3.3 Effects of Butyric Acid
In a typical ABE fermentation, acetic and butyric acids produced in acidogenesis were reassimilated via the CoA transferase, which transfers the CoA from acetoacetyl-CoA to acetate and butyrate to form acetyl-CoA and butyryl-CoA, respectively, with acetone, ethanol, and butanol as the final products (Long and Jones, 1984; Zhao et al., 2013). Previous studies have shown that overexpressing ctf AB encoding the CoA transferase could increase butanol production and the robustness of solventogenic clostridia (Lu et al., 2017; Yu et al., 2015) and butyric acid could upregulate ctf AB (Bahl et al., 1982; Lee SM et al., 2008). In this study, butyric acid was thus supplemented in the feed medium to promote solventogenesis in clostridial fermentation, resulting in ~20% higher butanol yield (0. 24 g/g vs. ~0.20 g/g) compared to without butyric acid addition. The increased butanol yield could also be attributed to reduced cell growth and acetone and ethanol production, which resulted in a much higher ratio of butanol present in the total solvents (~68% vs. ~60% w/w in typical ABE fermentation with C. acetobutylicum (Xu et al., 2015). Similar results were also reported for C. beijerinckii (Lee SM et al., 2008). The addition of butyric acid as a carbon source and precursor for butanol biosynthesis inhibited the conversion of butyryl-CoA to butyric acid and thus increased carbon flow toward butanol production. However, butyric acid at >5 g/L strongly inhibited cell growth and reduced cell viability and productivity in free cell fermentation (see Figs. 2 and 3). Nevertheless, cells immobilized in the FBB were resilient to butyric acid toxicity and could tolerate butyric acid at ~5 g/L, although butyric acid at 8 g/L completely halted cell metabolism in the FBB (Fig. S2).