Phosphate-solubilizing microorganisms associated with the rhizosphere of mangroves in a semiarid coastal lagoon

Abstract

The phosphate-solubilizing potential of the rhizosphere microbial community in mangroves was de- monstrated when culture media supplemented with in- soluble, tribasic calcium phosphate, and incubated with roots of black (Avicennia germinans L.) and white (La- guncularia racemosa (L.) Gaertn.) mangrove became transparent after a few days of incubation. Thirteen phosphate-solubilizing bacterial strains were isolated from the rhizosphere of both species of mangroves: Ba- cillus amyloliquefaciens, Bacillus licheniformis, Bacillus atrophaeus, Paenibacillus macerans, Vibrio proteolyti- cus, Xanthobacter agilis, Enterobacter aerogenes, Ente- robacter taylorae, Enterobacter asburiae, Kluyvera cryo- crescens, Pseudomonas stutzeri, and Chryseomonas lu- teola. One bacterial isolate could not be identified. The rhizosphere of black mangroves also yielded the fungus Aspergillus niger. The phosphate-solubilizing activity of the isolates was first qualitatively evaluated by the for- mation of halos (clear zones) around the colonies grow- ing on solid medium containing tribasic calcium phos- phate as a sole phosphorus source. Spectrophotometric quantification of phosphate solubilization showed that all bacterial species and A. niger solubilized insoluble phosphate well in a liquid medium, and that V. proteo- lyticus was the most active solubilizing species among the bacteria. Gas chromatographic analyses of cell-free spent culture medium from the various bacteria de- monstrated the presence of 11 identified, and several unidentified, volatile and nonvolatile organic acids. Those most commonly produced by different species were lactic, succinic, isovaleric, isobutyric, and acetic acids. Most of the bacterial species produced more than one organic acid whereas A. niger produced only suc- cinic acid. We propose the production of organic acids by these mangrove rhizosphere microorganisms as a possible mechanism involved in the solubilization of in- soluble calcium phosphate.