Co-fermentation of crotonate and chain elongation substrates towards mixed bioplastic/organic waste recovery

Biobased biodegradable plastics, such as polyhydroxyalkanoates (PHA) and polylactic acids (PLA), are increasingly considered as alternatives to fossil-based plastics. While these materials are primarily promoted for biodegradation in industrial composting or anaerobic digestion facilities, their recycling within more closed material loops remains limited. The carboxylate platform is emerging as a promising approach for valorizing organic waste streams, including the microbial recycling of biodegradable plastics. This study explored fermentation conditions to stimulate open-culture bioprocesses using representative biodegradable plastic monomers and typical organic residual substrates processed within the carboxylate platform. 17 batch experiments were conducted with model substrates, including crotonate, ethanol, lactate, and short-chain monocarboxylates. After 28 days of incubation, single crotonate (100 mM) fermentation primarily yielded 105 ± 4 mM acetate and 41 ± 3 mM n -butyrate. Adding 100 mM short-chain monocarboxylates did not inhibit crotonate conversion at an initial pH of 7.0 but extended conversion time. The co-fermentation of 88 ± 10 mM crotonate, 178 ± 20 mM ethanol, and net supplied 39 ± 8 mM acetate under neutral pH conditions produced the highest yield of n -butyrate at 188 ± 17 mM (i.e., 16.5 ± 1.5 g/L), followed by 11 ± 5 mM n -caproate. Co-fermentation of crotonate, ethanol, and lactate was feasible at an initial pH of 5.5, predominantly yielding acetate and n -butyrate. The thermodynamic analysis further supported the bioenergetic feasibility of crotonate conversion and its co-fermentation with ethanol and lactate and showed ample possibilities to improve the fermentation processes. These findings underscore the potential of open-culture fermentation for converting mixed bioplastic and organic waste into carboxylates. • The first study on open-culture crotonate fermentation identified neutral pH as favorable. • Crotonate fermentation and ethanol-based chain elongation co-occurred at neutral pH. • Lactate-based chain elongation induced crotonate conversion at mild acidic conditions (5.5 < pH < 6.0). • Microbial recycling of biobased biodegradable plastics via the carboxylate platform is promising.