Betaine-Enhanced Mixed Culture Production of Polyhydroxyalkanoates from Saline Organic Waste: Overcoming Osmotic Stress for Sustainable Bioplastics

Microbial production of biodegradable polyhydroxyalkanoates (PHAs) by a mixed culture (MC) from waste streams offers a sustainable solution to plastic pollution. A high-salinity environment can not only selectively enrich PHA-producing MCs by inhibiting non-PHA producers but also hinder microbial activity and PHA biosynthesis. This study investigated the effectiveness and underlying mechanisms of betaine addition in mitigating inhibition caused by high salinity without compromising selectivity, thereby enhancing the high-salinity MC PHA production. Betaine addition improved volatile fatty acid-to-PHA conversion efficiency and increased maximum PHA production by over 40% compared with the control (without betaine). It reshaped the microbial community, selectively enriching betaine-dependent, salt-tolerant PHA producers, such as Paracoccus. Metagenomic and metabolic analyses revealed betaine redirected cellular carbon flux toward PHA synthesis, evidenced by upregulated key synthesis genes. Betaine also alleviated osmotic stress by preferential cellular uptake, enhanced antioxidant defense, refined extracellular polymeric substance structure, and increased NADH/NADPH levels, thereby sustaining ATP generation and PHA synthesis. Life cycle assessment demonstrated that betaine-enhanced processes reduced environmental impact by 13–15% compared with the process without betaine. These findings identify betaine as an economical and effective strategy to overcome the inhibitory effects of high salinity in MC PHA production, enabling the conversion of saline organic wastes into valuable biodegradable polymers.