Solubilization of inorganic phosphate and production of organic acids by bacteria isolated from a Moroccan mineral phosphate deposit

Abstract

Three efficient inorganic-phosphate solubilizing bacteria (PSB) were isolated from a phosphate rock deposit of a Moroccan mine. The phosphate solubilization index of these isolates, determined in National Botanical Research Institute's phosphate (NBRIP) medium supplemented with tribasic calcium phosphate, ranging from 2.8 to 4.4. The medium pH dropped from 7.0 to 3.5 units after growth under continuous agitation for seven days. PSB6, the most efficient PSB, closely related to Enterobacter hormaechei subsp. steigerwaltii strain NM23-1, permitted the recovery of the maximum soluble orthophosphate concentration in the medium (505 mg/L) after a growth period of 60 to 72 h. PSB4 and PSB5 strains were identified as Enterobacter sp. Strain TSSAS2-48 and Bacterium DR172. The capacity to solubilize inorganic phosphate by these PSB can be attributed to the secretion of organic acids, to determine their presence in the cultures supernatant, reverse-phase high performance liquid chromatography was performed. The presence of 9 identified and three unidentified organic acids was consequently demonstrated. Gluconic acid was strongly produced by all strains, extending to 55.4 mM by PSB6, 46.4 mM by PSB5, and 44.9 mM by PSB4. Besides succinic, acetic, glutamic, oxaloacetic, pyruvic, malic and fumaric acids, a newly detected and identified organic acid was the alpha-ketoglutaric acid. To the best of our knowledge this is the first report mentioning alpha-ketoglutaric acid production by PSB strains