Ethanol and Acetate Production from Synthesis Gas using Microbial Catalysts
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Date
2007-03-08
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Abstract
The feasibility of inducing a resting stage in cultures of Clostridium ljungdahlii and Clostridium autoethanogenum was evaluated. Additionally, the effects of medium pH and benzyl viologen, a electron mediator, were tested separately for resting C. ljungdahlii cultures. Cells were grown on nitrogen-rich basal media, a modified Reinforced Clostridial Media and DSMZ 640 for C. ljungdahlii and C. autoethanogenum respectively, and then transferred to nitrogen-limited media to induce the resting stage. The performance of the resting cultures was evaluated by product formation (ethanol and acetate), culture stability, and cell viability. It was found that C. ljungdahlii was able to be induced into a stable non-growing stage with basal medium supplemented with vitamins and trace elements and devoid of the major nitrogen sources: yeast extract, beef extract, and proteose peptone, RCM.NA.SVE. However, there was no significant product formation in the resting cultures of C. ljungdahlii. Three pH levels, 6.8 (the control), 5.5, and 4.5, were tested for affects on C. ljungdahlii resting cell performance. The medium pH of resting cultures in NG.RCM.NA.SVE did not improve product formation of non-growing C. ljungdahlii cultures. The more acidic pH of the non-growing media reduced the culture viability from 100 % at pH 6.8 to 44.4 and 11.1 % at pH 5.5 and 4.5 respectively. Three levels of benzyl viologen concentration were tested for affects on C. ljungdahlii resting cell performance: 0, 50, and 100 part per million. The addition of benzyl viologen did not enhance the cultures' product formation capabilities. The benzyl viologen in the resting cell media also negatively affected the culture viability leaving only 33.3 and 55.5 % of the cultures viable with 100 and 50 ppm benzyl viologen respectively. C. autoethanogenum cultures were not able to be induced into stable non-growing cultures. With various nitrogen-source limitations from the basal medium cultures experienced either significant increases or decreases in culture density over time. The ethanol product selectivity in both non-growing and growing cultures of C. autoethanogenum in nitrogen-limited media was improved from the growing C. autoethanogenum in the basal medium. The ethanol to acetate production ratio was improved to 1:4.5 in two different nitrogen-limited media (one without yeast extract or trypticase peptone in which the culture density decreased, and only devoid of yeast extract in which the culture grew) from 1:7 in the basal growth medium. All cultures of C. autoethanogenum remained viable after transfer from the growth media to the non-growth media. From these studies it appears that ethanol is a primary metabolite for both C. ljungdahlii and C. autoethanogenum on sugar substrates.
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ethanol, Synthesis gas fermentation, Clostridium ljungdahlii, acetate, Clostridium autoethanogenum
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Degree
MS
Discipline
Biological and Agricultural Engineering
