Delayed fluorescence in algal growth inhibition tests
ABSTRACT A series of 72-hour growth inhibition tests with green alga Desmodesmus (Scenedesmus) subspicatus (ISO 8692) has been performed to test the delayed fluorescence (DF) parameters as possible endpoint measurements. Sensitivity
to five toxicants with direct and indirect effects on photosynthesis was tested, and the median effective concentration (EC50) values derived from the cell concentration, absorbance and DF were compared. The sensitivity of DF intensity (DFI) was comparable
with the two endpoints suggested in ISO 8692 for all five toxicants: potassium dichromate, 3,5-dichlorophenol (3,5-DCP), DCMU,
copper and cadmium. In the case of potassium dichromate and copper, DFI was more sensitive than the other endpoints in this
study. The analysis of DF relaxation kinetics showed a specific response to the herbicide DCMU. Additionally, a 24-hour test
was performed with the same toxicant concentrations (except copper), where DFI was measured 30 minutes, one hour and 24 hours
after the exposure. The influence of toxicants on DFI after a 24-hour exposure was comparable with the effects after 72 hours.
Only 3,5-DCP influenced DFI after 30 minutes. DF proved to be a simple, reliable and rapid measurement to assess toxicity
in algal tests, which can to some extent differentiate among various toxicants.