TOXICITY OF CHLORATE TO MARINE MICROALGAE

Recent concern over the release of chlorate into the aquatic environme nt has focused on its occurrence in industrial effluents and its prese nce in hypochlorite solutions used in drinking water disinfection. Whi le algae are known to be more sensitive than invertebrates and fish to chlorate, very little information on chlorate toxicity to marine micr oalgae is available. In particular, the mechanism of toxicity of chlor ate and the effect of ambient nitrate levels on chlorate uptake into a lgae has not been well studied. The effect of chlorate on growth and n itrate reductase activity in two species of marine microalgae Nitzschi a 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 inhibite d chlorate uptake and toxicity in both species. Under nitrate-limiting conditions typically found in oligotrophic waters (<0.005 mg l(-1)), 72-h EC50 values for chlorate were 1.9 and 11 mg l(-1) for Nitzschia a nd Dunaliella respectively. At high nitrate concentrations both algae were insensitive to chlorate, with 72-h EC50 values greater than 500 a nd 1000 mg l(-1) respectively. In Dunaliella only, when chlorate was i n excess of nitrate ion a molar basis), nitrate uptake was reduced due to inhibition of nitrate reductase activity. It is possible that in D unaliella, as in higher plants, chlorate is converted by the enzyme ni trate reductase to chlorite which is the toxic agent, In Nitzschia, ho wever, nitrate reductase activity was low even in the absence of chlor ate. At chlorate concentrations inhibitory to cell division, there was no effect on photosynthesis, ATP or cellular thiols in Nitzschia. A s mall decrease in dehydrogenase activity was found after a 24-h exposur e to chlorate, but this was not coupled to a decrease in ATP. It appea rs that chlorate toxicity to Nitzschia al low nitrate concentrations d oes not depend on chlorite as an intermediate. This study confirms tha t chlorate is toxic to marine microalgae, particularly in nitrate limi ted waters. Sensitivity to chlorate and its mechanism of toxicity appe ar to differ between species. (C) 1998 Elsevier Science B.V. All right s reserved.