Future trends in research on microbial phosphate solubilization: one hundred years of insolubility

Abstract

For over one hundred years, agricultural microbiologists and soil ecologists have studied the ability of a wide range of soil microorganisms to dissolve poorly soluble mineral phosphates. In the case of calcium phosphates, a significant body of evidence has been developed to show that Gram negative bacteria exhibiting superior mineral phosphate solubilizing (MPS) capabilities utilize the direct oxidase pathway. This pathway (also called nonphosphorylating oxidation) produces gluconic acid and 2-ketogluconic acid directly in the periplasmic space. These strong organic acids can dissolve poorly soluble calcium phosphates such as hydroxyapatite and rock phosphate ore (e.g. fluroapatite). Therefore, we propose that the conservation of the direct oxidation pathway in rhizobacteria may, at least in part, result from the mutualistic advantage provided by the MPS trait. This article contains a brief literature review, some examples of ongoing work in our lab and, finally, a proposal for a unified terminology for the classification of microorganisms capable of solubilizing or mobilizing P in the soil or other ecosystems.