Biochemical characteristics of heavy metal uptake by Escherichia coli NCP immobilized in kappa-carrageenan beads

摘要

A biosorbent was developed with Escherichia coli NCP immobilized in 2.25 percent k-carrageenan to remove heavy metals from solutions. The NCP is a recombinant E. coli, expressing the Neurospora crassa metallothionein gene that codes for a high-affinity, metal-binding peptide. The NCP has previously been shown to sequester low levels of cadmium (Cd) and other metals from solutions. k-Carrageenan gel encapsulated with the NCP forms an ellipsoidal bead with average diameters of 5.82=-0.31mm along the long axis and 2.70+-0.27mm along the short axis. This study reports physiological and biochemical characteristics of Cd uptake by the biosorbent. Encapsulated cells grew up to 1.82 X 10~9 cells within individual beads. However, a small amount of the immobilized cells continuously leaked out of the bead during the biosorption experiments when cell numbers reached 9 X 10~8 cells/bead. Scanning electron micrographs evidenced that encapsulated cells migrated toward the surface area, leaving small clusters of cells embedded in the gel matrix. The maximum uptake of Cd by the biosorbent was achieved within three hours, while cells were still in the exponential growth phase. Individual cells sequestered the maximum amount of Cd at 2-hour incubation, after which the specific uptake per cell declined significantly as cells appeared to escape from the bead. Results of the specific metal uptake by immobilized cells and free cells were compared to identify the transport limitation and availability of metals in polymer matrices. There was no apparent microbial limitation present in ellipsoidal-shaped, 2.25 percent k-carrageenan beads. Rather, the immobilized individual cells sequestered more Cd when they were grown in the LB medium than free cells dissolved in saline solutions. Results indicate that the immobilized recombinant cells function as more effective biosorbents than the free cells.