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.
Highlights
Toxic metal and metalloid pollution of soil and water can cause serious ecological and environmental problems
Toxic metals can exert a variety of effects on the morphology and physiology of fungi, including A. pullulans (Gadd and Griffiths 1980; Gadd and Mowll 1985; Gadd et al 1986; Newby and Gadd 1987; Ramsay et al 1999; Fomina et al 2000)
Mercury and selenium had a negative effect on the growth of A. pullulans, but lead and arsenic had no significant influence on the growth of A. pullulans at the concentrations tested
Summary
Toxic metal and metalloid pollution of soil and water can cause serious ecological and environmental problems. EPS is believed to play an important role in the tolerance of microorganisms exposed to toxic metals acting as a protective barrier and a matrix for geochemical transformations and bioprecipitation (Fomina et al 2005; Hou et al 2013; Naik et al 2012). This has been observed in a variety of microorganisms, such as sulfate-reducing bacteria (Yue et al 2015), iron-oxidizing bacteria (Liu et al 2017), and the fungi Phanerochaete chrysosporium (Cao et al 2018) and Beauveria caledonica (Fomina et al 2005)
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