Inhibition effect of initial Pb2+ concentration on Pb2+ accumulation by Saccharomyces cerevisiae and Aureobasidium pullulans

Influence of metals and metalloids on the composition and fluorescence quenching of the extracellular polymeric substances produced by the polymorphic fungus Aureobasidium pullulans

Abstract

Aureobasidium pullulans is a ubiquitous and widely distributed fungus in the environment, and exhibits substantial tolerance against toxic metals. However, the interactions between metals and metalloids with the copious extracellular polymeric substances (EPS) produced by A. pullulans and possible relationships to tolerance are not well understood. In this study, it was found that mercury (Hg) and selenium (Se), as selenite, not only significantly inhibited growth of A. pullulans but also affected the composition of produced EPS. Lead (Pb) showed little influence on EPS yield or composition. The interactions of EPS from A. pullulans with the tested metals and metalloids depended on the specific element and their concentration. Fluorescence intensity measurements of the EPS showed that the presence of metal(loid)s stimulated the production of extracellular tryptophan-like and aromatic protein-like substances. Examination of fluorescence quenching and calculation of binding constants revealed that the fluorescence quenching process for Hg; arsenic (As), as arsenite; and Pb to EPS were mainly governed by static quenching which resulted in the formation of a stable non-fluorescent complexes between the EPS and metal(loid)s. Se showed no significant interaction with the EPS according to fluorescence quenching. These results provide further understanding of the interactions between metals and metalloids and EPS produced by fungi and their contribution to metal(loid) tolerance.

Key points

• Metal(loid)s enhanced production of tryptophan- and aromatic protein-like substances.

• Non-fluorescent complexes formed between the EPS and tested metal(loid)s.

• EPS complexation and binding of metal(loid)s was dependent on the tested element.

• Metal(loid)-induced changes in EPS composition contributed to metal(loid) tolerance.

Lead resistance in micro-organisms

Lead (Pb) is an element present in the environment that negatively affects all living organisms. To diminish its high toxicity, micro-organisms have developed several mechanisms that allow them to survive exposure to Pb(II). The main mechanisms of lead resistance involve adsorption by extracellular polysaccharides, cell exclusion, sequestration as insoluble phosphates, and ion efflux to the cell exterior. This review describes the various lead resistance mechanisms, and the regulation of their expression by lead binding regulatory proteins. Special attention is given to the Pbr system from Cupriavidus metallidurans CH34, which involves a unique mechanism combining efflux and lead precipitation.