Monitoring corn stover processing by the fungus Ustilago maydis

Abstract Background A key aspect of sustainable bioeconomy is the recirculation of renewable, agricultural waste streams as substrates for microbial production of high-value compounds. One interesting approach is the bioconversion of corn stover, an abundant maize crop byproduct using the fungus Ustilago maydis. U. maydis is already used as a unicellular biocatalyst in the production of several industrially-relevant compounds. As a natural maize pathogen, the fungus is adapted to feed on nutrients derived from maize biomass. Here, we describe a small-scale bioreactor platform to investigate U. maydis processing of corn stover, combining online monitoring of fungal growth and metabolic activity profiles with biochemical analyses of the post-fermentation residue. Results By establishing and applying the online measuremet of important process parameters including scattered light, Gfp fluorescence, oxygen transfer rate and pH together with post-fermentation residue analysis we demonstrated that U. maydis utilizes specific carbohydrate sources within corn stover. More specifically, our findings reveal that it primarily grows by metabolizing soluble sugars, with only limited exploitation of the abundant lignocellulosics. The use of a lignin-deficient maize mutant, as well as the partial digestion of the lignocellulosic biomass with a commercial enzyme mixture, result in significant fungal performance enhancement, up to + 120%, likely as a result of the increased availability of specific lignocellulosic components. Furthermore, successful application of the established screening platform for the performance assessment of different engineered fungal strains was shown. Conclusions U. maydis can metabolize corn stover material as sole carbon source and the developed microtiter plate bioreactor platform enables quantification of the growth performance, as well as identification of the carbohydrate sources digested by the fungus. It serves as a first step for the process scale-up towards production of sustainable compounds from corn stover by U. maydis, and as a foundation for tailored carbohydrate active enzyme engineering in this fungus.