Heavy Metal Stress and Its Consequences on Exopolysaccharide (EPS)-Producing Pantoea agglomerans

Heavy metal induced stress on wheat: phytotoxicity and microbiological management

Among many soil problems, heavy metal accumulation is one of the major agronomic challenges that has seriously threatened food safety. Due to these problems, soil biologists/agronomists in recent times have also raised concerns over heavy metal pollution, which indeed are unpleasantly affecting agro-ecosystems and crop production. The toxic heavy metals once deposited beyond certain permissible limits, obnoxiously affect the density, composition and physiological activities of microbiota, dynamics and fertility of soil leading eventually to reduction in wheat production and via food chain, human and animal health. Therefore, the metal induced phytotoxicity problems warrant urgent and immediate attention so that the physiological activities of microbes, nutrient pool of soils and concurrently the production of wheat are preserved and maintained in a constantly deteriorating environment. To mitigate the magnitude of metal induced changes, certain microorganisms have been identified, especially those belonging to the plant growth promoting rhizobacteria (PGPR) group endowed with the distinctive property of heavy metal tolerance and exhibiting unique plant growth promoting potentials. When applied, such metal-tolerant PGPR have shown variable positive impact on wheat production, even in soils contaminated with metals, by supplying macro and micro nutrients and secreting active biomolecules like EPS, melanins and metallothionein (MTs). Despite some reports here and there, the phytotoxicity of metals to wheat and how wheat production in metal-stressed soil can be enhanced is poorly explained. Thus, an attempt is made in this review to better understand the mechanistic basis of metal toxicity to wheat, and how such phytotoxicity can be mitigated by incorporating microbiological remediation strategies in wheat cultivation practices. The information provided here is likely to benefit wheat growers and consequently optimize wheat production inexpensively under stressed soils.

Among many soil problems, heavy metal accumulation is one of the major agronomic challenges that has seriously threatened food safety.

Composition change and adsorption performance of EPS from Bacillus vallismortis sp. induced by Na2S

Sodium sulfide (Na₂S) was used as an inducer to regulate the components of Bacillus vallismortis sp. EPS (Extracellular Polymeric Substances). The main objective of this study was to improve the content of sulfhydryl protein and the adsorption property of EPS to Zn (Ⅱ) that as an typical heavy metal. The results showed that the maximum EPS production of 105.58 mg/g VSS coupling with doubled increase in protein in which the contant of -SH increased by 48.2% from 104.15 to 154.36 μmol/L were recorded in the presence of 20 mg/L Na₂S. Under this condition, the adsorption capacity of S-EPS (EPS with added exogenous Na₂S) for Zn (Ⅱ) was highest. The kinetics of the adsorption process of Zn (Ⅱ) by the S-EPS can be well fitted by the Langmuir isotherm model and the theoretical maximum adsorption amount of 979.09 mg/g EPS could be obtained. The results of 3D-EEM and FTIR analyses, illustrated that -SH, CO, and N-H/C-N played major roles in the removal of Zn (Ⅱ) by S-EPS. The results obtained in this study demonstrated that the addition of sulfur source could increase the content of sulfhydryl protein, and effectively regulate the content of chemical composition, expecially for the sulfhydryl of EPS, and thereby greatly improving the removal efficiency of heavy metals, which showed a great application prospect in the prevention and control of heavy metal pollution.

Role of extracellular polymeric substance (EPS) in toxicity response of soil bacteria Bacillus sp. S3 to multiple heavy metals

Heavy metal resistant bacteria are of great interest because of their potential use in bioremediation. Understanding the survival and adaptive strategies of these bacteria under heavy metal stress is important for better utilization of these bacteria in remediation. The objective of this study was to investigate the role of bacterial extracellular polymeric substance (EPS) in detoxifying against different heavy metals in Bacillus sp. S3, a new hyper antimony-oxidizing bacterium previously isolated from contaminated mine soils. The results showed that Bacillus sp. S3 is a multi-metal resistant bacterial strain, especially to Sb(III), Cu(II) and Cr(VI). Toxic Cd(II), Cr(VI) and Cu(II) could stimulate the secretion of EPS in Bacillus sp. S3, significantly enhancing the adsorption and detoxification capacity of heavy metals. Both Fourier transform infrared spectroscopy (FTIR) and three-dimensional excitation-emission matrix (3D-EEM) analysis further confirmed that proteins were the main compounds of EPS for metal binding. In contrast, the EPS production was not induced under Sb(III) stress. Furthermore, the TEM-EDX micrograph showed that Bacillus sp. S3 strain preferentially transported the Sb(III) to the inside of the cell rather than adsorbed it on the extracellular surface, indicating intracellular detoxification rather than extracellular EPS precipitation played an important role in microbial resistance towards Sb(III). Together, our study suggests that the toxicity response of EPS to heavy metals is associated with difference in EPS properties, metal types and corresponding environmental conditions, which is likely to contribute to microbial-mediated remediation.

Putative Role of Bacterial Biosorbent in Metal Sequestration Revealed by SEM-EDX and FTIR

Bacterial exopolysaccharides (EPS) play a critical role in sequestration of metals from contaminated environment. Considering these, this study was aimed at extracting EPS from metal tolerant Pseudomonas aeruginosa CPSB1 and Azotobacter chroococcum CAZ3 and to ascertain its role in metal removal. P. aeruginosa CPSB1 and A. chroococcum CAZ3 secreted 1306.7 and 1660 µg mL-1 EPS, respectively in the presence of 200 and 100 µg mL-1 Pb, respectively with glucose as C source. The binding of metal ions to bacterial EPS was validated by SEM and EDX. The functional group involved in metal chelation was revealed by FT-IR. The metal ions were adsorbed onto EPS and hence, EPS could play a crucial role in metal detoxification. Due to this novel trait, P. aeruginosa CPSB1 and A. chroococcum CAZ3 could be developed as bioinoculant to cleanup metal contaminated sites.