Metabolic flux and transcriptome analyses provide insights into the mechanism underlying zinc sulfate improved ß-1,3-D-glucan production by Aureobasidium pullulans.

The effects of zinc sulfate at various concentrations on ß-1,3-D-glucan (ß-glucan) and pullulan production were investigated in flasks, and 0.1 g/L zinc sulfate was found to be the optimum concentration favoring increased ß-glucan production. When batch culture of Aureobasidium pullulans CCTCC M 2012259 with 0.1 g/L zinc sulfate was carried out, the maximum dry biomass decreased by 16.9% while ß-glucan production significantly increased by 120.5%, compared to results obtained from the control without zinc sulfate addition. To reveal the mechanism underlying zinc sulfate improved ß-glucan production, both metabolic flux analysis and RNA-seq analysis were performed. The results indicated that zinc sulfate decreased carbon flux towards biomass formation and ATP supply, down-regulated genes associated with membrane part and cellular components organization, leading to a decrease in dry cell weight. However, zinc sulfate increased metabolic flux towards ß-glucan biosynthesis, up-regulated genes related to glycan biosynthesis and nucleotide metabolism, resulting in improved ß-glucan production. This study provides insights into the changes in the metabolism of A. pullulans in response to zinc sulfate, and can serve as a valuable reference of genetic information for improving the production of polysaccharides through metabolic engineering.

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

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.