Bioconversion of Some Agro-Residues into Organic Acids by Cellulolytic Rock-Phosphate-Solubilizing Aspergillus japonicus

Biological-based conversion of agricultural residues into bioactive compounds may be considered to be the basis for various vital industries. However, finding a suitable microorganism is a challenge in the bioconversion process. Therefore, this study was conducted to find local fungal isolates able to convert a combination of plant biomass residues into organic acids (OAs). Based on their cellulase and phytase activities and rock phosphate (RP) solubilization potential, an efficient 15 fungal isolates (named F1 to F15) were selected and identified by both morphological and molecular methods using the 18S rRNA sequencing technique. The best fungal isolate (F15) was identified as Aspergillus japonicus. After 4 weeks of incubation below solid-state fermentation (SSF) with a mix of sugarcane bagasse and faba bean straw (3:7), with 7.5% (v/w) fungal inoculum to the growth medium, the biodegradation process by the fungus reached its peak, i.e., maximum cellulolytic activity and RP solubilization ability. Under such fermentation conditions, seven organic acids were detected using HPLC, in the following order: ascorbic acid > oxalic acid > formic acid > malic acid > succinic acid > lactic acid > citric acid. Based on the results, Aspergillus japonicus (F15) could produce OAs and cellulose enzymes, and could be considered a new single-step bio-converter of sugarcane bagasse and faba bean straw residues into OAs. Furthermore, this fungus could be a new source of fungal cellulose, and could present a practical approach to reducing environmental contamination. Additional work is encouraged for more optimization of fermentation conditions.