Evaluating Baijiu Huangshui-Derived Caproate as a C6 Feedstock for Bioconversion to 6-Hydroxyhexanoic Acid and PHHx-Rich mcl-PHA

Abstract Organic waste streams from food processing and fermentation industries contain abundant fermentable carbon. Microbial chain elongation (CE) converts this carbon into medium-chain carboxylates like caproate. However, chain-elongation studies typically end with caproate recovery, while the further conversion of this waste-derived C6 carbon stream into defined downstream products has received limited attention. In this study, a caproate-rich broth─generated via CE from Baijiu brewing wastewater─served as a complex, waste-derived matrix. We used this matrix to explore the upgrading of caproate into C6 hydroxy acids and medium-chain-length polyhydroxyalkanoates (mcl-PHA). Driven by a defined AlkBGT ω-oxidation module, caproate was successfully converted to 6-hydroxyhexanoic acid (6-HHA), reaching a final titer of 250.7 mg/L. In parallel, an inoculation-dependent 3-hydroxyhexanoate (PHHx)-rich mcl-PHA phenotype was observed under the tested mixed-broth conditions following Paracoccus versutus inoculation. In the 5 L fermentation system, mean PHHx fractions ranged from 90.51% to 93.47% of total PHA, while maximum PHA accumulation reached 17.06 ± 1.41% of cell dry weight. 16S rRNA amplicon sequencing revealed community succession and the persistence of Paracoccus in the mixed system, while metatranscriptomics further detected PHA-related transcriptional signatures associated with the inoculation-dependent PHHx-rich PHA phenotype. Overall, this study establishes proof-of-feasibility for extending waste-derived caproate beyond terminal recovery as a shared C6 feedstock for two distinct bioconversion routes to 6-HHA and PHHx-rich mcl-PHA, while defining clear targets for improving product titer, selectivity, polymer accumulation, and process robustness.