Promotion of Fungal Bioactivity by Extracellular Polymeric Substances Under Phosphorus Deficiency

Extracellular polymeric substances (EPS) can work as a highly efficient nutrient source for many microorganisms. This study investigated the influences of EPS on the bioactivities of a typical phosphate-solubilising fungus, that is Aspergillus niger. The EPS were extracted from Rhodotorula mucilaginosa, with the addition of varying levels of inorganic phosphates. Under phosphorus (P) limitation, the cell wall thickness of A. niger increased from ~0.2 μm to ~0.4 μm. The addition of EPS relieved this phenomenon under the stress. In addition, it showed that EPS significantly promoted microbial growth with a four-fold increase in microbial biomass. For the treatment with the addition of sole inorganic phosphate, A. niger primarily secreted oxalic acid for phosphate solubilisation. However, for the treatment with sole EPS, the secretion of oxalic acid was inhibited, while the activity of acid phosphatase was promoted. Moreover, the concentration of oxalic acid was significantly reduced from approximately 90 to 1 mg/L during the co-existence of inorganic phosphate and EPS. Meanwhile, the activity of acid phosphatase was also enhanced. Therefore, A. niger can balance its strategy of P solubilisation, depending on the types of P sources. Furthermore, EPS was preferentially utilised over inorganic phosphate. Our findings highlight the potential of EPS for sustainable P supply.