Biotransformation of Carboxylic Acids to Alcohols: Characterization of Thermoanaerobacter Strain AK152 and 1-Propanol Production via Propionate Reduction

Abstract

Thermoanaerobacter strains have recently gained interest because of their ability to convert short chain fatty acids to alcohols using actively growing cells. Thermoanaerobacter thermohydrosulfuricus strain AK152 was physiologically investigated for its ethanol and other alcohol formation. The temperature and pH optimum of the strain was 70 °C and pH 7.0 and the strain degraded a variety of compounds present in lignocellulosic biomass like monosaccharides, disaccharides, and starch. The strain is highly ethanologenic, producing up to 86% of the theoretical ethanol yield form hexoses. Strain AK152 was inhibited by relatively low initial substrate (30 mM) concentration, leading to inefficient degradation of glucose and levelling up of all end-product formation. The present study shows that the strain produces alcohols from most of the tested carboxylic acids, with the highest yields for propionate conversion to propanol (40.7%) with kinetic studies demonstrating that the maximum conversion happens within the first 48 h of fermentation. Various physiological tests were performed to maximize the acid conversion to the alcohol which reveals that the optimum pH for propionate conversion is pH 6.7 which affords a 57.3% conversion. Kinetic studies reveal that propionate conversion is rapid, achieving a maximum conversion within the first 48 h of fermentation. Finally, by using 13C NMR, it was shown that the addition of propionate indeed converted to propanol.

Thermoanaerobacter Species: The Promising Candidates for Lig-nocellulosic Biofuel Production

“…strain X514, T. brockii ssp. Finnii and T. pseudethanolicus [104][105][106]. AdhE is the bifunctional enzyme including alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) activities, the deletion of which would reduce the ethanol yield by ~95% [107].…”

Biotechnological prospects of Thermoanaerobacter AK15; end-product formation from sugars, amino acids, lignocellulosic and macroalgae hydrolysates

“…Recent investigations have shown that bacteria within the genera of Thermoanaerobacter and Caldanaerobacter can dispose their electrons produced during glucose (and other sugars) oxidations not only to pyruvate to produce ethanol or lactate but may also use other electron acceptors like fatty acids which are converted to their corresponding alcohols [47][48][49]. This was tested for strain AK15 by cultivating the strain on glucose only and on glucose in the presence of butyrate.…”

“…No lactate and hydrogen were observed as end products at end of fermentation in the presence of butyrate This was also investigated for other volatile fatty acids, like propionate, branched chain fatty acids and pentanol, with similar results of the conversion of the fatty acid to their corresponding alcohol (results not shown). Thermoanerobacter pseudethanolicus has recently been shown to convert fatty acids to alcohols, during sugar degradation [25,48] as well as Thermoanerobacter thermohydrosulfuricus, strain AK152 [49]. Production of high carbon alcohols from complex biomass by adding cheap volatile fatty acids to the fermentation broth of the hydrolysates is indeed a new way of biofuel production that may be of great importance in the near future.…”

Influence of Inhibitory Compounds on Biofuel Production from Oxalate-Rich Rhubarb Leaf Hydrolysates Using Thermoanaerobacter thermohydrosulfuricus Strain AK91

“…Increased propionate concentrations resulted in increased formation of propanol in all cases. Recently, our research group has shown the capacity of Thermoanaerobacter species to convert fatty acids to their corresponding alcohols [20,27,28] under specific conditions. Instead of dispersing reducing equivalents to pyruvate and produce only ethanol (or lactate), these bacteria used the electrons produced to reduce fatty acids to alcohols.…”