BIOTECHNOLOGY TECHMQUES Volume 10 No. 8 (Augusf) pp.585488 Received as revised 1 June ROCK PHOSPHATE SOLUBILIZATION WITH GLUCONIC ACID PRODUCED BY IMMOBILIZED PENZCZLLZUM VARZABZLE P16 Nikolay Vassilev I*, Massimiliano Fenice and Federico Federici2 ‘Estacion Experimental de1Zaidin, CSIC, Prof. Albareda, i, Granada- 18008, Spain; 2Dipartamento di Agrobiologia e Agrochimica, University of Tuscia, Viterbo-0 1100, Italy. SUMMARY Penicillium variabile P 16 immobilized on polyurethane sponge produced gluconic acid in presence of rock phosphate, the latter being simultaneously solubilized during five repeated batches. A total production of 42, 60, and 90 g gluconic acid/l was obtained for 3, 7, and 14 g rock phosphate/l, respectively. Accordingly, soluble phosphorus concentration increased with gluconic acid production, reaching a maximum of 350 mg/l at the 3d batch in medium supplemented with 14 g rock phosphate/l. INTRODUCTION. Interest in solubilization of inorganic phosphate has been increasing over the last years, mainly because of the economical potential that rock phosphate may represent for the agriculture. The direct use of rock phosphate is restricted to acidic soils but even when conditions are optimal, yields are as a rule lower than those obtained with soluble phosphate. An attractive approach to rock phosphate solubilization is the application of microorganisms able to excrete organic acids. It has been repeatedly shown that low-molecular-weight organic acids can strongly increase phosphorus solution concentration by mechanisms involving chelation and exchange reactions (Kpomblekou and Tabatabai, 1994). Apart from increasing plant-available P-fraction in soil, microbial P-mobilization may play an important role in iron ore dephosphorilation (Parks et al., 1990). Although the advantages of immobilized organic acid-producing microorganisms are well-known (Vassilev and Vassileva, 1992), only their free forms have been used so far. The purpose of this work was to verify the potential application of P. variabile P16 immobilized on polyurethane sponge for the solubilization of rock phosphate. MATERIALS AND METHODS Microoreanism and culture conditions. PeniciElium variabile P16 was grown on malt extract agar. The growth medium contained (g/l of distilled water): NaN03, 5.0; KCl, 0.5; KH2P04, 1.0; FeS04.7H20, 0.01; mycological peptone (Oxoid), 3.0; CaC03, 35.0; glucose, 80.0. The production medium contained (g/l of distilled water): NaN03, 3.0; KCl, 0.5; FeS04.7H20, 0.01; mycological peptone, 2.0; glucose, 80.0; CaC03, 35.0. Sedimentary rock phosphate (12.8 %, w/w, P; lmm mesh) was added (either 3.0, 585