The impacts of different long-term fertilization regimes on the bioavailability of arsenic in soil: integrating chemical approach with Escherichia coli arsRp::luc-based biosensor.

An Escherichia coli arsRp::luc-based biosensor was constructed to measure the bioavailability of arsenic (As) in soil. In previous induction experiments, it produced a linear response (R (2) = 0.96, P < 0.01) to As from 0.05 to 5 µmol/L after a 2-h incubation. Then, both chemical sequential extraction, Community Bureau of Reference recommended sequential extraction procedures (BCR-SEPs) and E. coli biosensor, were employed to assess the impact of different long-term fertilization regimes containing N, NP, NPK, M (manure), and NPK + M treatments on the bioavailability of arsenic (As) in soil. Per the BCR-SEPs analysis, the application of M and M + NPK led to a significant (P < 0.01) increase of exchangeable As (2-7 times and 2-5 times, respectively) and reducible As (1.5-2.5 times and 1.5-2.3 times, respectively) compared with the no fertilization treated soil (CK). In addition, direct contact assay of E. coli biosensor with soil particles also supported that bioavailable As in manure-fertilized (M and M + NPK) soil was significantly higher (P < 0.01) than that in CK soil (7 and 9 times, respectively). Organic carbon may be the major factor governing the increase of bioavailable As. More significantly, E. coli biosensor-determined As was only 18.46-85.17 % of exchangeable As and 20.68-90.1 % of reducible As based on BCR-SEPs. In conclusion, NKP fertilization was recommended as a more suitable regime in As-polluted soil especially with high As concentration, and this E. coli arsRp::luc-based biosensor was a more realistic approach in assessing the bioavailability of As in soil since it would not overrate the risk of As to the environment.

Assessing the effect of phosphate and silicate on Cd bioavailability in soil using an Escherichia coli cadAp::luc-based whole-cell sensor.

An Escherichia coli cadAp::luc-based whole-cell sensor was constructed to measure cadmium (Cd) bioavailability and assess the immobilizing efficiency of phosphate and silicate on Cd. In previous induction experiments, a linear response (R(2) = 0.97, P < 0.01) from 0.1 to 5 µmol L(-1) of Cd was detected by this sensor after a 2 h incubation. The sensor was then used to estimate Cd bioavailability in soils spiked with different amounts of dipotassium phosphate (DKP, K2HPO4) or sodium silicate (SS, Na2SiO3·9H2O). The total Cd in soil-water extracts (TSWE) was determined with ICP-MS, and the bioavailable Cd in soil-water extracts (BSWE) and bioavailable Cd in soil-water suspensions (BSWS) were measured by the E. coli cadAp::luc-based whole-cell sensor. Final results showed that spiked SS (Si : Cd = 2 : 1, mol mol(-1)) reduced the different forms of Cd (TSWE, BSWE and BSWS) from 56.47 mg kg(-1), 42.11 mg kg(-1), and 206.72 mg kg(-1) to 16.63 mg kg(-1), 15.90 mg kg(-1), and 67.57 mg kg(-1), respectively. In other words, SS had 25.68%, 19.5%, and 9.54% better immobilizing efficiency, respectively, compared with DKP. All the results supported SS was more efficient than DKP at immobilizing Cd in soil, and higher soil pH and higher solubility of the immobilizing agents may have been the major factor affecting immobilizing efficiency. In addition, the total and bioavailable Cd in soil-water extracts was only 16.13-35.41% of the sensor contact assay-determined Cd (BSWS), which indicated that the whole-cell sensor-based contact assay was more practical in assessing the risk of Cd in soil after immobilization since it would not overrate the immobilizing capacity of the agents.

[Construction and properties of a microbial whole-cell sensor CB10 for the bioavailability detection of Cr6+].

A microbial whole-cell biosensor CB10 for the bioavailability assessing of Cr6+ was constructed by molecular biotechnology. The regulatory gene and promoter of CB10 was from the chromium resistance system of plasmid pMOL28 from Cupriavidus metallidurans CH34, and the reporter gene of CB10 was luc which was derived from Photinus pyralis. Finally, its response characteristic was discussed under different incubation conditions e. g. pH and temperature. The results showed that a microbial whole-cell biosensor CB10 had been successfully constructed which could respond to Cr6+ within 30 min, with a LOD for Cr6+ of 2 micromol x L(-1). When the incubation concentration of Cr6+ was between 20 micromol x L(-1) and 200 micromol x L(-1), the luc activity of the CB10 biosensor was in linear correlation with the concentration of Cr6+. When the concentration of heavy metal was in the range of 10-50 micromol x L(-1), the response of CB10 was relatively more specific. Moreover, high concentrations of Pb2+, Mn2+ and Sb2+ could also induce CB10. By analyzing the response characteristic of CB10 biosensor, we could draw the conclusion that 15-30 degrees C and pH 4-7 were appropriate for CB10, and 30 degrees C and pH 7 were the optimal conditions for the incubation of the CB10 biosensor. The microbial whole-cell biosensor CB10 for the detection of Cr6+ was fast-responding, specific, sensitive and stable under various conditions. In prospective, it could be used in the fast detection of Cr6+ in water and assessment of the bioavailability of Cr6+ in soil.

[Advance in the bioavailability monitoring of heavy metal based on microbial whole-cell sensor].

Microbial whole-cell biosensor is an excellent tool to assess the bioavailability of heavy metal in soil and water. However, the traditional physicochemical instruments are applied to detect the total metal. Furthermore, microbial whole-cell biosensor is simple, rapid and economical in manipulating, and is thus a highly qualified candidate for emergency detection of pollution incidents. The biological component of microbial whole-cell biosensor mostly consists of metalloregulatory proteins and reporter genes. In detail, metalloregulatory proteins mainly include the MerR family, ArsR family and RS family, and reporter genes mainly include gfp, lux and luc. Metalloregulatory protein and reporter gene are related to the sensitivity, specificity and properties in monitoring. The bioavailability of heavy metals is alterable under different conditions, influenced by pH, chelate and detection methods and so on. Increasing the accumulation of intracellular heavy metal, modifying the metalloregulatory proteins and optimizing the detecting conditions are important for improving the sensitivity, specificity and accuracy of the microbial whole-cell biosensor. The future direction of microbial whole-cell biosensor is to realize the monitoring of pollutions in situ and on line.