'Dark' CO fixation in succinate fermentations enabled by direct CO delivery via hollow fiber membrane carbonation.

Anaerobic succinate fermentations can achieve high-titer, high-yield performance while fixing CO through the reductive branch of the tricarboxylic acid cycle. To provide the needed CO, conventional media is supplemented with significant (up to 60 g/L) bicarbonate (HCO), and/or carbonate (CO) salts. However, producing these salts from CO and natural ores is thermodynamically unfavorable and, thus, energetically costly, which reduces the overall sustainability of the process. Here, a series of composite hollow fiber membranes (HFMs) were first fabricated, after which comprehensive CO mass transfer measurements were performed under cell-free conditions using a novel, constant-pH method. Lumen pressure and total HFM surface area were found to be linearly correlated with the flux and volumetric rate of CO delivery, respectively. Novel HFM bioreactors were then constructed and used to comprehensively investigate the effects of modulating the CO delivery rate on succinate fermentations by engineered Escherichia coli. Through appropriate tuning of the design and operating conditions, it was ultimately possible to produce up to 64.5 g/L succinate at a glucose yield of 0.68 g/g; performance approaching that of control fermentations with directly added HCO/CO salts and on par with prior studies. HFMs were further found to demonstrate a high potential for repeated reuse. Overall, HFM-based CO delivery represents a viable alternative to the addition of HCO/CO salts to succinate fermentations, and likely other 'dark' CO-fixing fermentations.