Fungi are ecologically ubiquitous organisms on earth and regarded as one of the prolific sources of natural products. Fungal endophytes may provide essential prerequisite molecules to plant biochemical pathways which allow the efficient synthesis of primary and secondary metabolites. This study characterized the influences of various combinations of process parameters namely, carbohydrate, nitrogen, and phosphorus sources on citric acid (CA) production by the isolated fungal endophyte Aspergillus fumigatus P3I6 from Citrus microcarpa. Aspergillus fumigatus P3I6 had higher CA concentration of 9.2 (± 0.9) g L-1 and 9.0 (± 5.0 × 10-15) g L-1 when supplemented with sucrose and white refined sugar, respectively, than A. niger NRRL 599. Response Surface Methodology (RSM) had shown that A. fumigatus P3I6 produced the highest CA (23.8 g L-1) in Combination 4 (18.0% sucrose, 0.3 g L-1 ammonium sulfate, and 5.0 g L-1 dipotassium phosphate (K2HPO4)). Analysis of variance showed that when K2HPO4 concentrations were increased, CA content in fermentation media was significantly elevated. Hence, K2HPO4 was the most critical variable in the quadratic model (p < 0.05); however, sucrose concentration still has its role in production. Aside from using A. niger in most fermentation processes, this discovered fungal strain can be potentially used in biotechnological applications.
Keywords: Aspergillus fumigatus; Aspergillus niger; citric acid; fungal endophytes; response surface methodology.