Steering yeast-bacteria synergy by biochar to achieve enhanced endogenous ethanol-driven chain elongation

Abstract

The recovery of medium-chain carboxylic acids (MCCA) from waste streams through chain elongation (CE) aligns with a circular economy concept. However, the need for electron donor (ethanol) supplements and the challenges of enriching chain-elongating bacteria have limited its application. This study adopted biochar to steer a novel yeast fungi-bacteria mixed microbiome to drive CE during food waste fermentation. The effects of hydrochar and pyrochar at dosages ranging from 5 g/L to 40 g/L on the CE process were compared. The highest MCCA concentrations without electron donor addition peaked at 21.46 ± 0.97 g chemical oxygen demand (COD)/L and 22.51 ± 1.53 g COD/L using 5 g/L of hydrochar and pyrochar. The Pearson correlation coefficient indicated strong linear relationships (R²: 0.95–0.99) between MCCA and ethanol, achieving an ethanol-driven CE process. Fungal yeast belonging to Wickerhamomyce and Saccharomycopsis were enriched using hydrochar (34.78 %) and pyrochar (41.42 %), contributing to endogenous ethanol generation. Key chain-elongating bacteria, including Clostridium_sensu_stricto_12 and Caproiciproducens, were enriched using hydrochar (30.59 %) and pyrochar (8.16 %). Metagenomic analysis revealed that hydrochar and pyrochar addition both up-regulated the genes involved in the CE pathway. In addition, the network topological metrics and mantel test results confirmed that more stable interactions among the microbiome were established under the hydrochar addition compared to pyrochar. Structural integrity analysis suggested an essential role of humic acid in hydrochar on CE. Finally, the roles of functional groups O-C=O in hydrochar and C=C in pyrochar involved in the yeast fungi-bacteria microbiome were proposed. This study provides new insights into MCCA recovery from waste streams.