Fermentation of xylose-rich substrates by the haloarchaeon Halorhabdus utahensis towards high value-added bioproducts

Research that focuses on the use of high value-added bioproducts for industrial applications is essential for the implementation of sustainable approaches forecasting a bio-based economy. The effective use of biomass feedstocks, particularly lignocellulosic materials, in large-scale applications will evolve from innovative research aimed at the development and implementation of biorefineries established for specific feedstocks. In this context, an important step is the concept of fractionating biomass into its core constituents (cellulose, hemicellulose and lignin) for further enhanced valorization. Contrary to the valorization of cellulose fraction, which has been extensively studied, there is a gap in the valorization of the hemicellulose fraction (xylose-rich substrate) towards bioproducts. In this context the present work aims to explore the ability of the haloarchaeon Halorhabdus utahensis (DSM-12940) to ferment xylose (or xylose-rich substrates) to high added-value bioproducts, such as pigments, exopolysaccharides (EPS), polyhydroxyalkanoates (PHAs), etc. Preliminary assays consisted on the growth of H. utahensis on 1L shake-flasks containing 200 mL of the DSMZ (Deutsche Sammlung von Mikroorganismen und Zellkulturen) culture medium (DSMZ 927) supplemented with 0.2% and 1% (w/v) xylose, at 37°C and 120 rpm agitation. The pH of the medium was checked twice a day, as acidification was observed during the growth, and its value was adjusted to the original (7.6) by adding some drops of 5 M NaOH. The microorganism was able to metabolize xylose at both concentration, attaining OD600 nm of 1.28 after 93 h in 0.2% xylose, and 3.11 after 140.5 h in 1% xylose, corresponding to a biomass yield of 4.75 g wet weight/L and 12.35 g wet weight/L, respectively. The orange-pink bacterial cultures indicate the presence of pigments, which must be further characterized. Moreover, under these experimental conditions, H. utahensis was able to synthesize exopolysaccharides (EPS), namely 23.5 mg/L and 47.5 mg/L in 0.2% and 1% xylose, respectively. To optimize the production conditions, a batch assay was carried out by growing H. utahensis on a bench-top 3L bioreactor (BioFlo III - New Brunswick Scientific, NJ, USA) in the culture medium DSMZ 927 with 1% xylose, at 37°C, pH 7.6, 250 rpm and 0.5 vvm aeration. In the reactor, this halophile microorganism was able to full xylose consumption reaching and OD600 nm of 2.73. The analysis of the bioproducts (pigments, EPS, PHAs) both on the biomass and supernatant produced in the bioreactor are ongoing. All these biobased produts (pigments, EPS, PHAs) have applications in different industrial sectors (food, pharmaceutical, cosmetic and environmental sectors). Thus, by addressing the current research gaps and commercial spheres in respect to adding-value to under-utilised xylan fractions from agro-industrial residues, this study may contribute to the overall objectives of biobased industries.