Algal growth inhibition test - feasibility and limitations for soil assessment

Unknown toxic substances in environmental samples can be detected using a combined procedure including chemical analyses and ecotoxicological tests. A commonly applied test system for the assessment of aqueous extracts of environmental samples is the growth inhibition test with algae. In the presented study the influence of nutrients present in aqueous extracts on algal growth was investigated and consequences for the test interpretation are discussed. Depending on the concentration of nitrogen and phosphorus either stimulation or inhibition can be observed. The influence of these nutrients can be compensated by preparing a second control (nutrient control) as reference for the sample. Besides nitrogen and phosphorus further unknown nutrients seem to have an influence resulting in an uncontrollable stimulation of growth. A test with autoclaved extract showed that the stimulating compounds were already in the extract and were not released by microorganisms during the test. A stimulation of algal growth due to nutrients may mask the effects of toxic substances. Therefore, false negative results can be obtained when using the test with algae for samples containing low concentrations of contaminants or high concentrations of unknown stimulatory substances. According to these findings it should be considered whether the chronic growth inhibition test with algae should/could be replaced by an acute test when dealing with aqueous extracts of solids rich in nutrients. Possible test designs are already available.