What contributes to the sensitivity of microalgae to triclosan?

Differential sensitivities of microalgae to triclosan have been reported, which may have significant implications for environmental risk assessment of this widely used biocide. Therefore, the aim of this study was to derive a mechanistic understanding of varying microalgal sensitivity to this substance. The toxicity of triclosan was evaluated using microalgal systems varying in biological complexity, exposure time and systematic position (a synchronized culture of the chlorophyte Scenedesmus vacuolatus, a diatom Nitzschia palea cultivated in suspension as well as attached to surfaces and periphyton communities). The results revealed (1) differences in sensitivity of the selected microalgal systems of three orders of magnitude and (2) highest sensitivity of the chlorophyte to triclosan in the range of environmental concentrations. To investigate algal sensitivity to triclosan in more detail, bioavailability was considered by investigating suspended and attached living algae. Differences in the generation time (in comparison to test duration) of the species were addressed by evaluating and modeling concentration-time-effect relationships. However, varying sensitivities of the selected microalgal systems remained unexplained. Comparison of species-specific toxic responses to calculated effect concentrations, derived from quantitative relationships for narcosis and uncoupling mode-of-action, leads us to the conclusion that triclosan may address multiple target sites in different microalgal species.