Methods for increasing the production of ethanol from microbial fermentation

A continuous method for producing ethanol from the anaerobic bacterial fermentation of a gaseous substrate comprising carbon monoxide, the method comprising:

• culturing in a fermentation bioreactor anaerobic, acetogenic C. ljungdahlii bacteria in a continuously fed liquid nutrient medium to provide a fermentation broth and supplying to said fermentation bioreactor said gaseous substrate comprising carbon monoxide (CO);
• maintaining a specific rate of CO uptake in said fermentation bioreactor at an amount of 0.3 to 2 mmol CO/gram dry cells weight of bacteria/minute in the fermentation broth after said bacteria achieve a stable cell concentration in said fermentation bioreactor, wherein said free acetic acid concentration in said fermentation bioreactor is less than 5 g/L free acetic acid,
• producing ethanol in a fermentation broth at a productivity greater than 10 g/L per day;
• and
• producing both ethanol and acetate in said fermentation broth in a ratio of ethanol to acetate ranging from 1:1 to 20:1.