (designated as JXJ-11 and JXJ-15 strains) exhibited higher activities in solubilizing phytin and the soluble phosphorus contents in the culture broths increased respectively by 219 mg·L^-1 and 216 mg·L^-1 after three days of incubation. The two strains showed lower activities of solubilizing calcium phosphate and the highest soluble phosphorus contents in the culture broths were only 21.79%-30.37% of phytin in media. Both JXJ-11 and JXJ-15 strains secreted acid, neutral and alkaline phosphatases to degrade insoluble phosphorus and probably produced organic acids (e.g., propionic and succinic acids) which reduced pH values of culture broths while increasing soluble phosphorus contents. Both JXJ-11 and JXJ-15 strains were gram-stain negative, non-endospore forming and positive for the production of hydrogen sulfide. While 16S rRNA gene sequence of JXJ-11 strain was closely related to Sphingomonas melonis DAPP-PG 224^T and S. aquatilis JSS7^T (99.79%), JXJ-15 strain was closely related to Klebsiella pneumoniae subsp. pneumoniae DSM 30104^T (99.73%). Based on the results above, JXJ-11 and JXJ-15 strains were identified as members of the genus Sphingomonas and Klebsiella, respectively. The action mechanisms of phosphate solubilization of JXJ-11 and JXJ-15 strains included secretion of organic acids and phosphatases. The study showed that JXJ-11 strain had a potential application value in developing microbial phosphatic fertilizers.