Application of the narcosis target lipid model to algal toxicity and deriving predicted-no-effect concentrations

The narcosis target lipid model (TLM) was developed to predict the toxicity of chemicals to aquatic organisms that act via narcosis. It is based on the hypothesis that target lipid is the site of toxic action within the organism, that octanol is the appropriate surrogate, and that target lipid has the same physical-chemical properties in all organisms. Here the TLM is extended to available freshwater green algal toxicity data to support a narcosis toxic mode-of-action (TMoA) effect assessment. For each species, significant linear relationships were observed between log(median effective concentration [EC50]) and log(K-OW) of the test chemicals. The slope of the log-log relationship statistically was similar to the universal narcosis slope of -0.945 that was derived from an earlier analysis of the TLM to nonalgal species. Critical target lipid body burdens (CTLBB), C-L* were estimated for each algal species from the intercepts of the regressions and found to be within the range (43-398 mumol/g octanol) reported previously, indicating that algae exhibit a similar sensitivity distribution relative to other aquatic species. The TLM is used to derive the predicted-no-effect concentrations (PNECs) using the hazardous concentration to 5% species (HC5) statistical extrapolation procedure. This calculation requires an analysis of the variability of the universal slope, the C-L*, and the acute-to-chronic ratio. The PNECs derived using this procedure were consistent with chronic-no-effect concentrations reported for narcotic chemicals. This is in contrast to PNECs derived from limited chemical-specific toxicity data and default application factors. It is concluded that coupling the TLM to the HC5 extrapolation procedure allows optimal use of available toxicity data for deriving environmental quality criteria with a narcotic TMoA.