Underlying Mechanism of Uncoupled Cell Growth and Ethanol Fermentation of Zymomonas mobilis using Different Nitrogen Sources

Abstract Background: Microbial growth needs C, N, P, S as well as metal ions such as magnesium, which is a major cofactor for enzymes involved in various metabolic activities. Yeast extract is widely used as nitrogen supply as well as vitamins and growth factors to sustain microbial growth in the culture medium. Zymomonas mobilis is a model ethanologenic bacterium for ethanol production, and has been developing as a chassis for diverse biochemical production. Although yeast extract is routinely used to prepare rich medium (RM) for Z. mobilis, the glucose consumption and ethanol production of Z. mobilis in RM were not coupled with cell growth in some studies. Results: In this study, the effects of different nitrogen sources as well as the supplementation of additional nitrogen source into RM and minimum medium (MM) on cell growth and ethanol fermentation of Z. mobilis were investigated to understand the uncoupled cell growth and ethanol fermentation for efficient carbon utilization and optimal ethanol productivity of Z. mobilis. Our results indicated that nitrogen sources such as yeast extract from different companies affected cell growth, glucose utilization, and the corresponding ethanol production. We also quantified the concentrations of major ion elements in different organic nitrogen sources using the quantitative analytic approach of Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), and demonstrated that metal ions such as magnesium in the media affected glucose consumption, cell growth, and ethanol fermentation. The effect of magnesium on gene expression was further investigated using RNA-Seq transcriptomics, and our result indicated that the lack of Mg2+ triggered stress responses while decreasing energy-consuming metabolism. Conclusions: Our work demonstrated that concentrations of metal ions such as magnesium and molybdenum in nitrogen sources are essential for vigorous cell growth, and the difference of Mg2+concentration in different yeast extract was one of the major factors affecting the coupling of cell growth and ethanol fermentation in Z. mobilis. We also revealed that genes responsive for Mg2+ deficiency in the medium were majorly related to stress responses and energy conservation. The importance of metal ions on cell growth and ethanol fermentation suggested that metal ions should become one of the parameters for monitoring the quality of commercial nitrogen sources and optimizing microbial culture medium for economic biochemical production.