초록
<P>Past studies on chain elongation by reactor microbiomes have shown promising productivities and specificities of<I>n</I>-caproate, which is a six-carbon (C6) medium-chain carboxylate (MCC). The production of small quantities of<I>n</I>-caprylate (C8) was also reported, but merely as a by-product of<I>n</I>-caproate production. Since<I>n</I>-caprylate is considerably more valuable than<I>n</I>-caproate by weight, the question was raised whether<I>n</I>-caprylate could be produced at a high specificity (product<I>vs.</I>all other carboxylate products). Here, we optimized the operating conditions of an anaerobic upflow bioreactor during a period of 186 days to accomplish this goal. We achieved an<I>n</I>-caprylate productivity of 19.4 g chemical oxygen demand (COD) L<SUP>−1</SUP>d<SUP>−1</SUP>(0.33 g L<SUP>−1</SUP>h<SUP>−1</SUP>), which is the highest ever reported. The product ratio of<I>n</I>-caprylate to<I>n</I>-caproate was 11 g COD per g COD, which was also greater than past studies. This ratio was even 25 g COD per g COD during an earlier operating period with a lower productivity, resulting in an<I>n</I>-caprylate specificity of 96% (based on COD). We accomplished this high<I>n</I>-caprylate productivity and specificity by: (1) feeding a substrate with a high substrate ratio of ethanol to acetate; (2) extracting the product from the bioreactor broth; and (3) acclimating an efficient chain-elongating microbiome. Because syngas-fermentation effluent contains a high ratio of ethanol to acetate, we propose coupling syngas fermentation with chain elongation to produce<I>n</I>-caprylate and to increase product value.</P>