Paper-disc method: An efficient assay for evaluating metal toxicity to soil algae

Abstract

The probabilistic ecological risk assessment using terrestrial toxicity data has been mainly based on microfauna or mesofauna. Soil algae, which are food source for microfauna and mesofauna, may be alternatively used for assessing soil toxicity. However, there are no internationally recommended guidelines for soil algal bioassays, and the collection of algae from the test soils has some limitations. In this study, we suggested the paper-disc method as an easy-to-use alternative. This method has been widely used for testing the antibacterial toxicity of various chemicals in agar media by measuring the diameter of the inhibition zone around the disc. We adapted the paper-disc method for screening the toxicity of copper (Cu) and nickel (Ni) to the soil alga Chlorococcum infusionum using various evaluation endpoints, such as growth zone, chlorophyll fluorescence, and photosynthetic activity. Chlorophyll fluorescence and photosynthetic activity decreased with the increasing concentrations of Cu+2 or Ni+2 contaminated soils. Algal growth zone was analyzed visually and showed similar results to those of chlorophyll fluorescence. The direct ethanol extraction method and indirect culture medium extraction method were similarly effective; however, the former was easier to perform, while the latter might facilitate the analysis of additional endpoints in future studies. Overall, the results suggested that the paper-disc method was not only a user-friendly assay for screening soil toxicity, but also effective due to its association with indirect soil quality indicators.