Toxicity of silver to two freshwater algae, Chlamydomonas reinhardtii and Pseudokirchneriella sub-capitata, grown under continuous culture conditions: influence of thiosulphate.

In a test of the biotic ligand model (BLM), the uptake and toxicity of silver, in the absence or presence of the inorganic ligand, thiosulphate, were assessed for two freshwater green algae, Chlamydomonas reinhardtii and Pseudokirchneriella sub-capitata, using turbidostat continuous cultures. In the initial experiments, run in the absence of thiosulphate, the influent Ag concentration was varied from 0 to 75 nM in steps; for each influent concentration, silver uptake was calculated and the algal growth rate was determined. Silver uptake rates at low Ag concentrations were similar for both algae (e.g., 14-19 nmolm(-2)h(-1), for influent Ag(+) concentrations of approximately 9 nM) but at higher exposures uptake by P. sub-capitata exceeded that of C. reinhardtii. Despite this higher uptake rate, in the absence of thiosulphate P. sub-capitata was not more sensitive to free silver; 50% growth inhibition was reached at influent free Ag(+) concentrations of 15+/-7 and 22+/-13 nM for C. reinhardtii and P. sub-capitata, respectively. In the second series of experiments, the free Ag(+) concentration was held constant ( approximately 9 nM in the influent; 2-3 nM in the effluent) while the concentration of the silver thiosulphate complex, AgS(2)O(3)(-), was increased from 9 to 90 nM in steps. Under such conditions, the BLM would predict that silver uptake and toxicity should remain constant. On the contrary, both silver uptake and silver toxicity increased, indicating that the anionic silver thiosulphate complex enters the algal cells via a membrane-bound sulphate transporter and contributes to uptake and toxicity. However, for both algae there were indications that silver assimilated in this manner was somewhat less toxic to the algal cell than silver that entered via cation transport only. Physiological indicators of stress revealed possible different intracellular targets for these two freshwater algae, proteins and enzymes for C. reinhardtii and the photosynthetic apparatus for P. sub-capitata.