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

Advanced Strategies for Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Production: PHA Synthase Homologous Overexpression in the Extremophile Haloferax mediterranei

Bioplastics such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) are promising alternatives to conventional plastics. However, the high production cost limits their industrial application. In this study, PHBV production was optimized in Haloferax mediterranei by the homologous overexpression of the key enzyme PHA synthase (PhaEC), resulting in the OEphaEC strain. The growth and PHBV production of OEphaEC compared with the parental strain (HM26) were evaluated in three culture media with different nitrogen sources (KNO3, NH4Cl, and casamino acids). The OEphaEC strain exhibited a 20% increase in PHBV production and a 40% increase in 3-hydroxyvalerate monomer (3HV) content in a defined medium with nitrate as a nitrogen source, as determined by GC-MS. Moreover, enzyme activity, measured spectrophotometrically, increased from 2.3 to 3.9 U/mg. Soluble and insoluble protein fractions were analysed to assess the overexpression of PHA synthase. Only PhaE was found in the insoluble protein fraction, where PHBV granules accumulate. Transmission electron microscopy (TEM) images confirmed a higher PHBV content in OEphaEC compared to the parental strain. These results demonstrate that the homologous overexpression of the key enzyme implicated in PHBV biosynthesis can enhance PHBV content, making its production competitive for industrial applications.

Bioconversion of bread waste into high-quality proteins and biopolymers by fermentation of archaea Haloferax mediterranei

The valorization of bread waste into high-quality protein and biopolymers using the halophilic microorganism Haloferax mediterranei presents a sustainable approach to food waste management and resource optimization. This study successfully coproduced protein and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) biopolymer with a biomass content of 8.0 ± 0.1 g L-1 and a productivity of 11.1 mg L-1 h-1. The fermentation process employed 3.0% w/v of enzymatically hydrolyzed bread waste. The amino acid profile of the cell biomass revealed a total content of 358 g kg-1 of biomass dry weight (DW), including 147 g kg-1 DW of essential amino acids. The protein quality, assessed through in-vitro enzyme digestion, indicated a high-quality protein with a digestibility value of 0.91 and a protein digestibility-corrected amino acid score (PDCAAS) of 0.78. The PHBV biopolymer component (36.0 ± 6.3% w/w) consisted of a copolymer of 3-hydroxybutyrate and 3-hydroxyvalerate in a 91:9 mol% ratio. This bioconversion process not only mitigates food waste but also generates valuable biomaterials.

Exploitation of wasted bread as substrate for polyhydroxyalkanoates production through the use of Haloferax mediterranei and seawater

The use of the halophile microorganism Haloferax mediterranei, able to synthesize poly(hydroxybutyrate-hydroxyvalerate) (PHBV), is considered as a promising tool for the industrial production of bioplastic through bioprocessing. A consistent supplementation of the growth substrate in carbohydrates and minerals is overall necessary to allow its PHBV production. In this work, wasted bread was used as substrate for bioplastic production by microbial fermentation. Instead of the consistent and expensive minerals supplement required for Hfx. mediterranei DSM1411 growth, microfiltered seawater was added to the wasted bread-derived substrate. The suitable ratio of wasted bread homogenate and seawater, corresponding to 40:60, was selected. The addition of proteases and amylase to the bread homogenate promoted the microbial growth but it did not correspond to the increase of bioplastic production by the microorganism, that reach, under the experimental conditions, 1.53 g/L. An extraction procedure of the PHBV from cells, based on repeated washing with water, followed or not by a purification through ethanol precipitation, was applied instead of the conventional extraction with chloroform. Yield of PHBV obtained using the different extraction methods were 21.6 ± 3.6 (standard extraction/purification procedure with CHCl3:H2O mixture), 24.8 ± 3.0 (water-based extraction), and 19.8 ± 3.3 mg PHAs/g of wasted bread (water-based extraction followed by ethanol purification). Slightly higher hydroxyvalerate content (12.95 vs 10.78%, w/w) was found in PHBV obtained through the water-based extraction compared to the conventional one, moreover, the former was characterized by purity of 100% (w/w). Results demonstrated the suitability of wasted bread, supplemented with seawater, to be used as substrate for bioplastic production through fermentation. Results moreover demonstrated that a solvent-free extraction, exclusively based on osmotic shock, could be used to recover the bioplastic from cells.