Solubilizing Phosphate Microorganisms isolated from rhizospheric and bulk soil of colonizer plants at an abandoned rock phosphate mine site.

I. Reyes, A. Valeri and Z. Valduz

Laboratorio de Biofertilizantes, Universidad Nacional Experimental del Táchira (UNET). Paramillo, San Cristóbal, VENEZUELA, 5001.

E-mail: isreyes@unet.edu.ve

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An abandoned “Monte-Fresco” rock phosphate mine in Táchira, Venezuela, was sampled to study the biodiversity of phosphate solubilizing microorganisms (PSM). Rhizosphere and bulk soil were sampled from three Compositae colonizer species growing at a disturbed site where pH and soluble P were higher than the values found at a near by undisturbed soil. Counting and isolating of PSM choosing strains showing high solubilization halos in a solid minimal medium with hydroxyapatite (MM-HA) as phosphate source (1) were characterized using biochemical tests and/or phosphate solubilizing evaluations with ammonia or nitrate as nitrogen sources. A larger number of PSM were found in the rhizospheric soil (1.87x10⁶ and 2.0x10⁴ UFC/ml of bacteria and fungi, respectively) than in the bulk soil (0.1x10⁶ and 0.13x10⁴ UFC/ml of bacteria and fungi, respectively). It was found that bacteria isolated from the rhizosphere soil showed higher solubilization halos than fungi but fungi from bulk soil. Fifteen strains of Gram-negative bacteria were able to show clear and large HA halos, with 1.5 to 4.6 mm after the colony border. Three of these strains were identified as encapsulated Azotobacter species that are nonsymbiotic nitrogen bacteria and able to use ammonium and nitrate to dissolve HA with glucose as a carbohydrate source. Six fungi strains belonging to the genus Penicillium and with high HA dissolution capacities were isolated from bulk soil. Four of these strains had similar colony characteristics than P. rugulosum IR-94MF1 (1) isolated from a bulk soil of the Monte Fresco mine when grown with different culture media. Using the MM-HA, glucose, and ammonium or nitrate these Penicillia were evaluated and it was found that three of these strains solubilized HA at different degrees with both nitrogen sources while two solubilized with ammonium. Different nitrogen and carbohydrate sources should be evaluated to farther characterize PSM (2).

1. Reyes, I., Bernier, L., Simard, R.R., Tanguay, P., and H. Antoun. 1999. Characteristics of phosphate solubilization by an isolate of a tropical Penicillium rugulosum and two UV-induced mutants. FEMS Microbiology Ecology 28(3): 291-295.

2. Reyes, I., Bernier, L., Simard, R.R., and H. Antoun 1999. Effect of nitrogen source on the solubilization of different inorganic phosphates by an isolate of Penicillium rugulosum and two UV-induced mutants. FEMS Microbiology Ecology 28(3): 281-290.