A new tropical algal test to assess the toxicity of metals in freshwaters

Supervising Scientist Report 133
Franklin N, Stauber J, Markich S & Lim R
Supervising Scientist, 1998
ISSN 1325-1554
ISBN 0 642 24336 0

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Executive summary

Copper (Cu) and uranium (U) are of potential ecotoxicological concern to tropical Australian freshwater biota as a result of mining impacts. No local data on the toxicity of these metals to tropical freshwater algae are currently available. The aim of this study was to develop a toxicity test for an Australian tropical freshwater alga that can be added to the suite of tests currently available for tropical freshwater invertebrates and fish. This toxicity test was used to investigate the toxicity of Cu and U to the alga Chlorella sp (new species) in a synthetic softwater and to specifically determine the effect of pH on metal toxicity over the range typically found in soft fresh surface waters in tropical northern Australia.

A growth inhibition toxicity test was successfully developed for this alga, which was isolated from Kakadu National Park, Northern Territory, prior to conducting the toxicity testing. Key environmental parameters including light, temperature and nutrients were optimised to obtain acceptable algal growth rates over 72 hours. HEPES buffer (2 mM at pH 6.5) was found to be a suitable and practical option for pH control that could be incorporated in the test protocol for Chlorella sp. The results obtained in this study confirmed a lack of toxic effects by HEPES on the algae, as well as negligible complexation with both Cu and U. Adequate pH control (ie

Based on the minimum detectable effect concentration (MDEC), Chlorella sp was more sensitive to Cu (0.7 µg L-1) than U (13 µg L-1), and more sensitive than other Australian tropical freshwater species, with an order of sensitivity: Alga ≥ Crustacea > Cnidaria > Mollusca > Chordata. The toxicity of Cu and U was highly pH-dependent. Copper concentrations needed to inhibit growth by 50% (72 h EC50) increased from 1.5 to 35 µg Cu L-1 as the pH decreased from 6.5 to 5.7. The 72 h EC50 for U increased from 44 to 78 µg U L-1 over the same pH range. Decreased toxicity at pH 5.7 was due to lower concentrations of cell-bound and intracellular Cu and U compared to that at pH 6.5. These results are explained in terms of the possible mechanism of competition between H+ and the metal ion at the cell surface.

The comparative sensitivity of Chlorella sp to Cu and U was also assessed. Chlorella sp was two times more sensitive to Cu than to U at pH 5.7 and up to 30 times more sensitive to Cu at pH 6.5 on a weight basis. However, on a molar basis, Chlorella sp was two times more sensitive to U than to Cu at pH 5.7. At pH 6.5, Cu was >8 times more toxic to the alga than U.

This species was sensitive enough to detect adverse effects of Cu at the ANZECC guideline values of 5 µg Cu L-1, making it a sensitive test organism for the assessment of Cu contamination of freshwaters. However, the unusual, often non-sigmoidal, concentration-response curve for Chlorella sp may reduce the reproducibility of the toxicity test. Despite this, Chlorella sp does possess a number of desirable characteristics for use in toxicity assessment and therefore is recommended to be used as part of a battery of toxicity tests with other local freshwater organisms. In particular, the alga's high sensitivity to Cu and U and environmental relevance make it a suitable choice for site-specific testing of mine wastewaters in tropical Australia. The findings obtained in this study have the potential to be incorporated into future revisions of the Australian water quality guidelines.