170 SORPTION AND TRANSFORMATION OF TOXIC METALS BY MICROORGANISMS

than uncoated particles, due to the similar solubility of the surfactant with the cell membrane (Lubick, 2008). Auffan et al. (2006) suggest that the chemical nature of the coating on the nanoγ -Fe2O3determines different cell responses in terms of cytotoxic-

ity. The morphological characteristics and sizes of the nanoparticles are another set of important factors that should be taken into consideration, and it is logical to postulate that the physiochemical and biochemical interactions of the spherical nanoparticles are different from those of the others with the bacteria. In addition, chemical and cat- alytic properties of nanomaterials should also be considered carefully in future research (Limbach et al., 2007).

The last but most interesting problem is the speciation of the nanoparticles in intra- cellular sites, the pathways of nanoparticle transport into bacterial cells, and the specific enzymes that assimilate or detoxify the metals or expel them from the cells. Although the generation of ROS under the lighted conditions in the cells of an organism is gener- ally believed to be a toxic mechanism to the host (Nel et al., 2006), Adams et al. (2006) point out that the inhibitory effects of nano TiO2, nano ZnO2, and nano SiO2to E. coli

and B. subtilis observed under dark conditions suggest that the new mechanisms may contribute additionally to toxicity. We believe that other mechanisms will be uncovered soon because new methodologies and novel analytical methods are being developed and made available. A comprehensive physicochemical and biochemical understanding of the interactions between nanometals and bacteria should soon be realized.

7.5 CONCLUSIONS

Both physicochemical and biochemical reactions are involved in interactions between metals and microorganisms. The physicochemical interaction is always referred to biosorption, which is the first step in the interaction process between the metals of concern and the microbial cells, including physical adsorption, ion exchange, and com- plexation. Functional groups such as carboxyl, phosphoryl, and amino on the surface of cellular components play a significant role in metal biosorption and immobilization. Although surface complexation models may partly explain some of the experimental observations in metal biosorption systems, a better comprehensive understanding of the physicochemical mechanism of interactions between metal ions and microorgan- isms should be based on direct proof from the metal speciation, existing format, and structure of the complexes formed on bacterial surfaces, which can be explored by x-ray analysis, such as XPS, XRD, EXAFS, and XANES. Biochemical interactions between microorganisms and toxic metals are more complicated processes, containing primarily oxidation and reduction, methylation and dealkylation, and precipitation. In the biotransformation of multivalent toxic metals such as As(V), Se(VI), Cr(VI), and U(VI), more interest has focused on identifying the enzymes involved in these pro- cesses. The biochemical diversity of the enzymes indicated in previous studies suggests the complication of the metal biotransformation processes. Additionally, differences in environmental factors such as substrates and humic acids may also lead to different enzymes and biochemical pathways involved in the metal biotransformation. With the rapid development of the nanotechnology, the transport and fate of nanometal has attracted more interest in recent years. Compared with aqueous metal ions, nanometal is less soluble, with larger particle sizes. However, due to the lack of thermodynamic data about nanoparticles in aqueousions, investigation of the interactions between bac- teria and nanoparticles is still scarce. Although significant progress has been made in

REFERENCES 171

the past 20 years in understanding the interactions between microorganisms and metal ions, further investigation into the physicochemical and biochemical mechanisms is still required to provide new insights into the fundamental biochemical mechanisms involved and potential application of metal bioremediation in cleaning up contaminated sites.

Acknowledgment

We thank Hui-Luo Cao for producting Figure 7.2.

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