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(M/EH124) pH buffering using MOPS does not affect copper and zinc toxicity for Daphnia magna and Selenastrum capricornutum. De Schamphelaere, Karel1, Heijerick, Dagobert1, Janssen, Colin1, 1 ABSTRACT- Aquatic ecotoxicologists are often confronted with problems associated with the use of pH buffers, especially when performing toxicity assays with metals in field-collected waters. Sampling of a surface water disrupts its chemical equilibrium which in most cases results in a pH increase (up to 2 pH units) of the sample and consequently in an altered metal toxicity. To adequately asses the toxicity of metals in natural waters, one therefore needs to add a pH buffer. Our initial choice of using MOPS (3-N-morfolino propane sulfonic acid) in bioassays was based on its property to be completely non-metal-complexing (Kandegedara and Rorabacher, Anal. Chem., 1999, 71, 3140-3144), its usefull pH range (6-8) and its recommended use for pH buffering in sediment toxicity testing (U.S. EPA). However, to our knowledge, it has never been fully tested if MOPS affects metal toxicity for standard test organisms like Daphnia magna and Selenastrum capricornutum. Firstly, we show that up to a concentration of 750 mg L-1 (which proved to be sufficient for pH buffering) MOPS did not affect 21-day net reproduction of D. magna or the 3-day growth rate of S. capricornutum, which means that both organisms physiology is not inhibited. Secondly, we conducted bioassays with copper and zinc spiked standard media (EEG) for the pH range 6 to 8. For D. magna the possible effect of 750 mg L-1 MOPS on copper and zinc 48-hour EC50s was investigated by performing parallel bioassays in NaHCO3 and MOPS buffered media. For S. capricornutum 3-day bioassays (growth rate) were performed in parallel with both MOPS pH buffering and daily manual pH adjustment with HCl. No significant differences in copper and zinc toxicity were observed between MOPS, NaHCO3 or HCl buffered test media. Thirdly, also no effect of MOPS on acute copper and zinc toxicity was observed in additional bioassays with natural waters. The results of this study strongly favour the use of MOPS whenever pH buffering is required for assessing metal toxicity tresholds in field-collected water samples. Key words: metal toxicity, Daphnia magna, Selenastrum capricornutum, pH buffering |
