The extreme halophilic archaeon Haloferax mediterranei can produce polyhydroxyalkanoates (PHAs) and extracellular polymeric substances (EPSs) simultaneously. In this study, we studied the influence of the ratio of carbon and nitrogen concentration (C/N) during feeding on PHA and EPS production in open fermentation by means of the developed model. The results indicated that nitrogen availability had significant effects on stain growth and PHA and EPS synthesis. The PHA volumetric productivity, EPS volumetric productivity and biomass increased with the increase of nitrogen concentration. Haloferax mediterranei synthesized EPSs with a faster rate than that for PHAs, indicating that EPSs were synthesized as priority energy and food reserves by Haloferax mediterranei. The deficiency of nitrogen promoted PHA accumulation. The highest PHA cell content of 47.22% mg PHA per mg CDW was found at a C/N ratio of 35. In contrast, the highest EPS productivity of 733.58 mg L⁻¹ was observed at a C/N ratio of 5. At a C/N ratio of 35, the percentage of the PHA yield coefficient in the sum of the PHA and EPS yield coefficient was 92.8%, indicating that the majority of the carbon was converted to PHA rather than EPS. Supported by the carbon flow and PHA and EPS productivity, PHA was the main product of Haloferax mediterranei despite the C/N ratio. The study proved that controlling the C/N ratio during feeding could be one solution to adjust the carbon flow to PHA and EPS synthesis by Haloferax mediterranei.

Microbial polyhydroxyalkanoate (PHA) production serves as a substitute for petroleum-based plastics. Enriching mixed microbial cultures (MMCs) with the capacity to store PHA is a key precursor for low-cost PHA production. This study investigated the impact of carbon types on enrichment outcomes. Three MMCs were separately fed by acetate sodium, glucose, and starch as an enriching carbon source, and were exposed to long-term aerobic dynamic feeding (ADF) periods. The PHA production capacity, kinetics and stoichiometry of the enrichments, the PHA composition, and the microbial diversity and community composition were explored to determine carbon and enrichment correlations. After 350-cycle enriching periods under feast-famine (F-F) regimes, the MMCs enriched by acetate sodium and glucose contained a maximum PHA content of 64.7% and 60.5% cell dry weight (CDW). The starch-enriched MMC only had 27.3% CDW of PHA. High-throughput sequencing revealed that non-PHA bacteria survived alongside PHA storing bacteria, even under severe F-F selective pressure. Genus of Pseudomonas and Stappia were the possible PHA accumulating bacteria in acetate-enriched MMC. Genus of Oceanicella, Piscicoccus and Vibrio were found as PHA accumulating bacteria in glucose-enriched MMC. Vibrio genus was the only PHA accumulating bacteria in starch-enriched MMC. The community diversity and composition were regulated by the substrate types.