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Your Location:Home > Thiourea Effect on other organisms in the laboratory and field

Aquatic environment

Numerous tests have been performed on the toxicity of thiourea to all trophic levels of aquatic organisms. Experimental test results for the most sensitive species are summarized in Table 5. Additional data on the toxicity of thiourea to aquatic organisms are cited in the BUA (1995) report. Among the tested organisms, green algae (Scenedesmus subspicatus) and water flea (Daphnia magna) proved to be the most sensitive freshwater species. The lowest EC50 value determined in a 96-h cell multiplication inhibition test was reported to be 3.8 mg/litre for S. subspicatus. For immobilization of D. magna, a 96-h EC50 value of 1.8 mg/litre was determined. In two long-term tests with D. magna, 21-day NOEC values of <0.25 mg/litre and 0.25 mg/litre were established for reproduction. It has to be taken into account that concentration–response curves in many acute tests on daphnia are very flat and difficult to reproduce, leading to very variable effect values (BUA, 1995). With respect to freshwater fish, all tests available for short-term exposure revealed LC50 values (48- and 96-h) at or above 100 mg/litre. Experimental results from long-term fish studies conducted to standard test procedures are not available. However, many authors have studied the effects of long-term exposure of teleosts and other kinds of fish to thiourea. Effects of thiourea (exposure concentrations: 20–330 mg/litre) on thyroid gland metabolism and the endocrine system have been described (Mackay, 1973; McBride & Van Overbeeke, 1975; Sathyanesan et al., 1978; Saxena & Mani, 1979).

Numerous investigations have been performed on the inhibition of microbial nitrification by thiourea (see Table 5), leading to very heterogeneous results. In short-term toxicity tests conducted with non-adapted activated sludge, inhibition of nitrification was observed at thiourea concentrations as low as 0.075 mg/litre (2- to 4-h IC75), whereas NAPM (1974a,b) determined an IC0 of 100 mg/litre for this end-point. Sensitivity is obviously highly dependent on the origin and adaptation of the specific microbial consortium. Tests on respiration inhibition revealed IC0 values of >100 mg/litre for activated sludge (NAPM, 1974a,b; Grünwald, 1984) and IC50 values of up to 4500 mg/litre. From the available studies, it can be concluded that microorganisms are able to adapt to thiourea.

Terrestrial environment

Laboratory tests on the toxicity of thiourea to terrestrial species have been performed with microorganisms, higher plants, and invertebrates (earthworms, nematodes, insects). Experimental test results for the most sensitive species are summarized below. Additional data on the toxicity of thiourea to terrestrial species are cited in the BUA (1995) report. Among the tested organisms, different stages of the red cotton bug (Dysdercus similis) proved to be most sensitive, exhibiting EC50 values of 0.03 and 0.025 mg/litre for egg survival and hatching, respectively.

Different fungi were found to be relatively insensitive to thiourea exposure. Complete growth inhibition was observed for Penicillium rugulosum after a 7-day exposure to 2000 mg thiourea/litre (Lashen & Starkey, 1970) and for Helminthosporium sativum and Fusarium oxysporum after a 15-day exposure to 750 mg/litre and 1000 mg/litre, respectively (Pandey et al., 1976).

Terrestrial plants proved to be generally more sensitive. Whereas thiourea concentrations below 12 mg/litre increased the growth of excised tomato roots (Lycopersicum esculentum) within 4 weeks of exposure in a defined basal medium, 18, 23, and 46 mg/litre reduced growth by about 45%, 60%, and 30%, respectively (Glazer & Orion, 1984). Friesel et al. (1984) obtained 14-day EC50 values of 15 mg/kg soil dry weight (turnip, Brassica rapa) and 190 mg/kg soil dry weight (common oat, Avena sativa) in a study conducted according to the draft of the OECD guideline "Growth Test with Higher Plants" (1981; adopted in 1984 as OECD Guideline 208). Rudolph & Boje (1985) reported 14-day EC50 values in the range of 205–618 mg/kg soil dry weight and 190–618 mg/kg soil dry weight for B. rapa and A. sativa, respectively. In greenhouse experiments, Günther & Pestemer (1990) determined a 10-day EC50 of 52.1 mg/kg for the end-point growth/germination of B. rapa. In experiments with 8 weeks of exposure of A. sativa to thiourea in soil solution, Günther & Pestemer (1990) observed that during the first 4 weeks of exposure, the EC50 value for growth reduction dropped from 170 mg/litre after 2 weeks over 80 mg/litre after 3 weeks to 30 mg/litre after 4 weeks. This value remained constant during the course of the following 4 weeks.

Friesel et al. (1984) investigated the toxicity of thiourea towards the earthworm Eisenia fetida according to the OECD draft "Guideline on Testing the Toxicity of Chemicals and Plant Protection Agents towards the Earth Worm" (adopted as OECD Guideline 207 in 1984). They determined a 28-day LC50 of 3550 mg/kg soil dry weight. Rudolph & Boje (1985) reported a 28-day LC50 of >1000 mg/kg soil dry weight for E. fetida.

Glazer & Orion (1984) investigated the effects of thiourea on the development of nematodes. Excised tomato roots, growing on basal medium and inoculated with eggs of Meloidogyne javanica, were exposed to thiourea concentrations in the range of 6–46 mg/litre. After 96 h of exposure, thiourea concentrations of 12 mg/litre inhibited nematode development. Only 36% matured to adults (in the untreated control: 90%) after an observation period of 4 weeks. For M. javanica (second larval stage), Tylenchulus semipenetrans (second larval stage), and Pratylenchus thornei (adult and juvenile organisms), no increased mortality was found after incubation in aqueous solutions of thiourea at concentrations up to 100 mg/litre for 96 h. The authors furthermore demonstrated that thiourea is taken up via the plant roots and that the nematicidal effect is systemic.

Bhide (1991) investigated the effect of different thiourea concentrations on eggs and nymphs of the red cotton bug (Dysdercus similis), a cotton plant pest. Solutions were applied topically to larval stages 1–5 and, for imagos, additionally in the diet. EC50 values of 0.03 mg/litre and 0.025 mg/litre were determined for egg survival and hatching, respectively. Thiourea concentrations in the range of 0.01–0.025 mg/litre reduced adult emergence by 50%. When nymphal instars were exposed topically, a thiourea concentration of 100 mg/litre proved to be lethal, with all the nymphs at all the various stages of development dying within 6 h.

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