Abstract
Chain elongation fermentation allows for the synthesis of biobased chemicals from complex organic residue streams. To expand the product spectrum of chain elongation technology and its application range we investigated 1) how to increase selectivity towards branched chain elongation and 2) whether alternative branched carboxylates such as branched valerates can be used as electron acceptors. Elongation of isobutyrate elongation towards 4-methyl-pentanoate was achieved with a selectivity of 27% (of total products, based on carbon atoms) in a continuous system that operated under CO2 and acetate limited conditions. Increasing the CO2 load led to more in situ acetate formation that increased overall chain elongation rate but decreased the selectivity of branched chain elongation. A part of this acetate formation was related to direct ethanol oxidation that seemed to be thermodynamically coupled to hydrogenotrophic carboxylate reduction to corresponding alcohols. Several alcohols including isobutanol and n-hexanol were formed. The microbiome from the continuous reactor was also able to form small amounts of 5-methyl-hexanoate likely from 3-methyl-butanoate and ethanol as substrate in batch experiments. The highest achieved concentration of isoheptanoate was 6.4 ± 0.9 mM Carbon, or 118 ± 17 mg/L, which contributed for 7% to the total amount of products (based on carbon atoms). The formation of isoheptanoate was dependent on the isoform of branched valerate. With 3-methyl-butanoate as substrate 5-methylhexanoate was formed, whereas a racemic mixture of L/D 2-methyl-butanoate did not lead to an elongated product. When isobutyrate and isovalerate were added simultaneously as substrates there was a large preference for elongation of isobutyrate over isovalerate. Overall, this work showed that chain elongation microbiomes can be further adapted with supplement of branched-electron acceptors towards the formation of iso-caproate and iso-heptanoate as well as that longer chain alcohol formation can be stimulated.
Highlights
Chain elongation fermentation is an emerging bioprocess by which medium chain carboxylates (MCCs) can be produced
Chain elongation fermentations provide a sustainable alternative by utilizing organic residue streams as substrate
The continuous reactor experiment was operated without any acetate in the influent with the intention to maximize i-C4 utilization during chain elongation and to maximize selectivity towards i-C6 formation
Summary
Chain elongation fermentation is an emerging bioprocess by which medium chain carboxylates (MCCs) can be produced. Chain elongation fermentations provide a sustainable alternative by utilizing organic residue streams as substrate. Branched Carboxylate Chain Elongation (Agler et al, 2011; Agler et al, 2012) These fermentations require electron acceptors in the form of short chain carboxylates (e.g., acetate, propionate, butyrate, valerate), that can be obtained after hydrolysis and acidification of organic residues. An electron donor such as ethanol is required to elongate the short chain carboxylates to MCCs (De Groof et al, 2019). Chain elongation fermentation technology could become an impactful recycling method that can aid in the development of a circular economy (De Groof et al, 2019)
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