Anaerobic　microalgae　fermentation,　leveraging　its　cost-effectiveness　and　the　adaptability　of　mixed　cultures,　holds　promise　for　carboxylate　biosynthesis.　Microalgae,　with　their　abundant　carbohydrates　and　proteins,　stand　out　as　an　optimal　substrate　for　this　process　among　various　options.　Furthermore,　microalgae　fermentation　not　only　shows　the　potential　to　mitigate　risks　associated　with　algae　blooms　but　also　aligns　with　the　need　for　sustainable　practices.　However,　the　limited　utilization　rate　of　microalgae　in　anaerobic　fermentation　poses　challenges　to　achieving　high　production　rates　of　desired　products.　In　this　study,　we　implemented　a　bioaugmentation　process　with　yeast　to　enhance　carboxylate　production　performance　of　microalgae　fermentation.　The　results　demonstrated　a　fourfold　increase　in　carboxylate　yield　with　the　addition　of　yeast.　In-situ　ethanol　production　facilitated　the　conversion　of　short-chain　carboxylic　acids　into　medium-chain　carboxylates,　achieving　a　yield　of　46.3　mM-C/g　VS.　The　presence　of　yeast　significantly　enhanced　substrate　utilization　from　20　%　to　80　%,　steering　the　metabolic　pathway　towards　chain　elongation.　Metagenomic　analysis　further　revealed　metabolic　shifts　following　yeast　addition,　particularly　an　increased　abundance　of　genes　involved　in　acetyl-CoA　production.　Notably,　the　aldehyde:ferredoxin　oxidoreductase　(AOR)　pathway　emerged　as　a　key　driver　in　butanol　production.　These　findings　highlight　the　improved　performance　of　anaerobic　microalgae　fermentation　with　yeast,　enabling　efficient　production　of　higher　value　bioproducts　while　eliminating　the　need　for　external　electron　donors.　©　2024　The　Author(s)