Prospecting the biodiversity of the fungal family Ustilaginaceae for the production of value-added chemicals

Abstract

Background: Ustilaginaceae (belonging to the smut fungi) are commonly known for their plant pathogenicity. Although these microbes lead to yield reduction of cereal production, they can also have an economically positive side. Ustilaginaceae naturally produce a versatile range of value-added chemicals with potential applications in the food, pharmaceutical, and chemical industry.

Results: In this study 68 Ustilaginaceae of 13 species were screened for the production of organic acids, polyols, and glycolipids from glucose to characterize their biodiversity and identify potential novel strains for biocatalysis of these valuable chemicals. Ustilago cynodontis, Ustilago maydis, Ustilago avenae, and Sporisorium exsertum were identified as promising production organisms for itaconate, malate, succinate, and erythritol, respectively. The influence of buffer concentration (pH) on acid production was investigated. Selected strains with best itaconate and malate production were characterized in more detail in bioreactor experiments obtaining total acid concentrations of up to 47 ± 1 g L^-1.

Conclusion: The identification and detailed characterization of these producers of valuable chemicals highlights the potential of these unicellular smut fungi for industrial applications and is a further step towards the biotechnological utilization of Ustilaginaceae.

Keywords: Erythritol; Glycolipid; Itaconate; Malate; Organic acid; Polyole; Succinate; Ustilaginaceae; Ustilago maydis.

Figure 1

Overall biodiversity of different Ustilaginaceae. Concentration of itaconate (A), malate (B) and succinate (C) produced by different Ustilaginaceae were measured after 96 h (itaconate, succinate) or 48 h (malate) cultivation in CaCO₃ buffered screening medium. Black bars are U. maydis strains, white bars other Ustilaginaceae. The values are the arithmetic mean of two biological determinations. Error bars indicate deviation from the mean. Strain numbers are available in Additional file 1.

Figure 2

The influence of the buffer concentration on the acid production. pH (▽) and concentration of itaconate (■), malate (▲) and erythritol (◇) produced by A: U. maydis 2229 (solid lines) and B: U. cynodontis 2217 (dashed lines) after 120 h and C: U. maydis 2162 (dotted lines) after 72 h cultivation in screening medium containing different MES concentrations. The values are the arithmetic mean of two biological determinations. Error bars indicate deviation from the mean.

Figure 3

Light microscopy images of U. cynodontis 2217 (A, C) and U. maydis 2162 (B, D) cells. A and B: controlled batch fermentation in a bioreactor containing batch medium (200 g L⁻¹ glucose, 4 g L⁻¹ NH₄Cl, 30°C, 80% DOT, at pH 6.0). C and D: System Duetz cultivations containing screening medium (45.5 g L⁻¹ glucose, 0.8 g L⁻¹ NH₄Cl, 33 g L⁻¹ CaCO₃, 30°C, 80% DOT, magnification 400×).

Figure 4

Controlled batch fermentation of the best itaconate and malate producers. A: OD₆₀₀ (Δ), emitted CO₂ amount (without symbols), concentration of glucose (●) and ammonium (□) and B: concentration of itaconate (■), malate (▲) and succinate (▼) during fermentation in a bioreactor containing batch medium (200 g L⁻¹ glucose, 4 g L⁻¹ NH₄Cl, 30°C, 80% DOT, at pH 6.0) with U. maydis 2229 (solid lines) and U. maydis 2162 (dotted lines). The values are the arithmetic mean of two biological determinations. Error bars indicate deviation from the mean. For the CO₂ values, all deviations from the means were under 10%.