The solubilization of potassium-bearing rock powder by Aspergillus niger in small-scale batch fermentations

The fungus Aspergillus niger was cultivated in culture medium with an alkaline ultramafic rock powder to evaluate the solubilization of potassium for biofertilizer production. The assays were carried out with 2 strains (CCT4355 and CCT911) in small-scale batch fermentations using 125, 500, 1000, and 2000 mL Erlenmeyer flasks, with a nominal volume of 40%, and rock powder at 0.4%, shaken at 160 r/min, incubated at 30 degrees C, and sampled every 7 days for 35 days. The amount of soluble K(+), the pH of the culture medium, and the acidity were determined. Both strains solubilized K(+) from the rock powder to the same extent (approximately 62%-70% after 35 days) in the 125 mL flasks; however, the percent solubilization decreased at higher volumetric scales. The results also indicated a difference in strain sensitivity to the increase in volumetric scales in batch fermentation. When filter-sterilized air was injected into the medium, the K(+) percent solubilization obtained after 4 days of cultivation was similar to that obtained after a 28 day period. The acid production by the fungus may be a mechanism of rock solubilization, in spite of the elevation in pH values probably caused by the increasing hydrolysis of the silicates. Both strains of A. niger are recommended for solubilizing potassium from ultramafic rocks, but it is necessary to optimize the oxygen transfer, which seemed to affect the rock solubilization at higher volumetric scales.