Monitoring toxicity of an azo dye methyl red and a heavy metal Cu, using plant and animal bioassays

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Toxicities of an azo dye methyl red and a heavy metal copper (Cu) were quantified, using growth and mortality as end points, in four plant species and three animal species by subjecting them to short-term (4 days for animals, 10 days for plants) static bioassays. Lemna aequinoctialis Welwitch (EC50: 7–16 ppm) was found to be the most sensitive species for methyl red, Ceratophyllum demersum L. (EC50: 25 ppm) and Lactuca sativa L. (EC50: 56 ppm) were intermediate, while Oryza sativa L. shows reduction in seedling vigor (9–27%) of <50%, being the least sensitive amongst the tested plant species. Methyl red toxicity is almost 3–5-fold higher in growing medium (pH = 5.8–6.0), even at high nutrient levels, while Cu toxicity is higher in nutrient-poor alkaline medium at alkaline pH (8.3–8.7; EC50: Ceratophyllum = 104–200 ppb; Lemna = 100–170 ppb) compared to nutrient-rich acidic medium (pH = 5.4–5.7; EC50: Ceratophyllum = 2600–3175 ppb; Lemna = 4350–4715 ppb). Rice tolerance (EC50: 6500 ppb) was found to be higher than hydrophytes while lettuce was most tolerant to Cu. Fish sensitivity toward the test chemicals was almost parallel to Ceratophyllum and Lemna [Gambusia affinis Baird and Gerard (LC50: 250 ppb for Cu) and Poecilia reticulata Peters (LC50: 24 ppm for methyl red)]. Similar to the plants, dye toxicity increased markedly (LC50: 7 ppm) in the acidic medium (pH = 6.0). Amongst the tested organisms, Daphnia was found to be most sensitive to methyl red (EC50: 6 ppm) while its sensitivity to Cu (EC50: 230 ppb) was similar to hydrophytes and fish. Initially, the combination of dye and Cu (at their sublethal concentrations) had additive effects in duckweed, while dye concentration ruled afterward. These results indicate that hydrophytes and animals are equally sensitive toward the test chemicals. Dye toxicity in hydrophytes and fish was pH dependent, while in the case of Cu, it is related to the nutrient status of the growth medium of plants.