RESEARCH ARTICLES CURRENT SCIENCE, VOL. 114, NO. 4, 25 FEBRUARY 2018 845 *For correspondence. (e-mail: sandeep@unigoa.ac.in) Phosphate solubilization mechanisms in alkaliphilic bacterium Bacillus marisflavi FA7 Neha Prabhu 1 , Sunita Borkar 2 and Sandeep Garg 1, * 1 Department of Microbiology, Goa University, Panjim 403 206, India 2 Department of Microbiology, P.E.S’s Shri Ravi S. Naik College of Arts and Science, Ponda 403 401, India This study reports the mechanisms of phosphate solu- bilization present in alkaliphilic Bacillus marisflavi FA7. The strain obtained from sediment samples of mangrove ecosystem exhibited different mechanisms to solubilize inorganic phosphate and mineralize organic phosphate under alkaline conditions. It re- duced the pH of the medium that showed near perfect correlation with tri-calcium phosphate solubilization. Organic acids produced by the strain were detected in broth. Maximum decrease in pH of the medium was observed with NH 4 Cl as an inorganic nitrogen source. This indicated involvement of proton extrusion mechanism toward phosphate solubilization during ammonium uptake. It produced exopolysaccharide, but failed to produce siderophore. Bacillus marisflavi FA7 produced extracellular alkaline phosphatase hav- ing molecular weight of 175–200 kDa. The pH opti- mum for maximum enzyme activity was 10.1 and K m of 1.13 M p-nitrophenolphosphate. This is the first study to report the highest tri-calcium phosphate solubilization by an alkaliphilic bacterium. Keywords: Alkaliphilic bacterium, Bacillus, mecha- nism, phosphate solubilization, phosphatase. PHOSPHORUS, one of the major essential macronutrients required for plant growth, is taken up by plants as an orthophosphate ion 1 . Phosphate containing compounds present in soil can be classified into three groups, viz. soluble orthophosphate, insoluble inorganic phosphate and insoluble organic phosphate. Orthophosphate ions re- act with numerous organic and inorganic constituents of soil and therefore become least mobile and unavailable for uptake by plants. Hence, a large quantity of phosphate fertilizer is applied to achieve maximum plant productiv- ity. However, it is observed that orthophosphate ions of the applied fertilizer react with cations of the soil to pre- cipitate into inorganic tri-calcium phosphate, iron phos- phate and aluminium phosphate. Phosphorus containing organic compounds enter the soil by decomposition of plant, animal and microbial matter. Different forms of organic phosphorus compounds include nucleic acids, phospholipids, phosphoproteins, inositol phosphates, phosphonates, etc. 2 . These forms are not taken up by plants due to their high molecular weight. Transformation of phosphate from orthophosphate ions into inorganic or organic phosphate occurs in a cyclic form. The global cycling of insoluble inorganic and organic soil phosphates is attributed to microorganisms 3 . Several mechanisms responsible for solubilization of inorganic phosphates and mineralization of organic phosphates have been reported 4–15 . Organic acid production is the most reported mechanism for solubilization of inorganic phosphate 4,5 . Production of sulphuric, nitric and carbonic acids has been reported to solubilize inorganic phos- phate 6 . Acidification of the medium as a result of H + excretion originating from NH + 4 has been suggested as an alternate mechanism of inorganic phosphate solubiliza- tion 7,8 . The high molecular weight microbial exopolysac- charides have been shown to play an indirect role in phosphate solubilization 9,10 . The rate of dissolution due to exopolysaccharide appears to be dependent on the micro- bial source and concentration of polymer. Since sideropho- res chelate iron, it has been attempted to correlate the amount of siderophore released to the amount of phos- phate solubilized from insoluble iron phosphates 11–13 . However, siderophore production has not been widely implicated as a phosphate solubilization mechanism. Other postulated mechanisms include production of hydrogen sulphide by sulphur reducing bacteria. H 2 S reacts with phosphate containing minerals, thus releasing phosphate 14 . Organic phosphate solubilization is called mineralization of organic phosphorus 15 . It occurs due to production of extracellular or membrane bound enzymes like phosphatase, phytase, phosphonatase, and C–P lyase by microorganisms in soil. Phosphatase enzymes render high molecular weight organic phosphate into low mole- cular weight compounds by the hydrolysis of ester- phosphate bonds, leading to the release of phosphate ions. Based on their pH optima, these enzymes are classified as acid, neutral and alkaline phosphatase 16,17 . Phytase en- zyme hydrolyses phytic acid or myo-inositolphosphate compounds. Phosphonatase and C–P lyase hydrolyse ester bonds of phosphonates (e.g. phosphoenol pyruvate, phosphonoacetate) and converts phosphonates into hydro- carbons and phosphate ions for assimilation 18 . Phosphate solubilizing bacterial bio-fertilizers have been developed for plants of agriculture, horticulture and forestry importance. These bio-fertilizers work best in