MICROBIAL INTERACTIONS WITH CESIUM - IMPLICATIONS FOR BIOTECHNOLOGY

The continuing release of caesium isotopes into the environment has highlighted the necessity for efficient removal of Cs from industrial waste effluents prior to discharge. Existing technologies, e.g. zeolite ion-exchange for Cs removal, can be expensive and microbial metal adsorption/accumulation may represent a cheap alternative. The distinct chemical properties of Cs+, which dictate a high degree of metabolism-dependent uptake via monovalent cation transport systems, indicate that different approaches are required for biological Cs removal to those which are generally adopted for other metals/radionuclides. The low toxicity of Cs+ eliminates one potential problem in the use of live cells for Cs removal. High levels of Cs+ accumulation have been reported in a number of microorganisms, but uptake levels vary markedly in different organisms and are strongly influenced by a number of physico-chemical and mechanical parameters, e.g. the use of batch or continuous-flow systems, biomass immobilization (which tends to increase Cs adsorption at the expense of metabolism-dependent accumulation), pH and particularly the prevalence of other monovalent cations such as K+ and Na+ Inherent differences in Cs+ uptake capacities of different microorganisms appear to be largely attributable to differences in the affinity of monovalent cation transport systems for Cs+. The application of rigorous screening procedures involving the use of autoradiography has great potential for isolation of microorganisms with particularly high affinities for Cs+. Alternatively, manipulation of the physiological status of microorganisms can dramatically alter the transport of Cs+ and other monovalent cations. Hyper- and hypo-osmotic shock, respectively, have so far proved to be the most successful treatments for stimulating Cs removal and recovery. Other manipulations, at both the cellular and molecular level, which are known to influence K+ fluxes but have yet to be characterized for Cs+, are outlined here.