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.