The purpose of this test is to determine the effects of a test substance or environmental samples on the growth of freshwater green algae. These algae play a very important role as primary producers in aquatic systems. Exponentially growing green algae are exposed to the test substance or environmental sample over a period of 72h and inhibition of growth is analysed. In spite of the relatively brief test duration, effects over several generations can be assessed.
The planktonic freshwater algae species Desmodesmus subspicatus or Raphidocelis subcapitata (former name: Pseudokirchneriella subcapitata) are exposed to a dilution series of the test substance or environmental sample and incubated under defined light and temperature conditions for 72 h. Three replicates for each test concentration and six control replicates without test substance are studied in parallel. Growth of the algae is analysed after 24, 48 and 72 h incubation by measuring a surrogate parameter for biomass, such as cell counts, fluorescence, optical density etc. The test endpoint, inhibition of growth, is calculated by comparing the logarithmic increase in biomass during the exposure period (average specific growth rate) for each test concentration with the growth rate in the control batch. Dose-effect relationships are calculated by means of regression analysis and from this the 50% effect concentration regarding growth (ErC50) is determined. When testing waste water by means of a graduated dilution, the test medium with the highest concentration at which an inhibition <5% is observed is termed the lowest ineffective dilution (LID) and reported as final result of the test.