Solubilization of inorganic calcium phosphates—Solubilization mechanisms

Two species [Penicillium aurantiogriseum and Pseudomonas sp. (PI1889)] having high abilities in solubilizing inorganic phosphates (hydroxylapatite and brushite) were used to examine solubilization mechanisms. No direct contact between microorganisms and calcium phosphates (Ca-Ps) were necessary for effective solubilization. The P-concentration in solution did not increase according to a sigmoid curve type. Therefore, sampling time is of particular importance for estimating P mobilization. The widespread theory that solubilization is always caused by the release of organic acids may prove wrong for our two organisms. In our opinion the most probable reason for solubilization without acid production is the release of protons accompanying respiration or NH4+ assimilation. It was shown that solubilization of Ca-Ps with two different solubility products may lead to a short-term increase of the amount of at least one CaP. Precipitation and subsequent resolubilization of different organic or inorganic phosphates result in hardly predictable P-concentrations in culture solution.