Fast algal eco-toxicity assessment: Influence of light intensity and exposure time on Chlorella vulgaris inhibition by atrazine and DCMU

In order to develop a rapid assay suitable for algal eco-toxicity assessments under conditions representative of natural ecosystems, this study evaluated the short-term (<1 h) response of algae exposed to atrazine and DCMU using oxygen productivity measurements. When Chlorella vulgaris was exposed to these herbicides under 'standard' low light intensity (as prescribed by OECD201 guideline), the 20 min-EC50 values recorded via oxygen productivity (atrazine: 1.32±0.07 μM; DCMU: 0.31±0.005 μM) were similar the 96-h EC50 recorded via algal growth (atrazine: 0.56 μM; DCMU: 0.41 μM), and within the range of values reported in the literature. 20 min-EC50 values increased by factors of 3.0 and 2.1 for atrazine and DCMU, respectively, when light intensity increased from 60 to 1400 μmol m−2 s−1 of photosynthetically active radiation, or PAR. Further investigation showed that exposure time significantly also impacted the sensitivity of C. vulgaris under high light intensity (>840 μmol m−2 s−1 as PAR) as the EC50 for atrazine and DCMU decreased by up to 6.2 and 2.1 folds, respectively, after 50 min of exposure at a light irradiance of 1400 μmol m−2 s−1 as PAR. This decrease was particularly marked at high light intensities and low algae concentrations and is explained by the herbicide disruption of the electron transfer chain triggering photo-inhibition at high light intensities. Eco-toxicity assessments aiming to understand the potential impact of toxic compounds on natural ecosystems should therefore be performed over sufficient exposure times (>20 min for C. vulgaris) and under light intensities relevant to these ecosystems.