Salt-stimulation of caesium 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 incubated in the presence than in the absence of 0.5 M-NaCl. An approximate 70% removal of external Cs⁺ resulted after 15 h incubation of cells with 50 μM-CsCl and 0.5 M-NaCl. LiCl also had a stimulatory effect 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 (10⁶ cells)^-1. Little effect on Cs⁺ uptake resulted from the presence of Mg²⁺ or Ca²⁺ or from varying the external pH, and Cs⁺ was relatively non-toxic towards C. salina. At increasing cell densities (from 4 x 10⁵ to 1 x 10⁷ cells ml^-1), decreasing amounts of Cs⁺ were accumulated per cell although the rate of Cs⁺ removal from the external medium was still greatest at the higher cell densities examined. Freely suspended C. salina and cell-loaded 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 hypoosmotic shock, loss of cellular Cs⁺ occurred allowing easy Cs⁺ recovery. This loss exceeded 90% of cellular Cs⁺ when cells were washed with solutions containing ≤ 50 mM-NaCl between consecutive Cs⁺ uptake periods; these cells subsequently lost their ability to accumulate large amounts of Cs⁺. Maximal Cs⁺ uptake (approximately 85.1% removal after three 15 h incubations) occurred when cells were washed with a solution containing 500 mMNaCl and 200 mM-KCl between incubations. The relevance of these results to the possible use of C. salina in a salt-dependent biological Cs-removal process is discussed.