Individual and combined toxicity of nitriles and aldehydes to Raphidocelis subcapitata

A closed-system algal toxicity test with no headspace was applied to evaluate the toxicity of 11 aldehydes and two nitriles to Raphidocelis subcapitata. Algal growth rate and dissolved oxygen (DO) production were used as the test endpoints. Compared to literature data, our test results based on the endpoint of DO production is 2.5 to 257 times more sensitive than the conventional algal batch tests. In addition, our analyses show that different relative toxicity relationships may be observed when different test methods or different endpoints are applied. The test alga was found to be quite sensitive to aldehydes, and a quantitative structure-activity relationship (QSAR) relationship was established based on the n-octanol/water partition coefficient (log Kow). This study also evaluated the combined effects of aldehydes and nitriles. Four synergistic joint actions related to malononitrile were identified. On the other hand, most of the combined effects between aldehydes and acetonitrile were antagonistic. In general, we find that greater-than-additive effects mainly are associated with toxicants displaying flat dose-response curves, and less-than-additive effects may be related to certain steep-slope chemicals. Model analyses show that the above mixture toxicity behavior may be due to response addition or response multiplication joint action modes.