Impact of carbon monoxide partial pressures on methanogenesis and medium chain fatty acids production during ethanol fermentation

Abstract

Medium-chain fatty acids (MCFA) are important biofuel precursors. Carbon monoxide (CO) is a sustainable electron and carbon donor for fatty acid elongation, since it is metabolized to MCFA precursors, it is toxic to most methanogens, and it is a waste product generated in the gasification of waste biomass. The main objective of this work was to determine if the inhibition of methanogenesis through the continuous addition of CO would lead to increased acetate or MCFA production during fermentation of ethanol. The effects of CO partial pressures (P ; 0.08-0.3 atm) on methanogenesis, fatty acids production, and the associated microbial communities were studied in batch cultures fed with CO and ethanol. Methanogenesis was partially inhibited at P ≥ 0.11 atm. This inhibition led to increased acetate production during the first phase of fermentation (0-19 days). However, a second addition of ethanol (day 19) triggered MCFA production only at P ≥ 0.11 atm, which probably occurred through the elongation of acetate with CO-derived ethanol and H :CO . Accordingly, during the second phase of fermentation (days 20-36), the distribution of electrons to acetate decreased at higher P , while electrons channeled to MCFA increased. Most probably, Acetobacterium, Clostridium, Pleomorphomonas, Oscillospira, and Blautia metabolized CO to H :CO , ethanol and/or fatty acids, while Peptostreptococcaceae, Lachnospiraceae, and other Clostridiales utilized these metabolites, along with the provided ethanol, for MCFA production. These results are important for biotechnological systems where fatty acids production are preferred over methanogenesis, such as in chain elongation systems and microbial fuel cells.

The occurrence and ecology of microbial chain elongation of carboxylates in soils

“…Genomic analyses from human gut samples suggest Oscillospira has a fermentative metabolism and produces butyrate, likely utilizing glucuronate, a sugar generated by the human host or obtained from diet containing animal products [ 67 ]. Sequences of Oscillibacter and/or Oscillospira at high relative abundance, even greater than 20% of the total sequences, have been reported in chain-elongating bioreactors converting acetate and ethanol to butyrate, hexanoate, and octanoate at pH 5.2 [ 49 , 55 ], in butyrate- and hexanoate-producing bioreactors fed with synthesis gas [ 68 ], or in acetate and ethanol-fed and CO-fed batch cultures [ 31 , 34 , 56 ]. Nonetheless, the metabolism of these microorganisms under chain-elongating conditions remains unknown.…”

“…The concentrations of C2–C8 carboxylates, ethanol, butanol, and hexanol in microcosm liquid were analyzed using a high-performance liquid chromatograph (HPLC, Shimadzu LC-20AT) with a photodiode array detector at 210 nm and a refractive index detector [ 31 – 33 ]. The HPLC was equipped with an Aminex HPX-87H column (Bio-Rad, Hercules, CA, USA).…”

One-step production of C6–C8 carboxylates by mixed culture solely grown on CO

“…However, pure cultures are inclined to be sensitive to environmental disturbance and susceptible to contamination, and hence, are not perfect for industrial application with regard to waste valorization [ 12 , 13 ]. Esquivel-Elizondo et al [ 14 ]. gained MCCAs production from combination of ethanol and CO, where CO was used to inhibit the methanogenesis and was regarded to contribute as potential electron donor at partial pressure between 0.11 and 0.3 atm.…”

Recirculation of H2, CO2, and ethylene improves carbon fixation and carboxylate yields in anaerobic fermentation

“…In this direction, operation with CO or syngas mixtures may help improve gas consumption, chain elongation, solventogenesis, and volumetric rates 43 . Besides, the presence of CO can help inhibit methanogens 44 .…”