Toxicity of chlorate and chlorite to selected species of algae, bacteria, and fungi.

The present study confirms that chlorate is toxic only to brown algae and not to species of other ecologically relevant taxa. The brown alga Ectocarpus variabilis exhibited a LOEC of 0.005 mM (0.4 mg ClO3-/liter) and an LC50 of 0.012 mM, when cultured with nitrate as a sole source of nitrogen. The toxicity to species other than brown algae as measured in growth inhibition tests ranged from 0.75 mM (96-h NOEC) for Selenastrum capricornutum to > or = 7.48 mM (48-h NOEC) for the fungus Trichoderma hamatum. The nitrogen source, nitrate or ammonium, did not significantly influence the toxicity to the nonsensitive species. The tests on brown algae found that as compared with ammonium, the toxicity to nitrate-grown cultures is higher by a factor of about 10. This confirms the hypothesis that nitrate reductase is involved in the toxic effects of chlorate on brown algae. Chlorite, tested as a potential toxic metabolite of chlorate, demonstrated high toxicity to many of the taxa tested and only low toxicity to E. variabilis. It may be concluded that brown algae are exceptionally sensitive to chlorate. It may also be concluded that various nitrogen sources could not induce toxicity in nonsensitive species. From these experiments no conclusions could be drawn as to the potential role of chlorite in chlorate toxicity. Furthermore it may be concluded that E. variabilis is a suitable laboratory test species for further investigations into the mechanism of chlorate toxicity to brown algae.

The ecotoxicity of chlorate to aquatic organisms: a critical review.

In order to assess the risk posed by chlorate in aquatic ecosystems, data on the effects of chlorate on aquatic organisms (microorganisms, algae, invertebrates, and fish) and mesocosm studies have been collated and critically reviewed. The geometric mean E(L)C50 values for both freshwater and marine species were (as ClO3-): microorganisms, 38,583 mg.liter-1; microalgae, 563 mg.liter-1; invertebrates, 2442 mg.liter-1; fish, 3815 mg.liter-1. Marine macro red algae were insensitive to chlorate, whereas marine macro brown algae (e.g., Fucus sp.) appeared to be exceptionally sensitive to chlorate, adverse long-term effects having been reported at concentrations as low as 0.015 mg ClO3-.liter-1. Evidence for the mechanism by which chlorate is thought to be particularly toxic to these species is also reviewed. It is concluded that, based on the species reported, chlorate is nontoxic (acute toxicity > 100 mg.liter-1) to most of the freshwater and marine species examined. However, chlorate is highly toxic (acute toxicity < 0.1 mg.liter-1) to certain macro brown algal species. For macro brown algae, the NOEC after 6 months was reported to be approximately 0.005 mg ClO3-.liter-1. It is also concluded that an improved understanding of the actual mode of action of chlorate in sensitive species is desirable. Together with further information on the environmental fate of chlorate, this will improve the risk assessment for chlorate in the aquatic environment.