Optimization of oxalic acid production by fungi for biotechnological solubilization of rock phosphate

This study describes a biotechnological strategy for producing and applying oxalic acid to solubilize phosphorus (P) from rock phosphate (RP). We evaluated six fungal species (Aspergillus niger FS1, Penicillium islandicum FS41, Pleurotus ostreatus PO1, Rhizoctonia solani Rhiz555, Sclerotium rolfsii Sr25, and Sclerotinia sclerotiorum Scl134) and three culture media (potato dextrose broth, Tsao and Strasser media) to maximize oxalic acid production. Among the fungal isolates tested and culture media, S. rolfsii Sr25 and Tsao medium showed efficient oxalic acid production. Tsao medium was optimized following a response surface methodology after initial screening of factors affecting RP solubilization. The optimized concentrations were 1 g L-1 NaNO3, 100 g L-1 glucose, 2 g L-1 KH2PO4, 4.5 g L-1 yeast extract, and 25 mg L-1 MgSO4 center dot 7H(2)O used for 20 days of incubation. Under these conditions, 71 mmol L-1 oxalic acid was obtained, representing a three-fold increase over production under non-optimized conditions (20 mmol L-1). Under optimized conditions, oxalic acid produced by S. rolfsii Sr25 reacted with low-solubility RP and solubilized 100 % of the P contained in ore. Thus, using S. rolfsii Sr25 to produce oxalic acid seems a promising biotechnological alternative for P solubilization from RP.