Acetic Acid Production by Clostridium thermoaceticum in pH-Controlled Batch Fermentations at Acidic pH

Abstract

Four strains of the homofermentative, obligately anaerobic thermophile Clostridium thermoaceticum were compared in pH-controlled batch fermentation for their tolerance to acetic acid, efficiency of converting glucose to acetic acid and cell mass, and growth rate. At pH 6 (and pH 7) and initial acetic acid concentrations of less than 10 g/liter, the four strains had mass doubling times of 5 to 7 h and conversion efficiencies to acetic acid and cell mass of about 90% (70 to 110%) and 10%, respectively. At pH 6 and initial acetic acid concentrations of greater than 10 g/liter, only two of the strains grew, the mass doubling time increased to 18 h, and the conversion efficiencies to acetic acid and cell mass remained unchanged. Both of these strains had been selected for their ability to grow in the presence of acetate at neutral pH. The highest acetic acid concentrations reached were about 15 and 20 g/liter at pH 6 and 7, respectively. C. thermoaceticum is apparently more sensitive to free acetic acid than to either acetate ion or pH. It was also shown that, at pH 6 and 7, the redox potential must be at least as low as −300 and −360 mV, respectively, for growth to occur.

Uncoupling by Acetic Acid Limits Growth of and Acetogenesis by Clostridium thermoaceticum

“…Acid and solvent-producing clostridia, which may offer economically attractive sources for chemicals currently derived from fossil fuels, have elicited renewed interest recently (32,34). One focus of current research is the physiological factors that limit synthesis of the end product (10).…”

“…The anaerobic thermophile Clostridiium thermoaceticum produces large amounts of acetic acid by glucose homofermentation (7). Since organisms, under pH control conditions, grow faster and produce more acetic acid near neutral pH than in more acid media (32,33), they can be classified as neutrophiles (28). However, in batch culture without pH control, the pH of the medium decreases as the cells grow and ferment the sugar, until growth and then acetic acid production cease at about pH 5.…”

“…However, in batch culture without pH control, the pH of the medium decreases as the cells grow and ferment the sugar, until growth and then acetic acid production cease at about pH 5. Several authors have concluded that it is not the low external pH per se that limits growth, but that the limiting factor is the acetic acid produced (32,33). The undissociated acetic acid is considered more inhibitory than the acetate anion (32, 33), but how the end product exerts its effect is not understood.…”

Dissimilation of Carbon Monoxide to Acetic Acid by Glucose-Limited Cultures of Clostridium thermoaceticum

“…When the initial cultivation gas phase was 100% CO2 (culture A), 2.1 mol of acetic acid was produced per mol of glucose consumed. This was less than the theoretical maximum of 3, which has recently been approached in pH-controlled batch cultures (39,42), and also lower than the 2.3 to 2.7 typically observed in standard batch culture (12). Indeed, PM fermentations yielded stoichiometries approximating 2.5 (data not shown).…”

“…Clostridium thermoaceticum (12) is a homoacetate-fermenting bacterium which has been extensively studied for its unique physiological properties and potential applications (23,25,30,38,39,42,44,46,47). Historically, industrial interest in this acetogen stems from its ability to convert 1 mol of glucose in near stoichiometry to 3 mol of acetic acid.…”