Twenty-four Actinobacteria strains, isolated from Arundo donax, Eucalyptus camaldulensis and Populus nigra biomass during natural biodegradation and with potential enzymatic activities specific for the degradation of lignocellulosic materials, were identified by a polyphasic approach. All strains belonged to the genus Streptomyces (S.) and in particular, the most highly represented species was Streptomyces argenteolus representing 50% of strains, while 8 strains were identified as Streptomyces flavogriseus (synonym S. flavovirens) and Streptomyces fimicarius (synonyms Streptomyces acrimycini, Streptomyces baarnensis, Streptomyces caviscabies, and Streptomyces flavofuscus), and the other four strains belonged to the species Streptomyces drozdowiczii, Streptomyces rubrogriseus, Streptomyces albolongus, and Streptomyces ambofaciens. Moreover, all Streptomyces strains, tested for endo and exo-cellulase, cellobiase, xylanase, pectinase, ligninase, peroxidase, and laccase activities using qualitative and semi-quantitative methods on solid growth medium, exhibited multiple enzymatic activities (from three to six). The 24 strains were further screened for endo-cellulase activity in liquid growth medium and the four best endo-cellulase producers (S. argenteolus AE58P, S. argenteolus AE710A, S. argenteolus AE82P, and S. argenteolus AP51A) were subjected to partial characterization and their enzymatic crude extracts adopted to perform saccharification experiments on A. donax pretreated biomass. The degree of cellulose and xylan hydrolysis was evaluated by determining the kinetics of glucose and xylose release during 72 h incubation at 50°C from the pretreated biomass in the presence of cellulose degrading enzymes (cellulase and β-glucosidase) and xylan related activities (xylanase and β-xylosidase). The experiments were carried out utilizing the endo-cellulase activities from the selected S. argenteolus strains supplemented with commercial β-gucosidase and xylanase preparations from Genencore (Accellerase BG and Accellerase XY). Cellulose and xylan conversion, when conducted using commercial (hemi)cellulases, gave glucose and xylose yields of 30.17 and 68.9%, respectively. The replacement of the cellulolytic preparation from Genencor (Accellerase 1500), with the endo-cellulase from S. argenteolus AE58P resulted in almost 76% of the glucose yield obtained in the presence of the commercial counterpart. Due to the promising results obtained by using the enzymatic crude extracts from S. argenteolus AE58P in the pretreated A. donax saccharification experiments, the proteins putatively responsible for endo-cellulase activity in this strain were identified by proteomics. Several proteins were confidently identified in different Streptomyces spp., eight of which belong to the class of Carbohydrate active enzymes. Overall results highlighted the biotechnological potential of S. argenteolus AE58P being an interesting candidate biocatalyst-producing bacterium for lignocellulose conversion and production of biochemicals and bioenergy.
Bio-based succinic acid production from lignocellulosic biomass is one of the attractive and prominent alternative technologies to overcome issues associated with the utilization of fossil sources. In this context, it is necessary to find new microorganisms that are able to efficiently ferment this recalcitrant feedstock. The ecological approach developed in this study enabled the isolation of Basfia succiniciproducens BPP7 from a complex rumen ecosystem. This new wild-type strain was able to synthesize up to 6.06 ± 0.05 g/L of succinate (corresponding to 0.84 ± 0.017 g of succinate per gram of consumed glucose + xylose and to 0.14 ± 0.001 g of succinate per gram of glucans + xylans present in the biomass before hydrolysis) from Arundo donax hydrolysate in separate hydrolysis and fermentation (SHF) experiments. Higher titers of succinic acid were obtained through the optimization of growth conditions. The optimal medium composition identified on the smaller scale was then used for 2.5-L batch experiments, which used A. donax hydrolysate and yeast extract as the main C and N sources, respectively. A maximal titer of 9.4 ± 0.4 g/L of succinic acid was obtained after 24 h. The overall results clearly demonstrate the potential of B. succiniciproducens BPP7 for succinate production.
In this study, a high-throughput sequencing approach was applied to discover novel biocatalysts for lignocellulose hydrolysis from three dedicated energy crops, Arundo donax, Eucalyptus camaldulensis and Populus nigra, after natural biodegradation. The microbiomes of the three lignocellulosic biomasses were dominated by bacterial species (approximately 90%) with the highest representation by the Streptomyces genus both in the total microbial community composition and in the microbial diversity related to GH families of predicted ORFs. Moreover, the functional clustering of the predicted ORFs showed a prevalence of poorly characterized genes, suggesting these lignocellulosic biomasses are potential sources of as yet unknown genes. 1.2%, 0.6% and 3.4% of the total ORFs detected in A. donax, E. camaldulensis and P. nigra, respectively, were putative Carbohydrate-Active Enzymes (CAZymes). Interestingly, the glycoside hydrolases abundance in P. nigra (1.8%) was higher than that detected in the other biomasses investigated in this study. Moreover, a high percentage of (hemi)cellulases with different activities and accessory enzymes (mannanases, polygalacturonases and feruloyl esterases) was detected, confirming that the three analyzed samples were a reservoir of diversified biocatalysts required for an effective lignocellulose saccharification.
The world economy is moving toward the use of renewable and nonedible lignocellulosic biomasses as substitutes for fossil sources in order to decrease the environmental impact of manufacturing processes and overcome the conflict with food production. Enzymatic hydrolysis of the feedstock is a key technology for bio-based chemical production, and the identification of novel, less expensive and more efficient biocatalysts is one of the main challenges. As the genomic era has shown that only a few microorganisms can be cultured under standard laboratory conditions, the extraction and analysis of genetic material directly from environmental samples, termed metagenomics, is a promising way to overcome this bottleneck. Two screening methodologies can be used on metagenomic material: the function-driven approach of expression libraries and sequence-driven analysis based on gene homology. Both techniques have been shown to be useful for the discovery of novel biocatalysts for lignocellulose conversion, and they enabled identification of several (hemi)cellulases and accessory enzymes involved in (hemi)cellulose hydrolysis. This review summarizes the latest progress in metagenomics aimed at discovering new enzymes for lignocellulose saccharification.
Cereals are becoming interesting substrates to be fermented to obtain new functional foods. For this reason, an enzymatic pretreatment of a wheat flour suspension using amylase was investigated in order to guarantee the feasibility to ferment a cereal‐based substrate at a solid content higher than that used in past experimentations. Trials with and without amylase pretreatment, using a 52% w/v wheat flour water suspension, were carried out to evaluate the fermentation feasibility; mixing tests with a food dye were performed to verify the suspension homogeneity. The pretreated suspension, whose starch was hydrolyzed for about 80% by α‐amylase, was then fermented (37°C, 24h) by Lactobacillus paracasei CBA‐L74. Starting from pH value 5.75, microbial concentration 1.89 × 106 CFU/ml and lactic acid 0 mg/L, final values of 3.93, 3 × 108 CFU/ml and 6,251 mg/L were found after 24 hr of fermentation. The untreated suspension went to gelation and was impossible to ferment because its consistency was like a dough and the mixing system was not suitable to ensure mixing. Practical applications The aim of the present study was the handling and the fermentation of cereal‐based substrate at high solid content (52% w/v), through a preliminary amylase pretreatment. The fermentation of such a solid content could guarantee the production of a greater quantity of solid fermented matrix and this could considerably reduce drying times with greater productivity at a low cost.
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