CESIUM ACCUMULATION BY MICROORGANISMS - UPTAKE MECHANISMS, CATION COMPETITION, COMPARTMENTALIZATION AND TOXICITY

The continued release of caesium radioisotopes into the environment ha s led to a resurgence of interest in microbe-Cs interactions. Caesium exists almost exclusively as the monovalent cation Cs+ in the natural environment Although Cs+ is a weak Lewis acid thai exhibits a low tend ency to form complexes with Ligands, its chemical similarity to the bi ologically essential alkali cation Kt facilitates high levels of metab olism-dependent intracellular accumulation. Microbial Cs+ (K+) uptake is generally mediated by monovalent cation transport systems located o n the plasma membrane. These differ widely in specificity for alkali c ations and consequently microorganisms display large differences in th eir ability to accumulate Cs+; Cs+ appears to have an equal or greater affinity than K+ for transport in certain microorganisms. Microbial C s+ accumulation is markedly influenced by the presence of external cat ions, e.g. K+, Na+, NH4+ and H+, and is generally accompanied by an ap proximate stoichiometric exchange for intracellular K+. However, stimu lation of growth of Ki-starved microbial cultures by Cs+ is limited an d it has been proposed that it is not the presence of Cs+ in cells tha t is growth inhibitory but rather the resulting loss of K+. Increased microbial tolerance to Cs+ may result from sequestration of Cs+ in vac uoles or changes in the activity and/or specificity of transport syste ms mediating Cs+ uptake. The precise intracellular target(s) for Cs+ i nduced toxicity has yet to be clearly defined, although certain intern al structures, e.g. ribosomes, become unstable in the presence of Csand Cs+ is known to substitute poorly for K+ in the activation of many K+-requiring enzymes.