Building a Synthetic Microaerobic Co-Culture for Upgrading C1 Substrates Using a High Throughput Bioreactor Platform.

Many important microbial consortia, such as the Human gut microbiota, consist of both aerobes and anaerobes that experience fluctuating levels of oxygen, yet little is known about how oxygen availability impacts community dynamics and metabolic interactions. We have developed a synthetic aerobe-anaerobe co-culture of E. coli and Clostridium ljungdahlii, in order to study these novel interactions. In this system, E. coli maintains the environment at steady low-oxygen conditions that support growth of C. ljungdahlii, while respiring on the fermentation byproducts of C. ljungdahlii. We have successfully established and analyzed the co-culture heterotrophically in both batch reactors and chemostats, and recently have shown that C. ljungdahlii can grow autotrophically under these conditions, despite using the highly oxygen-sensitive Wood-Ljungdahl pathway for CO2 fixation. To scale these experiments, we have developed low-cost parallel mini-bioreactor platform in which gas mixes with different oxygen concentrations can be sparged to individual vials, and are now using this to understand the impact of varying environmental conditions on each microbe. Beyond understanding community dynamics, computational modeling suggests microaerobic co-culture of a mixed aerobic/anaerobic community could be broadly useful in metabolic engineering, enabling the coupling of high yield associated with anaerobic metabolism with the productivity of aerobic metabolism in a single reactor through cross-feeding. We present recent progress toward engineering our co-culture for continuous carbon-negative production of high-value chemicals from CO2/H2 to exemplify this concept. This work provides fundamental insight into the role of oxygen in microbial consortia, as well as a platform for analyzing impact of atmospheric composition.