Respiration is essential for aerobic growth of Zymomonas mobilis ZM4

Zymomonas mobilis is an alpha-proteobacterium that is a promising platform for industrial scale production of biofuels or valuable products due to its efficient ethanol fermentation and low biomass generation. Z. mobilis has also intriguing physiology, sometimes difficult to explain by the rules and strategies commonly observed in other bacteria. One of the most mysterious features of Z. mobilis is its growth in oxic conditions. Z. mobilis is an aerotolerant bacterium that encodes a complete respiratory electron transport chain but the benefit of respiration for growth in oxic conditions has never been confirmed, despite decades of research. Quite the opposite, growth and ethanol production of WT Z. mobilis is poor in oxic conditions indicating that it does not benefit from oxidative phosphorylation. Additionally, in previous studies, aerobic growth improved significantly when respiratory genes were disrupted (ndh) or acquired point mutations (cydA, cydB) even if respiration was significantly reduced by these changes. Here, we obtained clean deletions of respiratory genes ndh and cydAB, individually and in combination, and showed, for the first time, that deletion of cydAB completely inhibited O2 respiration and dramatically reduced growth in oxic conditions. Both respiration and aerobic growth were restored by expressing a heterologous, water-forming NADH oxidase, noxE. This result shows that the main role of the electron transport chain in Z. mobilis is reducing the toxicity of molecular oxygen, helping to explain why it is beneficial for Z. mobilis to use electron transport chain complexes that contribute little to oxidative phosphorylation. ImportanceA key to producing next generation biofuels is to engineer microbes that efficiently convert non-food materials into drop-in fuels and to engineer microbes effectively we must understand their metabolism thoroughly. Zymomonas mobilis is a bacterium that is a promising candidate biofuel producer but its metabolism remains poorly understood, especially its metabolism when exposed to oxygen. Although Z. mobilis respires with oxygen, its aerobic growth is poor and disruption of genes related to respiration counterintuitively improves aerobic growth. This unusual result has sparked decades of research and debate regarding the function of respiration in Z. mobilis. Here, we used a new set of mutants to determine that respiration is essential for aerobic growth and likely protects the cells from oxidative damage caused by molecular oxygen. These results indicate that respiration has a non-canonical function in Z. mobilis and expand our understanding of the role of respiration in metabolism and oxidative stress.