Effect of pH on the ultrastructure of the conidial wall of Aspergillus Niger aggregate conidia and its relationship to zearalenone adsorption

Abstract

The purpose of the present study was to evaluate the effect of pH on the ultrastructure of the conidial wall of dead conidia harvested from the non-toxicogenic Aspergillus Niger aggregate strain RC084 and its relationship to zearalenone (ZEA) adsorption capacity. Moreover, mathematical models were applied to explain the interaction between conidia and ZEA absorbance. A ZEA adsorption test was performed using a concentration of 1107 dead conidia/ml at pH 2 and 6 at 37 C for 30 min. Unbound ZEA was quantified by high-pressure liquid chromatography. The ZEA adsorption was strongly dependent on the pH of the medium; the highest values were 2.210-6 μg ZEA/conidia at pH 6. Isotherms representing the amount of bound ZEA as a function of ZEA concentration in equilibrium after adsorption have typical S- or L-shapes. The experimental data could be fitted to the Frumkin-Fowler-Guggenheim (FFG) and Hill theoretical models. The ultrastructure of the conidial wall was studied by infrared spectroscopy (IR) and transmission electron microscopy (TEM). The representative IR spectra showed that pH did not produce significant changes in the different chemical groups. However, ultrastructure studies by TEM detected considerable changes in the organisation of the conidial wall. This is the first study showing that the loss of the outermost electron dense layer, responsible for the ornamentations on the conidial surface. Adsorption is favoured at pH 6.