A newly identified Comamonas serinivorans strain for PHA production from sterile-filtered dark-fermentation-derived volatile fatty acids: isolation, metabolic profiling and industrial prospects.

Dark-fermentation for hydrogen production leaves a substantial fraction of substrate carbon unexploited, with a large fraction remaining as volatile fatty acids (VFAs), mainly acetate and butyrate. To improve process valorisation, this study investigates the microbial conversion of fermentation-derived VFAs into polyhydroxyalkanoates (PHAs). A PHA-producing strain was isolated from dairy wastewater sludge using a feast-famine enrichment strategy and identified as Comamonas serinivorans ATH based on 16S rRNA gene sequencing. The strain exhibited strict specialisation towards organic acids, efficiently assimilating VFAs but not carbohydrates. Under nutrient limitation, intracellular PHA accumulation reached up to 93% of cell dry weight. When cultivated in a sterile-filtered dark-fermentation stream, the strain consumed both acetate and butyrate and produced PHA with a yield of 0.54 g/g of consumed VFAs under non-optimised conditions. GC-MS analysis confirmed that Comamonas serinivorans ATH produces a polymer mainly composed of 3-hydroxybutyrate monomers. These findings demonstrate the feasibility of coupling hydrogen-producing dark-fermentation with PHA synthesis, supporting an integrated biorefinery approach for sustainable bioplastic production from waste-derived carbon streams.