Enrichment performance and salt tolerance of polyhydroxyalkanoates (PHAs) producing mixed cultures under different saline environments

The key to the resource recycling of saline wastes in form of polyhydroxyalkanoates (PHA) is to enrich mixed cultures with salt tolerance and PHA synthesis ability. However, the comparison of saline sludge from different sources and the salt tolerance mechanisms of salt-tolerant PHA producers need to be clarified. In this study, three kinds of activated sludge from different salinity environments were selected as the inoculum to enrich salt-tolerant PHA producers under aerobic dynamic feeding (ADF) mode with butyric acid dominated mixed volatile fatty acid as the substrate. The maximum PHA content (PHAm) reached 0.62 ± 0.01, 0.62 ± 0.02, and 0.55 ± 0.03 g PHA/g VSS at salinity of 0.5%, 0.8%, and 1.8%, respectively. Microbial community analysis indicated that Thauera, Paracoccus, and Prosthecobacter were dominant salt-tolerant PHA producers at low salinity, Thauera, NS9_marine, and SM1A02 were dominant salt-tolerant PHA producers at high salinity. High salinity and ADF mode had synergistic effects on selection and enrichment of salt-tolerant PHA producers. Combined correlation network with redundancy analysis indicated that trehalose synthesis genes and betaine related genes had positive correlation with PHAm, while extracellular polymeric substances (EPS) content had negative correlation with PHAm. The compatible solutes accumulation and EPS secretion were the main salt tolerance mechanisms of the PHA producers. Therefore, adding compatible solutes is an effective strategy to improve PHA synthesis in saline environment.