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

This paper investigates Aspergillus niger's behaviour in the presence of mobile Al species by evaluating the changes in oxalate exudation at various aluminium contents. When the fungus was exposed to Al, no significant changes in oxalate production were observed until 100 mg.L 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 aluminium was observed, respectively, indicating the phosphate's significance instead of Al in oxalate production. Our results suggest that the low phosphate bioavailability, which most likely resulted from its interaction with Al, 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, while at 1.85 mM of available phosphate only 80.7 μmol.g 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.