Water quality objectives for mixtures of toxic chemicals: problems and perspectives.

The need to develop water quality objectives not only for single substances but also for mixtures of chemicals seems evident. For that purpose, the conceptual basis could be the use of the two existing biometric models: concentration addition (CA) and independent action (IA), which is also called response addition. Both may allow calculation of the toxicity of mixtures of chemicals with similar modes of action (CA) or dissimilar modes of action (IA), respectively. The joint research project Prediction and Assessment of the Aquatic Toxicity of Mixtures of Chemicals (PREDICT) within the framework of the IVth Environment and Climate Programme of the European Commission, provided the opportunity to address (a) chemometric and QSAR criteria to classify substances as supposedly similarly or dissimilarly acting; (b) the predictive values of both models for the toxicity of mixtures at low, statistically nonsignificant effect concentrations of the individual components; and (c) the predictability of mixture toxicity at higher levels of biological complexity. In this article, the general outline, methodological approach, and some preliminary findings of PREDICT are presented. A procedure for classifying chemicals in relation to their structural and toxicological similarities has been developed. The predictive capabilities of CA and IA models have been demonstrated for single species and, to some extent, for multispecies testing. The role of very low effect concentrations in multiple mixtures has been evaluated. Problems and perspectives concerning the development of water quality objectives for mixtures are discussed.

Time-dependent toxicity in the long-term inhibition assay with Vibrio fischeri.

The significance of the duration of exposure for the detection of toxicity was evaluated in a 24 h long-term bioluminescence inhibition assay with Vibrio fischeri. Bioluminescence was measured over the time course of 24 h using microplates. The undisturbed luminescence of controls in this assay exhibited characteristic dynamics: a decrease within a period of 12 h with minimal luminescence followed by a continuous increase of luminescence beyond the starting value. To evaluate the toxic influence of compounds chosen to reflect immediate and delayed toxicity to V. fischeri, the bioluminescence was measured for 24 h at 30 min intervals. Luminescence inhibition patterns were recorded for subdinoseb, pentachlorophenol and 2,4,5-trichlorophenol) and for substances causing delayed toxicity (chloramphenicol, nalidixic acid and phosphomycin). The toxic influence of substances with immediate toxicity was observed directly after application, whereas the toxicity patterns of substances with delayed toxicity developed specifically over the time according to the different involved mechanisms of action. It is concluded that knowledge about time to toxicity in bioassays is necessary in order to identify suitable test endpoints as well as to recognize potential hazards related to time-dependent toxicity.

Bioassays with Vibrio fischeri for the assessment of delayed toxicity.

The standardized bioluminescence assay with Vibrio fischeri underestimates the aquatic toxicity of chemicals which interfere with metabolic pathways supporting long term processes like growth and reproduction due to its short incubation time (30 min). Therefore this short term assay was compared with two alternative bioassays with prolonged incubation times using the same test organism: the growth inhibition assay (7 h) and the long term bioluminescence assay (24 h). Two sets of compounds were selected to reflect acute and delayed toxicity. The first group comprised pentachlorophenol, dodecylpyridiniumbromide and 3,4-dichloroaniline and the second nalidixic acid, chloramphenicol and streptomycinsulfate. The effects of compounds with acute toxicity are determined with similar sensitivity in all bioassays. Substances with delayed toxicity show only minor or no toxicities in the standardized short term bioassay but severe effects in both long term bioassays independent of the parameter used. It is concluded that the standardized short term bioluminescence assay exhibits serious limitations for the assessment of aquatic toxicity. The long term bioassays, however, may help to overcome these limitations.