Direct and indirect impact of the bacterial strain Pseudomonas aeruginosa on the dissolution of synthetic Fe(III)- and Fe(II)-bearing basaltic glasses

Abstract

This study investigates the direct and indirect bacterial contributions that influence the dissolution of basaltic glass. In this regard, three different types of glasses – with or without Fe, in the reduced Fe(II) or oxidized Fe(III) states – were prepared on the basis of a simplified basaltic glass composition. In order to prevent the direct contact between the glasses and the model siderophore-producing strain Pseudomonas aeruginosa, the glass samples were isolated in dialysis bags and immersed at 25 °C and pH 6.5 in bacterial cultures. Throughout the dissolution experiments, the following parameters were monitored: determination of bacterial growth, quantification of siderophore (i.e. pyoverdine) production, microscopic observation of the glass surface and determination of dissolution kinetics. Isolating the glass from the bacterial suspension only triggered the biosynthesis of siderophores in the Fe(III)-bearing glass dissolution experiments. Siderophores were produced in the presence of Fe(II)-bearing and Fe-free glass, independently on the experimental setup. The siderophore production appeared to be either continuous in the absence of Fe (glass-free control, Fe-free glass dissolution experiments) or stopped as soon as the bacteria entered their stationary phase when an Fe source was present (Fe(II) and Fe(III)-bearing glass dissolution experiments). The increase in the dissolution rates of each glass was correlated to the complex stability constants of the siderophore with the metallic cations in presence (KFe2+ < KAl3+ ≪ KFe3+). Among the three glasses, only the Fe(III)-bearing one seemed to be significantly impacted by the dialysis process: its dissolution rate was doubled by isolating the glass grains from the cells. These results particularly allow to separate the impact of such bacterial exudates from physical contact effects: they showed the efficiency of pyoverdine in increasing the dissolution of an Fe(III)-bearing glass and evidenced that a direct bacterial cell attachment to the surface of such a glass results in a more moderate enhancement of its dissolution process. This work is a new contribution regarding the high affinity of microorganisms for basaltic glasses as an Fe-source. It highlights the role of Fe(III) accessibility upon the bacterial cells as a key parameter regulating their activity and their efficiency in accelerating the dissolution.