Evaluating haloarchaeal culture media for ultrahigh-molecular-weight polyhydroxyalkanoate biosynthesis by Haloferax mediterranei

Abstract

A series of culture media for haloarchaea were evaluated to optimize the production of ultrahigh-molecular-weight (UHMW) poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) by Haloferax mediterranei. Cells of H. mediterranei grew (> 1 g/L of dry cell weight) and accumulated PHBV upon flask cultivation in 10 medium types with neutral pH and NaCl concentration > 100 g/L. Molecular weight and compositional analysis revealed that the number-average molecular weight (M_n) of PHBV produced with six selected types of media ranged from 0.8 to 3.5 × 10⁶ g/mol and the 3-hydroxyvalerate (3HV) composition ranged from 8 to 36 mol%. Cultivation in two NBRC media, 1214 and 1380, resulted in the production of PHBV with an M_n of more than 3.0 × 10⁶ g/mol and a weight-average molecular weight of more than 5.0 × 10⁶ g/mol, indicating the production of UHMW-PHBV. These culture media contained small amount of complex nutrients like yeast extract and casamino acids, suggesting that H. mediterranei likely produced UHMW-PHBV on poor nutrient condition. Haloferax mediterranei grown in NBRC medium 1380 produced PHBV with the highest 3HV composition. A solvent-cast film of UHMW-PHBV with 26.4 mol% 3HV produced from 1-L flask cultivation with NBRC medium 1380 was found to be flexible and semi-transparent. Thermal analysis of the UHMW-PHBV cast film revealed melting and glass-transition temperatures of 90.5 °C and - 2.7 °C, respectively. KEY POINTS: • Haloarchaeal culture media were evaluated to produce UHMW-PHBV by H. mediterranei. • UHMW-PHBV with varied molecular weight was produced dependent on culture media. • Semi-transparent film could be made from UHMW-PHBV with 26.4 mol% 3HV.

Keywords: Biodegradable polymers; Haloarchaea; Haloferax mediterranei; Poly(3-hydroxybutyrate-co-3-hydroxyvalerate); Polyhydroxyalkanoates; Ultrahigh-molecular-weight.