caprylate reactors. caprylate reactors

Chain-elongation microbiomes can be used to convert ethanol- or lactic-rich organic wastes to medium chain carboxylic acids. This technology is mostly still at the laboratory and startup scale, and the microbiome involved in this process has not been fully characterized. Here, three 5-L bioreactors were fed with ethanol and acetate, operated at a pH of 5.5, and a temperature of 30ºC, and with inline product extraction to promote the formation of the medium chain carboxylates, n-caproate and n-caprylate. Illumina 16S rRNA gene sequencing, shotgun metagenomics, and metaproteomics were used to characterize the bioreactor microbiomes during the operating period. In the bioreactors, despite similar imposed operating conditions, we observed different microbial communities and different n-caprylate productivities. In the bioreactor with the lowest n-caprylate conversion efficiency, relatively high hydrogen partial pressures were observed. Subsequent sparging of nitrogen gas reduced the hydrogen partial pressure in that bioreactor and resulted in a higher n-caprylate conversion efficiency. However, across all the bioreactors a common relationship between hydrogen partial pressures and n-caprylate conversion efficiency was not found. An Oscillospira OTU in the family Ruminococcaceae dominated in the bioreactor with the highest n-caprylate specificities and productivities. Metagenomics analysis revealed genes for enzymes involved in reverse beta oxidation, though the full set of genes for reverse beta oxidation was not found in any of the partial genomes recovered.