Influence of calcium and natural organic matter on the toxicity of an organophosphorus insecticide

Natural organic matter (NOM) and calcium (Ca2+) are capable to alter resorptivity and toxicity of organic chemicals in aquatic systems. Therefore, we exemplarily studied the impact of NOM and Ca2+ on the toxicity of the organo-phosphorous insecticide trichlorfon (TCF) to early life stages of zebrafish (Danio rerio). Embryo and larvae were exposed to different TCF concentrations for 144 h. Combinations of different Ca2+ and NOM concentrations were used during the exposure. The results show an enhanced survival of fish in high Ca2+ concentrations (p<0.001), but an increased mortality at low Ca2+ conditions. We suggest that TCF acts as a complexing agent, which is able to bind Ca2+ ions to its functional groups. There appear to be two potential modes of action. First, the positive charge of the TCF-Ca2+ complex avert from uptake, or, additionally, TCF is protected from breakdown to the most toxic compound dichlorvos (DDVP). If NOM is added to the test solution the toxicity is enhanced, even if Ca2+ is present in the water. This effect is significant in low Ca2+ water due to the absorption of Ca2+ ions by NOM that are not longer available for detoxification.