Microbial Consortia: Promising Probiotics as Plant Biostimulants for Sustainable Agriculture

Recently, issues concerning the sustainable and harmless disposal of organic solid waste have generated interest in microbial biotechnologies aimed at converting waste materials into bioenergy and biomaterials, thus contributing to a reduction in economic dependence on fossil fuels. To valorize biomass, waste materials derived from agriculture, food processing factories, and municipal organic waste can be used to produce biopolymers, such as biohydrogen and biogas, through different microbial processes. In fact, different bacterial strains can synthesize biopolymers to convert waste materials into valuable intracellular (e.g., polyhydroxyalkanoates) and extracellular (e.g., exopolysaccharides) bioproducts, which are useful for biochemical production. In particular, large numbers of bacteria, including Alcaligenes eutrophus, Alcaligenes latus, Azotobacter vinelandii, Azotobacter chroococcum, Azotobacter beijerincki, methylotrophs, Pseudomonas spp., Bacillus spp., Rhizobium spp., Nocardia spp., and recombinant Escherichia coli, have been successfully used to produce polyhydroxyalkanoates on an industrial scale from different types of organic by-products. Therefore, the development of high-performance microbial strains and the use of by-products and waste as substrates could reasonably make the production costs of biodegradable polymers comparable to those required by petrochemical-derived plastics and promote their use. Many studies have reported use of the same organic substrates as alternative energy sources to produce biogas and biohydrogen through anaerobic digestion as well as dark and photofermentation processes under anaerobic conditions. Therefore, concurrently obtaining bioenergy and biopolymers at a reasonable cost through an integrated system is becoming feasible using by-products and waste as organic carbon sources. An overview of the suitable substrates and microbial strains used in low-cost polyhydroxyalkanoates for biohydrogen and biogas production is given. The possibility of creating a unique integrated system is discussed because it represents a new approach for simultaneously producing energy and biopolymers for the plastic industry using by-products and waste as organic carbon sources.

The aims of this study were to evaluate the microbial diversity of different lignocellulosic biomasses during degradation under natural conditions and to isolate, select, characterise new well-adapted bacterial strains to detect potentially improved enzyme-producing bacteria. The microbiota of biomass piles of Arundo donax, Eucalyptus camaldulensis and Populus nigra were evaluated by high-throughput sequencing. A highly complex bacterial community was found, composed of ubiquitous bacteria, with the highest representation by the Actinobacteria, Proteobacteria, Bacteroidetes and Firmicutes phyla. The abundances of the major and minor taxa retrieved during the process were determined by the selective pressure produced by the lignocellulosic plant species and degradation conditions. Moreover, cellulolytic bacteria were isolated using differential substrates and screened for cellulase, cellobiase, xylanase, pectinase and ligninase activities. Forty strains that showed multienzymatic activity were selected and identified. The highest endo-cellulase activity was seen in Promicromonospora sukumoe CE86 and Isoptericola variabilis CA84, which were able to degrade cellulose, cellobiose and xylan. Sixty-two percent of bacterial strains tested exhibited high extracellular endo-1,4-sz-glucanase activity in liquid media. These approaches show that the microbiota of lignocellulosic biomasses can be considered an important source of bacterial strains to upgrade the feasibility of lignocellulose conversion for the /`greener/' technology of second-generation biofuels.

Twenty-four Actinobacteria strains, isolated from Arundo donax, Eucalyptus camaldulensis and Populus nigra biomasses 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 Streptomyces (S.) genus and in particular, the most representative species was S. argenteolus including the 50% of strains; while 8 strains were identified as members of species S. flavogriseus (synonym S. flavovirens) and S. fimicarius (synonyms S. acrimycini, S. baarnensis, S. caviscabies and S. flavofuscus), and the other 4 strains belonged to the species S. drozdowiczii, S. rubrogriseus, S. albolongus and S. 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 multi-enzymatic activities (from three to six). The twenty-four 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 of Arundo donax pretreated biomass. The different degrees of cellulose and xylan hydrolysis was evaluated by determining the kinetics of glucose and xylose release during 72 h incubation at 50°C of the pretreated biomass in the presence of cellulose degrading enzymes (cellulase and β-glucosidase) and xylan related activities (xylanase and β-xylosidase). Cellulose and xylan conversion, when conducted using commercial (hemi)cellulolytic, 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 allowed to reach almost the 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. Overall results highlighted the biotechnological potential of S. argenteolus AE58P being an interesting candidate biocatalysts-producing bacterium for the lignocellulose conversion and biochemicals and bioenergy production.