Abstracts
Résumé
Une étude écotoxicologique a été menée à l'égard du Tébuthiuron (TB), un phytocide homologué inhibant la croissance de végétation nuisible, afin de mieux cerner son impact sur le milieu aquatique susceptible d'être affecté par des épandages terrestres. La toxicité du TB a été évaluée en réalisant des bioessais à trois paliers écologiques, soit avec la truite arc-en-ciel (Salmo gairdneri), avec l'algue Selenaslrum capricornutum et avec la bactérie (Photobacterium phosphoreum) du système Microtox®. Parmi ces trois indicateurs, les algues se sont montrées les plus sensibles (C150 = 0,08 mg • L-1), suivies des truites (CL50 = 115 mg • L-1) et enfin, des bactéries du système Microtoxe (C150 - 328 mg • L-1). Des résidus maximaux variant de 0,091 à 0,18 mg • L-1 rapportés pour le TB en milieu aquatique, suite à des applications expérimentales, laissent donc croire que seules les algues pourraient étre victimes d'une agression marquée. D'autre part, les essais réalisés avec le SOS Chromotest ont démontré que le TB était faiblement génotoxique sans activation métabolique. En revanche, des algues exposées à 1 mg • L-1 de TB durant 4 heures n'ont pu ni (dés)actlver ni bioaccumuier l'herbicide. En général, notre enquéte corrobore certaines données générées pour fins d'homologation de ce produit, lesquelles concluaient en faveur de son innocuité relative à l'égard de l'environnement aquatique. Les effets chroniques que pourrait avoir une longue exposition de faibles concentrations de TB sur certains paliers écologiques devraient cependant faire l'objet d'lnvestigations futures.
Mots-clés:
- Tébuthiuron,
- herbicide,
- toxicité,
- génotoxicité
Abstract
A wide array of chemical products are commonly used to inhibit the growth of a diversity of undesirable vegetation types for numerous purposes. In forestry applications, herbicides can improve ranges, contribute to sylviculture and facilitate rights-of-way management, for example. Among congeners of the substituted urea class herbicides, Tebuthiuron (TB) has proven efficient for such activities. Since its commercial appearance in 1974, this broad-spectrum weed killer was employed to control a variety of herbaceous and woody plants. When applied on soil before or during the onset of plant emergence, TB irreversibly affects photosynthesis after being absorbed by roots and translocated to its target sites. Prior to its registration as a herbicide, TB had undergone extensive (eco)toxicological testing which had generally indicated low potential for environmental concern, with regards to terrestrial and avian fauna. Although TB is generally purported to be unproblematic towards fish, the overall impact of substituted urea class herbicides is still not fully documented, as far as various members of the aquatic community are concerned. The experimental results presented herein - specifically on TB - contribute both confirmatory as well as some new information in this respect.
In our study, TB toxicity was investigated at three ecological levels by undertaking acute bioassays with rainbow trout (Salmo gairdeneri), algae (Selenastrum capricornutum) and bacteria (Photobacterium phosphoreum). Among these bio-indicators, algae proved to be the most sensitive (EC50 = 0.08 mg • L-1), followed by rainbow trout (LC50 = 115 mg • L-1) and bacteria (EC50 = 328 mg • L-1). Since maximum TB residues lying between 0.091 and 0.18 mg • L-1 have been reported for aquatic systems following experimental terrestrial applications, our toxicity results suggest that only algae could be adversely affected following acute exposure to the herbicide.
Additional tests performed with the SOS Chromotest, a bacterial colorimetric assay for detecting DNA-damaging agents, first showed that TB is weakly genotoxic without metabolic activation. Since recent genotoxicity studios have revealed that vegetal systems can either detoxify, activate or uptake specific chemicals, we then explored this possibility by exposing S. capricornutum (106 cells • mL-1) to 1 mg • L-1 of TB for 4 h. Results of this acute exposure indicated an absence of positive (detoxication) or negative (activation, accumulation) phytoplanktonic interactions. Indeed, the genotoxic characteristics of TB, before and after algal exposure, were unaltered, as demonstrated by SOS Chromotest assays. In this same experiment, a similar assay on TB-exposed algal cells (i.e. SOS Chromotest on an algal cell solvent extract) detected no genotoxic activity.
In conclusion, our study corroborates existing data generated for TB registration purposes and essentially supports the notion that this chemical is relatively harmless towards the aquatic environment under normal use conditions. Nevertheless, an important caveat remains concerning chronic affects on specific organisms, which could result from long term exposure to low concentrations of TB. Since such potential effects have not yet been adequately addressed, further studios are warranted in this area.
Keywords:
- Tebuthiuron,
- herbicide,
- toxicity,
- genotoxicity
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