Toxicity of cyanide and cyanide complexes to the marine diatom Nitzschia closterium

The effect of sodium cyanide (NaCN), iron-cyanide complexes and a blast furnace effluent on the growth of the unicellular marine diatom Nitzschia closterium was investigated. Due to the volatile nature of HCN, the free and total cyanide concentrations in all test solutions were monitored daily to ensure that average daily losses of free cyanide did not exceed 5%. The iron-cyanide complexes were less toxic than the NaCN, with 72-h EC50 values of 57, 127 and 275 μg total-CN litre−1 for NaCN, K3Fe(CN)6 and K4Fe(CN)6, respectively. The estimated lowest observed effect concentration (LOEC) for NaCN was 10 μg litre−1. The toxicity of the cyanide complexes was largely due to free cyanide (HCN + CN−) with possible additional toxicity caused by dissociation of the complexes at the algal cell membrane. The toxicity of an effluent from a blast furnace drawdown episode also correlated well with the toxicity of free cyanide.