Sv. Avery et al., TRANSPORT KINETICS, CATION INHIBITION AND INTRACELLULAR LOCATION OF ACCUMULATED CESIUM IN THE GREEN MICROALGA CHLORELLA-SALINA, Journal of General Microbiology, 139, 1993, pp. 827-834
Caesium accumulation by Chlorella salina, from buffer (pH 8.0) supplem
ented with 50 muM-CsCl and Cs-137 continued for approximately 15 h and
displayed first-order kinetics, indicating a single rate-limiting tra
nsport process. Efflux of Cs+ from Cs+-loaded cells occurred in two di
stinct phases: a rapid initial loss, representing approximately 11 % o
f total cellular Cs+, corresponded to release from the cell surface, w
hereas a second, slower, phase of efflux corresponded to loss from the
cytoplasm and vacuole. Analysis of subcellular Cs+ compartmentation r
evealed that most Cs+ was accumulated into the vacuole of C. salina, w
ith lesser amounts being associated with the cell surface or located i
n the cytoplasm. Uptake of Cs+ into the vacuole was correlated with a
stoichiometric exchange for K+. However, no loss of K+ from the cell s
urface or cytoplasm was evident nor was Cs+ or K+ associated with inso
luble intracellular components. Calculated values for the Cs+ flux acr
oss the vacuolar membrane were approximately equal to, or higher than,
values for total cellular influx. Cs+ influx obeyed Michaelis-Menten
kinetics over the lower range of external Cs+ concentrations examined
(0.01-0.25 mm) and a single transport system with a K(m) approximately
0.5 mM was evident. The effects of other monovalent cations on Cs+ in
flux implied that K+ and Rb+ were competitive, and NH4+ non-competitiv
e/uncompetitive inhibitors of Cs+ uptake. The order of inhibition was
Rb+ > K+ > NH4+. We propose that a single, relatively non-selective, r
ate-limiting transport system for Cs+ influx is located on the cytopla
smic membrane of C. salina, while a more permeable vacuolar membrane f
acilitates transport of Cs+ into the vacuole.