Phosphate-solubilising rhizobacteria associated with Coffea arabica L. in natural coffee forests of southwestern Ethiopia

Phosphate-solubilising rhizobacteria associated with Coffea arabica L. in natural coffee forests of southwestern Ethiopia were investigated. The main purpose was to screen for potential microbial biofertilisers by assessing isolated strains for phosphate solubilisation efficiency and organic acid production in different media. Initial screening was performed on Pikovskaya’s agar (PA). Quantitative colorimetric estimations of mobilised phosphate were made in different broth media in the presence of two phosphate sources. HPLC was employed for the detection of organic acids. From a total of 395 rhizobacterial isolates tested for P solubilisation, over 72% (mostly Pseudomonas spp.) formed visible dissolution haloes on PA. Two Erwinia species and a P. chlororaphis strain produced the largest solubilisation indices and also solubilised hydroxyapatite strongly in broth medium. Solubilisation of hydroxyapatite (HAP)/tricalcium phosphate (TCP) by all isolates coincided with a decrease in medium pH. HPLC analyses of culture supernatants confirmed the presence of several organic acids, with 2-ketogluconic acid dominating. The production of organic acids by these coffee-associated phosphobacteria could be considered the major mechanism involved in the solubilisation of insoluble HAP/TCP. Certain isolates deserve particular attention for bioinoculant development due to their remarkable efficiency of insoluble phosphate solubilisation. The present study could therefore be important with respect to screening of Coffea arabica-associated rhizobacteria that possess direct plant growth-promoting traits for extending the use of indigenous microbes as microbial biofertilisers.