Traits of selected Clostridium strains for syngas fermentation to ethanol

Syngas fermentation is an anaerobic bioprocess that could become industrially relevant as a biorefinery platform for sustainable production of fuels and chemicals. An important prerequisite for commercialization is adequate performance of the biocatalyst (i.e., sufficiently high production rate, titer, selectivity, yield, and stability of the fermentation). Here, we compared the performance of three potential candidate Clostridium strains in syngas-to-ethanol conversion: Clostridium ljungdahlii PETC, C. ljungdahlii ERI-2, and Clostridium autoethanogenum JA1-1. Experiments were conducted in a two-stage, continuously fed syngas-fermentation system that had been optimized for stable ethanol production. The two C. ljungdahlii strains performed similar to each other but different from C. autoethanogenum. When the pH value was lowered from 5.5 to 4.5 to induce solventogenesis, the cell-specific carbon monoxide and hydrogen consumption (similar rate for all strains at pH 5.5), severely decreased in JA1-1, but hardly in PETC and ERI-2. Ethanol production in strains PETC and ERI-2 remained relatively stable while the rate of acetate production decreased, resulting in a high ethanol/acetate ratio, but lower overall productivities. With JA1-1, lowering the pH severely lowered rates of both ethanol and acetate production; and as a consequence, no pronounced shift to solventogenesis was observed. The highest overall ethanol production rate of 0.301 g · L(-1)  · h(-1) was achieved with PETC at pH 4.5 with a corresponding 19 g/L (1.9% w/v) ethanol concentration and a 5.5:1 ethanol/acetate molar ratio. A comparison of the genes relevant for ethanol metabolism revealed differences between C. ljungdahlii and C. autoethanogenum that, however, did not conclusively explain the different phenotypes.