Large-scale microbial industrial fermentations have significantly higher absolute pressure and dissolved CO2 concentrations than otherwise comparable laboratory-scale processes. Yet the effect of increased dissolved CO2 (dCO2) levels is rarely addressed in the literature. In the current work, we have investigated the impact of industrial levels of dCO2 (measured as the partial pressure of CO2, pCO2) in an Escherichia coli-based fed-batch process producing the human milk oligosaccharide 2'-fucosyllactose (2'-FL). The study evaluated the effect of high pCO2 levels in both carbon-limited (C-limited) and carbon/nitrogen-limited (C/N-limited) fed-batch processes. High-cell density cultures were sparged with 10%, 15%, 20%, or 30% CO2 in the inlet air to cover and exceed the levels observed in the industrial scale process. While the 10% enrichment was estimated to achieve similar or higher pCO2 levels as the large-scale fermentation it did not impact the performance of the process. The product and biomass yields started being affected above 15% CO2 enrichment, while 30% impaired the cultures completely. Quantitative proteomics analysis of the C-limited process showed that 15% CO2 enrichment affected the culture on the protein level, but to a much smaller degree than expected. A more significant impact was seen in the dual C/N limited process, which likely stemmed from the effect pCO2 had on nitrogen availability. The results demonstrated that microbial cultures can be seriously affected by elevated CO2 levels, albeit at higher levels than expected.
Keywords: 2′-focussyllatose; Escherichia coli; HMO; dissolved carbon dioxide; fermentation; large-scale fermentation; physiology; proteome; scale-down.