Recent advances and perspectives in efforts to reduce the production and application cost of microbial flocculants

Microbial flocculants are macromolecular substances produced by microorganisms. Due to its non-toxic, harmless, and biodegradable advantages, microbial flocculants have been widely used in various industrial fields, such as wastewater treatment, microalgae harvest, activated sludge dewatering, heavy metal ion adsorption, and nanoparticle synthesis, especially in the post-treatment process of fermentation with high safety requirement. However, compared with the traditional inorganic flocculants and organic polymeric flocculants, the high production cost is the main bottleneck that restricts the large-scale production and application of microbial flocculants. To reduce the production cost of microbial flocculant, a series of efforts have been carried out and some exciting research progresses have been achieved. This paper summarized the research advances in the last decade, including the screening of high-yield strains and the construction of genetically engineered strains, search of cheap alternative medium, the extraction and preservation methods, microbial flocculants production as an incidental product of other biological processes, combined use of traditional flocculant and microbial flocculant, and the production of microbial flocculant promoted by inducer. Moreover, this paper prospects the future research directions to further reduce the production cost of microbial flocculants, thereby promoting the industrial production and large-scale application of microbial flocculants.

Applied microbiology and biotechnology uncovering the biosynthetic pathway of polysaccharide-based microbial flocculant in Agrobacterium tumefaciens F2

The low yield as bottleneck problem limits the application of microbial flocculant in water treatment. However, genetic information of microbial flocculant-producing strains can guide the regulation of microbial flocculant production, but it remains unknown. Agrobacterium tumefaciens F2 produced polysaccharide-based microbial flocculants in the fermentation medium but none in Luria Bertani medium; hence, the transcriptome was used to analyze the potentially associated genes with the production of microbial flocculants. Glucose, mannose, rhamnose, and galactose are the main sugar monomers, and genes (manA, glmM, manC, rfb genes, exo genes, etc.) with changed expression levels related to sugar monomers metabolism potentially participated in the biosynthesis of polysaccharide-based microbial flocculants. exoC, exoP, and manC were confirmed to participate in the biosynthesis via constructing the mutants F2-dexoC, F2-dexoP, and F2-dmanC. An exo_F2 gene cluster was annotated due to the high percentage of matches between the genome sequences of strains F2 and C58, and exo genes in their genome sequences showed the similarity of 86~92%. The hypothetical pathway for the biosynthesis of polysaccharide-based microbial flocculants in strain F2 was proposed, laying the basis for the production yield regulation. KEY POINTS: • An exo_F2 gene cluster in the polysaccharide biosynthesis was annotated. • exoC, exoP, and manC genes participated in the polysaccharide biosynthesis. • A hypothetical biosynthesis pathway of polysaccharide in flocculant was proposed. Graphical abstract.

The adsorption behavior and mechanism investigation of Pb(II) removal by flocculation using microbial flocculant GA1

In this work, microbial flocculant GA1 (MBFGA1) was used to remove Pb(II) ions from aqueous solution. A series of experimental parameters including initial pH, MBFGA1 dose, temperature and initial calcium ions concentration on Pb(II) uptake was evaluated. Meanwhile, the flocculation mechanism of MBFGA1 was investigated. The removal efficiency of Pb(II) reached up to 99.85% when MBFGA1 was added in two stages, separately. The results indicated that Pb(II) adsorption could be described by the Langmuir adsorption model, and being the monolayer capacity negatively affected with an increase in temperature. The adsorption process could be described by pseudo-second-order kinetic model. Fourier transform-infrared spectra and environmental scanning electron microscope analysis indicated that MBFGA1 had a large number of functional groups, which had strong capacity for removing Pb(II). The main mechanisms of Pb(II) removal by MBFGA1 could be charge neutralization and adsorption bridging.