Aspergillus niger enhances oxalate production as a response to phosphate deficiency induced by aluminium(III)

Abstract

This paper investigates Aspergillus niger's behaviour in the presence of mobile Al³⁺ species by evaluating the changes in oxalate exudation at various aluminium contents. When the fungus was exposed to Al³⁺, no significant changes in oxalate production were observed until 100 mg.L^-1 aluminium was reached resulting in oxalate production decrease by 18.2%. By stripping the culture medium completely of phosphate, even more prominent decrease by 34.8% and 67.1% at 10 and 100 mg.L^-1 aluminium was observed, respectively, indicating the phosphate's significance instead of Al³⁺ in oxalate production. Our results suggest that the low phosphate bioavailability, which most likely resulted from its interaction with Al³⁺, stimulated the overproduction of oxalate by A. niger. Furthermore, when the fungus was incubated in aluminium-free media supplemented with 0.1 mM of phosphate, oxalate production increased up to 281.5 μmol.g^-1, while at 1.85 mM of available phosphate only 80.7 μmol.g^-1 of oxalate was produced. This indicates that oxalic acid is produced by fungus not as a mean to detoxify aluminium, but as an attempt to gain access to additional phosphate.

Keywords: Aluminium; Aspergillus niger; Oxalate; Phosphate.