SALT-STIMULATION OF CESIUM ACCUMULATION IN THE EURYHALINE GREEN MICROALGA CHLORELLA-SALINA - POTENTIAL RELEVANCE TO THE DEVELOPMENT OF A BIOLOGICAL CS-REMOVAL PROCESS

Accumulation of Cs+ by Chlorella salina was 28-fold greater in cells i ncubated in the presence than in the absence of 0.5 m-NaCl. An approxi mate 70 % removal of external Cs+ resulted after 15 h incubation of ce lls with 50 muM-CsCl and 0.5 m-NaCl. LiCl also had a stimulatory effec t on Cs+ uptake, although mannitol did not. Cs+ influx increased with increasing external NaCl concentration and was maximal between 25-500 mm-NaCl at approximately 4 nmol Cs+ h-1 (10(6) cells)-1. Little effect on Cs+ uptake resulted from the presence of Mg2+ or Ca2+ or from vary ing the external pH, and Cs was relatively non-toxic towards C. salina . At increasing cell densities (from 4 x 10(5) to 1 x 10(7) cells ml-1 ), decreasing amounts of Cs+ were accumulated per cell although the ra te of Cs+ removal from the external medium was still greatest at the h igher cell densities examined. Freely suspended C. salina and cell-loa ded alginate microbeads accumulated similar levels of Cs+, however, 46 % of total Cs+ uptake was attributable to the calcium-alginate matrix in the latter case. When Cs+-loaded cells were subjected to hypoosmoti c shock, loss of cellular Cs+ occurred allowing easy Cs+ recovery. Thi s loss exceeded 90% of cellular Cs+ when cells were washed with soluti ons containing less-than-or-equal-to 50 mm-NaCl between consecutive Cs + uptake periods; these cells subsequently lost their ability to accum ulate large amounts of Cs+. Maximal Cs+ uptake (approximately 85.1% re moval after three 15 h incubations) occurred when cells were washed wi th a solution containing 500 mM-NaCl and 200 mM-KCl between incubation s. The relevance of these results to the possible use of C. salina in a salt-dependent biological Cs-removal process is discussed.