A novel bioreactor to study the dynamics of co-culture systems

• A new, in-house developed membrane-separated co-culture bioreactor.
• A dual continuous/pseudo-continuous operation mode enabled by an in-house developed cell-retention module.
• The bioreactor has the capability of independent control of different oxygen condition for each individual strain.
• The bioreactor can easily track and control biomass development for each individual strain.
• The bioreactor can maintain chemostat for various stable OUR conditions.

Co-culture strategy has drawn increasing interest in the last a few years to ferment the mixture of glucose and xylose or lignocellulosic hydrolysate. However, existing research has been mostly qualitative which mainly examined the ethanol production performance of the co-culture system such as final ethanol yield and productivity, with little or no attempt made to understand the dynamic interactions between the two microbes. This is partially due to the difficulties associated with monitoring and control of co-culture systems. In this work, we developed a bioreactor and associated protocols and control strategies to facilitate quantitative and systematic study of co-culture systems. In particular, the reported equipment, operation protocols and control strategies can deliver chemostat operation under controlled stable operation conditions by achieving stable oxygen utilization rates at various levels. In addition, the developed membrane-separated co-culture bioreactor enables independent control of the dissolved oxygen (DO) levels in each chamber, and easy tracking of individual biomass of each strain. The new protocol is a dual continuous/pseudo-continuous operation mode, which allows the control of constant biomass by adjusting the operation time of each mode, dilution rate and feed concentration. Experimental results on the co-culture system of Saccharomyces cerevisiae and Scheffersomyces stipitis are provided to demonstrate the capabilities of the developed co-culture bioreactor. With the developed equipment and protocol, simultaneous and complete utilization of both glucose and xylose was achieved around 70 h into the experiments we conducted, and was maintained as long as 800 h. Such complete utilization can be maintained even longer if desired. In addition, different OUR conditions (ranging 0.0036–0.0045 mmol O2/gDCW/h) were tested under controlled chemostat. Under the different operation conditions tested, ethanol yields and conversion rates varied in the range of 0.12–0.44 g/g and 0.22–1.95 g EtOH/L/h, respectively, which are in line with results reported in the literature.