Structural composition and metabolic activities of the microbiome involved in the production of n-caproate from food waste

Recent applications of anaerobic digestion technology are moving towards production of high value-added products, such as carboxylic acids, from renewable resources in order to achieve a sustainable and circular bio-economy by minimizing waste production. Among the possible high value-added chemicals, this work focused on the biological production of medium chain fatty acids (e.g., caproate) from real food waste (FW). The long-term performance of chain elongation process was thoroughly investigated in a single-stage fermentation process, without addition of external electron donor (e.g., ethanol,lactate). Different organic load rates (OLRs) (i.e. 5,15,20 gCOD L-1d-1) were tested by using two different feeding strategies (a feeding cycle of 4 days with stop feeding cycle of 3 days and a discontinuous feeding every 3/4 days) both with a hydraulic retention time of 4 days and pH controlled at 6±0.2. The use of FW extract provided bioavailable sugars essential for primary fermentation, fostering lactate production together with volatile fatty acids useful for the subsequent chain elongation (Fig.1). Caproic acid biological productions was influenced by the applied OLRs, reaching its highest value (5.8gL-1) with a significant COD conversion of 21% at OLR =15gCOD L-1d-1 with discontinuous feeding. A cutting-edge biomolecular approach was used for describing the structural composition and metabolic activities of chain-elongating microbiome. The application of in situ hybridization techniques and high-throughput 16S rRNA gene sequencing revealed a core microbiome composed of lactate-producing Actinomyces, Atopobium, Lactobacillus and Olsenella, and caproate-producing Pseudoramibacter or Caprociproducens. In agreement with process data, the existence of a stable long-term lactate-based chain elongation process was fully supported by metagenomics analysis revealing the presence of key enzymes for the production of lactate, as well as several enzymes for chain elongation involved in the reverse ?-oxidation pathway.