Effects of salinity regulation strategies on the enrichment of polyhydroxyalkanoate (PHA) producing mixed cultures: Microbial community succession and metabolic mechanisms

The environmental impact of petroleum-based plastics has spurred interest in biodegradable alternatives like polyhydroxyalkanoate (PHA). Recycling PHA from saline organic wastewater through mixed culture (MC) processes represents a sustainable waste-to-resource approach. Although it is possible to enrich PHA producers by exploiting characteristics of high salinity that inhibit non-PHA producers, high salinity also inhibits PHA synthesis. Effectively enriching PHA producers under high-salinity conditions without compromising PHA synthesis remains a critical challenge. This study investigated the effects of two salinity regulation strategies, gradient salt addition and direct high salt application, on the enrichment stage of PHA producing MCs. The results show that gradient salinity increase fails to effectively select high-salinity tolerant PHA producers whereas direct high-salinity application strategy proves to be more effective. The enhancement mechanism of PHA synthesis under direct high-salinity application strategy is attributed to increased secretion of electron transfer- related substances in extracellular polymeric substances, along with improved microbial antioxidant capacity, ATP synthesis, electron transfer and quorum sensing, and sustained selective pressure promotes an increased relative abundance of PHA producers. The PHA producing MC enrichment strategy of direct high-salinity application emerges as a superior approach for resource recovery and PHA synthesis in high-salinity organic wastewater, offering a scalable pathway to enhance PHA production, utilize saline waste resources, and mitigate plastic pollution.