Effects of C/N in the substrate on the simultaneous production of polyhydroxyalkanoates and extracellular polymeric substances by Haloferax mediterranei via kinetic model analysis

Abstract

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.