Short communication Fungal solubilization of rock phosphate is enhanced by forming fungal–rhizobial biofilms H.S. Jayasinghearachchi, Gamini Seneviratne * Biological Nitrogen Fixation Project, Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka Received 10 February 2005; received in revised form 19 May 2005; accepted 6 June 2005 Abstract Biosolubilization of rock phosphate (RP) using a Penicillium spp., an Aspergillus spp., Pleurotus ostreatus, Bradyrhizobium elkanii SEMIA 5019 and their fungal–rhizobial biofilms was investigated. Eppawala Rock Phosphate (ERP, total P concentration 17.6%), a RP from a deposit in Sri Lanka was used. Penicillium spp.–B elkanii SEMIA 5019 biofilm released the highest amount of P from the ERP with the highest P release-to-P uptake ratio. The P release of Penicillium spp. alone was significantly lower than that of its biofilm. Similarly, P. ostreatus–B. elkanii SEMIA 5019 biofilm showed a higher P release than P. ostreatus alone. However, P. ostreatus alone or its biofilm showed lower P release-to-P uptake ratios indicating relatively higher P uptake compared to the P release. The Aspergillus spp., showed a moderate P release. Large bradyrhizobial cell clusters attached to the mycelial mat of Penicillium spp. and P. ostreatus were observed under light microscope after 12 and 25 days of incubation, respectively. The present study, identified an effective method of fungal–rhizobial biofilm mediated solubilization of RP. q 2005 Elsevier Ltd. All rights reserved. Keywords: Rock phosphate; Biosolubilization; Biofilm formation; Bradyrhizobium; Pleurotus; Penicillium Phosphorus (P) is one of the major nutrient elements limiting agricultural production in the world. It is added to the soil in the form of phosphate fertilizers, a part of which is utilized by plants and the rest is rapidly converted into insoluble complexes in the soil (Vassilev and Vassileva, 2003). This leads to the need of frequent application of phosphate fertilizers, but its use on a regular basis has become a costly affair and also environmentally undesirable (Reddy et al., 2002). Therefore, the necessity to develop economical and eco-friendly technologies is steadily increasing (Vassilev and Vassileva, 2003). Natural phos- phate rocks have been recognized as a valuable alternative for P fertilizers (Reddy et al., 2002). Many different biotechnological methods that have been tested to improve microbial organic acid production and, simultaneously, RP solubilization such as application of agro-industrial residues, solid state fermentation, and liquid submerged fermentation, etc. were reviewed recently (Vassilev and Vassileva, 2003). Biofilm formation is a prominent feature of microbial growth in nature. Biofilms have been observed in a number of environments, but little is known about their use for the mineral availability to the plants. Mycelial colonization and biofilm formation by bradyrhizobia with common soil fungi was reported recently (Seneviratne and Jayasinghearachchi, 2003). Nitrogenase activity in the developed bioflms was also detected (Jayasinghearachchi and Seneviratne, 2004a). Further, these biofilms enhance N and P availabilities when inoculated to soil (Seneviratne and Jayasinghearachchi, 2005). This preliminary study investigates the use of fungal– bradyrhizobial biofilms for biosolubilization of poorly soluble RP and thereby, to enhance simultaneously phosphorus availability and soil fertility. Penicillium spp. isolated from garden soil, Aspergillus spp. from compost, Pleurotus ostreatus mushroom and Bradyrhizobium elkanii SEMIA 5019 nodulating soybean were used in this study. Bradyrhizobial cultures were maintained in Yeast Manitol Broth (YMB: Somasegaran and Hoben, 1994). They were incubated on a rotary shaker Soil Biology & Biochemistry 38 (2006) 405–408 www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2005.06.004 * Corresponding author. Tel.: C94 81 2232 002; fax: C94 81 2232 131. E-mail address: gaminis@ifs.ac.lk (G. Seneviratne).