Acetate accumulation enhances mixed culture fermentation of biomass to lactic acid
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Lactic acid is a high-in-demand chemical, which can be produced through fermentation of lignocellulosic feedstock. However, fermentation of complex substrate produces a mixture of products at efficiencies too low to justify a production process. We hypothesized that the background acetic acid concentration plays a critical role in lactic acid yield; therefore, its retention via selective extraction of lactic acid or its addition would improve overall lactic acid production and eliminate net production of acetic acid. To test this hypothesis, we added 10 g/L of acetate to fermentation broth to investigate its effect on products composition and concentration and bacterial community evolution using several substrate-inoculum combinations. With rumen fluid inoculum, lactate concentrations increased by 80 ± 12 % (cornstarch, p < 0.05) and 16.7 ± 0.4 % (extruded grass, p < 0.05) while with pure culture inoculum (Lactobacillus delbrueckii and genetically modified (GM) Escherichia coli), a 4 to 23 % increase was observed. Using rumen fluid inoculum, the bacterial community was enriched within 8 days to >69 % lactic acid bacteria (LAB), predominantly Lactobacillaceae. Higher acetate concentration promoted a more diverse LAB population, especially on non-inoculated bottles. In subsequent tests, acetate was added in a semi-continuous percolation system with grass as substrate. These tests confirmed our findings producing lactate at concentrations 26 ± 5 % (p < 0.05) higher than the control reactor over 20 days operation. Overall, our work shows that recirculating acetate has the potential to boost lactic acid production from waste biomass to levels more attractive for application.
KeywordsLactic acid Acetate Lignocellulosic biomass Mixed culture
The authors would like to thank Charlotte Melis and Professor Veerle Fievez (ILVO, UGent) for providing the rumen fluid; Marjan de Mey (UGent) for providing the E. coli strain (3KO: E. coli K12 MG1655 δ (ackA-pta) δ (poxB); Lien Saey (InBio, UGent) for the analysis using liquid chromatography; Tim Lacoere for the figure illustration; Amanda Luther for proofreading of the manuscript; Sunil Patil, Emma Hernandez Sanabria and Chiara Ilgrande (LabMET, UGent) for their support; the Department of Bioengineering, Ghent University and the Department of Industrial Biological Science, Ghent University Campus Kortrijk.
Compliance with ethical standards
This article does not contain any studies with human participants or animals performed by any of the authors.
Conflict of interest
The authors declare that they have no competing interests.
This work was funded by Special Research Fund (BOF, project number: DEF13/AOF/010) of the University of Ghent (Belgium). The funder has no role in study design, data collection and interpretation or the decision to submit the work for publication. MC was supported by Ghent University Multidisciplinary Research Partnership (MRP) – Biotechnology for a sustainable economy (01 MRA 510W). KR and HR are supported by the European Research Council Starter Grant ELECTROTALK.
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