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Global issues over the unsustainable use of resources demand novel bio-based processes. The production of edible mushrooms from forest and agricultural residues offers efficient bio-based solutions. Spent mushroom substrate (SMS), a by-product from mushroom cultivation, can be valorized as a source of fermentable sugars. Similarly, the stalks of Jerusalem artichoke (Helianthus tuberosus), a crop of increased interest, represent under-investigated bioresources to be considered for biorefinery applications. In this study, SMS and Jerusalem artichoke stalks (JAS) were used for the first time as substrates to produce exopolysaccharides (EPS) by cultivating halotolerant bacteria (coded SU4M and SU3A) isolated from a hypersaline environment. Cultivations were performed using either cellulosic hydrolysates obtained by enzymatic saccharification or synthetic media. Acid-catalyzed hydrothermal pretreatment was applied to enhance the enzymatic saccharification of JAS, while no pretreatment was needed to saccharify SMS. The use of a SMS hydrolysate-based medium resulted in the highest biomass concentration (3.8 g L−1) and EPS production (852.1 mg L−1) among the media used in shake-flask fermentations. The fermentations in SMS hydrolysates resulted in comparable EPS volumetric productivity (17.8 ± 0.7 mg L−1 h−1) to that of synthetic medium (16.8 ± 0.8 mg L−1 h−1), but higher than that of the JAS hydrolysates (13.3 ± 0.6 mg L−1 h−1). Cultivation in SMS hydrolysates was particularly advantageous, as it did not require either pretreatment or external nutrient supplementation. The highest volumetric productivity of EPS production was achieved in a bioreactor when SMS hydrolysate was used, reaching 100 mg L⁻¹ h⁻¹. This study demonstrates the potential of spent mushroom substrate and Jerusalem artichoke stalks, two underexploited agroresidues, as sustainable substrates for EPS production, thereby contributing to the strengthening of the regional bioeconomy in rural areas.

Microbial exopolysaccharides (EPSs) have attracted increasing attention due to their versatile applications across diverse areas. However, large-scale production of EPSs remains challenging due to the high production costs, primarily driven by the use of synthetic carbon sources. This study demonstrates the potential of quinoa stalk hydrolysates as a sustainable alternative for EPS production using a halotolerant bacterial strain that was isolated from a hypersaline environment and termed SU4M. The bacterial isolate was identified through 16S rRNA and gyrB sequencing as a Bacillus swezeyi strain, and was then cultivated in quinoa stalk hydrolysates. The hydrolysates were produced by acid-catalyzed hydrothermal pretreatment using either sulfuric acid or phosphoric acid, followed by enzymatic saccharification. Fermentation experiments conducted in both shake flasks and bioreactors demonstrated that B. swezeyi SU4M utilized glucose from the hydrolysates efficiently, resulting in significantly higher biomass (5.1 ± 0.1 g L−1) and EPS production (1.2 ± <0.1 g L−1) compared to synthetic media (4.3 ± 0.1 g L−1 and 1.1 ± <0.1 g L−1). The kinetic analysis revealed distinct substrate consumption rates and growth patterns, with hydrolysates enhancing EPS yields under single-pulse fed-batch conditions. Advanced characterization techniques, including compositional analysis, Fourier transform infrared (FTIR) spectroscopy, 1H and 1H-13C heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR), high-performance size-exclusion chromatography (HPSEC), and thermogravimetric analysis (TGA), confirmed that the EPSs derived from hydrolysates were heteropolysaccharides with close structural similarities to those obtained from synthetic media. These findings underscore the potential of quinoa stalk hydrolysates as a biobased alternative to synthetic media as a substrate for EPS production.

The global production of fossil-based plastics has reached critical levels, and their substitution with bio-based polymers is an urgent requirement. Poly(3-hydroxybutyrate) (PHB) is a biopolymer that can be produced via microbial cultivation, but efficient microorganisms and low-cost substrates are required. Halomonas boliviensis LC1, a moderately halophilic bacterium, is an effective PHB producer, and hydrolysates of the residual stalks of quinoa (Chenopodium quinoa Willd.) can be considered a cheap source of sugars for microbial fermentation processes in quinoa-producing countries. In this study, H. boliviensis LC1 was adapted to a cellulosic hydrolysate of quinoa stalks obtained via acid-catalyzed hydrothermal pretreatment and enzymatic saccharification. The adapted strain was cultivated in hydrolysates and synthetic media, each of them with two different initial concentrations of glucose. Cell growth, glucose consumption, and PHB formation during cultivation were assessed. The cultivation results showed an initial lag in microbial growth and glucose consumption in the quinoa hydrolysates compared to cultivation in synthetic medium, but after 33 h, the values were comparable for all media. Cultivation in hydrolysates with an initial glucose concentration of 15 g/L resulted in a higher glucose consumption rate (0.15 g/(L h) vs. 0.14 g/(L h)) and volumetric productivity of PHB (14.02 mg/(L h) vs. 10.89 mg/(L h)) than cultivation in hydrolysates with 20 g/L as the initial glucose concentration. During most of the cultivation time, the PHB yield on initial glucose was higher for cultivation in synthetic medium than in hydrolysates. The produced PHBs were characterized using advanced analytical techniques, such as high-performance size-exclusion chromatography (HPSEC), Fourier transform infrared (FTIR) spectroscopy, 1H nuclear magnetic resonance (NMR) spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). HPSEC revealed that the molecular weight of PHB produced in the cellulosic hydrolysate was lower than that of PHB produced in synthetic medium. TGA showed higher thermal stability for PHB produced in synthetic medium than for that produced in the hydrolysate. The results of the other characterization techniques displayed comparable features for both PHB samples. The presented results show the feasibility of producing PHB from quinoa stalks with H. boliviensis.