Toxicity of chlorate to marine microalgae

Abstract

Recent concern over the release of chlorate into the aquatic environment has focused on its occurrence in industrial effluents and its presence in hypochlorite solutions used in drinking water disinfection. While algae are known to be more sensitive than invertebrates and fish to chlorate, very little information on chlorate toxicity to marine microalgae is available. In particular, the mechanism of toxicity of chlorate and the effect of ambient nitrate levels on chlorate uptake into algae has not been well studied. The effect of chlorate on growth and nitrate reductase activity in two species of marine microalgae Nitzschia closterium and Dunaliella tertiolecta was investigated as part of a risk assessment for the construction of a new bleached eucalypt kraft (BEK) pulp mill on the south-east coast of Australia. Nitrate inhibited chlorate uptake and toxicity in both species. Under nitrate-limiting conditions typically found in oligotrophic waters (<0.005 mg l −1), 72-h EC 50 values for chlorate were 1.9 and 11 mg l −1 for Nitzschia and Dunaliella respectively. At high nitrate concentrations both algae were insensitive to chlorate, with 72-h EC 50 values greater than 500 and 1000 mg l −1 respectively. In Dunaliella only, when chlorate was in excess of nitrate (on a molar basis), nitrate uptake was reduced due to inhibition of nitrate reductase activity. It is possible that in Dunaliella, as in higher plants, chlorate is converted by the enzyme nitrate reductase to chlorite which is the toxic agent. In Nitzschia, however, nitrate reductase activity was low even in the absence of chlorate. At chlorate concentrations inhibitory to cell division, there was no effect on photosynthesis, ATP or cellular thiols in Nitzschia. A small decrease in dehydrogenase activity was found after a 24-h exposure to chlorate, but this was not coupled to a decrease in ATP. It appears that chlorate toxicity to Nitzschia at low nitrate concentrations does not depend on chlorite as an intermediate. This study confirms that chlorate is toxic to marine microalgae, particularly in nitrate limited waters. Sensitivity to chlorate and its mechanism of toxicity appear to differ between species.