Effects of nutrient trace metal speciation on algal growth in the presence of the chelator [S,S]-EDDS

This study tests the hypothesis that the apparent toxicity of strong chelators in standard algal growth inhibition tests (e.g. method OECD 201, EC C.3., ISO 8692) is related to essential trace metal bioavailability. This hypothesis was investigated for the chelator [S,S]-ethylene diamine disuccinate ([S,S]-EDDS) and the green alga Chlorella vulgaris. Metal speciation calculations were used to help design the algal growth experiments and interpret the data. Results suggest that interaction of the chelator with trace metals alters the free metal concentration and affects algal population growth, as opposed to a direct interaction between the alga and the chelator (toxicity sensu stricto). Even low levels of[S,S]-EDDS (i.e. 3 mg l(-1) or less) reduce the free p(Cu) and p(Zn) (p(Metal) = -log[Metal]) in standard OECD medium below 16 and 11, respectively, which are the minimum levels required to support algal growth. Nutrient deficiency was overcome by supplementing the medium with appropriate amounts of the trace metals Cu, Zn and Co, but not by increasing the hardness of the medium. A short-term photosynthesis inhibition experiment with the alga Selenastrum capricornutum in metal-free medium showed only a minimal effect of [S,S]-EDDS on the C-14-CO2 fixation rate. About 10% inhibition was observed at 100 mg [S,S]-EDDS l(-1), i.e. the EC(50) for CO2 fixation is greater than 100 mg l(-1). Results from this study illustrate that the standard algal growth inhibition test is not well suited to the assessment of algal toxicity (sensu stricto) of strong chelators. The no-effect level and EC(50) value are probably overestimated by at least one order of magnitude for [S,S]-EDDS. The study also illustrates the importance of speciation calculations when assessing algal inhibition by chelators.