Transport kinetics, cation inhibition and intracellular location of accumulated caesium in the green microalga Chlorella salina

Abstract

Caesium accumulation by Chlurella salina, from buffer (pH 8.0) supplemented with 50 pM-CsCl and 13' Cs, continued for approximately 15 h and displayed first-order kinetics, indicating a single rate-limiting transport process. Efflux of Cs+ from Cs+-loaded cells occurred in two distinct phases: a rapid initial loss, representing approximately 11 % of total cellular Cs+, corresponded to release from the cell surface, whereas a second, slower, phase of efflux corresponded to loss from the cytoplasm and vacuole. Analysis of subcellular Cs+ compartmentation revealed that most Cs+ was accumulated into the vacuole of C. salina, with lesser amounts being associated with the cell surface or located in the cytoplasm. Uptake of Cs+ into the vacuole was correlated with a stoichiometric exchange for K+. However, no loss of K+ from the cell surface or cytoplasm was evident nor was Cs+ or K+ associated with insoluble intracellular components. Calculated values for the Cs+ flux across the vacuolar membrane were approximately equal to, or higher than, values for total cellular influx. Cs+ influx obeyed MichaelisrMenten kinetics over the lower range of external Cs+ concentrations examined (0.01-0.25 mM) and a single transport system with a K, -0.5 mM was evident. The effects of other monovalent cations on Cs+ influx implied that K+ and Rb+ were competitive, and NHt non-competitive/uncompetitive inhibitors of Cs+ uptake. The order of inhibition was Rb+ > K+ > NHT. We propose that a single, relatively non-selective, rate-limiting transport system for Cs+ influx is located on the cytoplasmic membrane of C. salina, while a more permeable vacuolar membrane facilitates transport of Cs+ into the vacuole.

Salt-stimulation of caesium accumulation in the euryhaline green microalga Chlorella salina: potential relevance to the development of a biological Cs-removal process

“…The low level of inhibition of Cs' uptake by other monovalent cations in C. saEina (Avery et al, 1993), and by the divalent cations Mg2+ and Ca2+ reported here, suggests that problems frequently caused by the presence of competing ions on metal removal processes (Gadd, 1990) may not be so important in the present system. Furthermore, in contrast to the marked pH-dependence of Cs' accumulation by the cyanobacterium Synechocystis PCC 6803 (Avery et al, 1991), high Cst uptake levels were maintained over a range of external pH values in C. salina.…”

“…Although no improvement in Cs' uptake efficiency resulted from cell immobilization, advantages of this process over many other living cell systems include its pH-independence, the low toxicity of Cs+ towards C. salina and the non-destructive treatments that may be employed for complete recovery of ~C C U M Ulated Cs'. Furthermore, despite the probable influence of external K+ on Cs+ accumulation following successive incubation periods, competition by K+ is low in C. salinu (Avery et al, 1993) compared to certain other microorganisms (Avery et al, 1992a), and the influence of other naturally abundant ions like Ca2+ and Mgz+ was negligible. The application of recently gained knowledge to the present system, e.g.…”

“…In Escherichia coli, severaI different K' transport systems occur (Walderhaug et al, 1987) although those under osmotic control are distinct from the system that mediates Cs+ uptake (Macaskie, 199 1). However, Pick et a/. (1986) detected a single K' system in the halotolerant microalga Dunalieila salina, and recent work has demonstrated that a single mechanism also mediates Cs" uptake in Chlorella salina (Avery et al, 1993). It is likely 0001-7996 0 1993 SGM therefore that any factor that influences K+ uptake in these organisms will also affect Cs+ uptake.…”

Characterization of Cesium Uptake Mediated by a Potassium Transport System of Bacteria in a Soil Conditioner

“…Therefore Cs + uptake is likely to take place in soil under natural conditions. It is thought that cellular Cs + uptake is mediated by K + transport systems, as in Rhodococcus, 5,17) E. coli 6) and Chlorella salina, 3) Cs-contaminated soils indicate that this approach may be effective for removing 137 Cs from soil.…”

“…[2][3][4] Also, Cs + uptake by bacteria is inhibited by potassium ion (K + ). 2,5) Thus, Cs + might be accumulated via K + transport system(s).…”