*5.1.1 Organic acid production*

The key mechanism of phosphate solubilisation is the secretion of the organic acids that result from the carbon metabolism of PSM, which is closely related to the concentration of soluble phosphate [3, 4, 24]. Low-molecular-weight organic acids are synthetised during glucose oxidation through direct periplasmic oxidation and intracellular phosphorylation. The synthesis of organic acids from glucose by the cells of PSMs in phosphate deficient conditions is higher than in phosphate sufficient conditions, which correlates with the effect of soluble phosphate on organic acid production [56].

The release of these organic acids into the environment is accompanied by a decrease in pH and chelate of the cations (such as Al3+, Fe3+, and Ca2+) bound to phosphate ions to release the phosphate [3, 8]. These organic acids compete with the phosphate binding sites of the medium and allow phosphates to be available in free [15] as HPO4 −2 and HPO4 −3 [10]. However, there is no correlation between pH and the amount of solubilised phosphates [56]. Gluconic acid is the frequent secretion among all organic acids released by PSM (**Table 2**) [3, 10, 54]. PSF may be even more important than PSB since they typically produce and excrete more acids [56].

## *5.1.2 Inorganic acid production*

Inorganic acid-producing bacteria also involve with phosphate solubilisation through acidification. Bacteria, engaged in nitrification and sulphur-oxidation, which have the ability to produce inorganic acids, are involved with phosphate solubilisation by secreting nitric, carbonic, sulphuric [3] and hydrochloric acids [4, 24]. Nitrifying bacteria such as *Nitrosovibrio*, *Nitrosomonas*, *Nitrobacter*, *Nitrosospira* frequently secrete nitric acid [61] and sulphur oxidising bacteria such as *Thiobacillus thiooxidans* [62] produce sulphuric acid directly involve to solubilisation of phosphate which is required to be further studied. Acidification of the media by secreting inorganic acids and H+ substitution reactions release the phosphates by converting insoluble phosphate to its soluble form [4, 24]. However, the efficacy of phosphate solubilisation by inorganic acids is lower than the efficiency of organic acids [3, 8].

## *5.1.3 Proton extrusion*

Another alternative mechanism to solubilise phosphate is extrusion of proton, which minimise the requirement to synthesis acids [3]. Excretion of H+ through H2CO3 production, NH4 + assimilation and liberation of organic acid anions facilitate the solubilisation of phosphates by acidifying the media [3, 8, 10]. The release of H+ to the extracellular surface of PSM through the exchange of cation or the ATPase activity




*Managing Soil and Plant Nutrients: Role of Microbial Phosphate Solubilisation DOI: http://dx.doi.org/10.5772/intechopen.112640*

#### **Table 2.**

*Major examples of the production of organic acids by PSM.*

with H+ translocation, lead to the solubilisation of P [56]. *Pseudomonas fluorescens* [63], *Bacillus* sp., *Azospirillum* sp. [64] significantly exhibit this mechanism of H+ extrusion in the process of phosphate solubilisation.
