Acartia tonsa eggs as a biomonitor to evaluate bioavailability/toxicity of persistent contaminants in anoxic/sulfidic conditions: The case of cadmium and nickel.

The evaluation of toxicity due to persistent pollutants in anoxic aquatic environments has met with various problems, as most test organisms can not withstand oxygen lack and exposure to free sulfide. We evaluated the suitability of the eggs of the brackish copepod Acartia tonsa for bioassays in anoxic/sulfidic conditions: when exposed to deep hypoxia and free sulfide, the eggs become quiescent and are able to resume hatching after restoring normoxic conditions. Tests with cadmium and nickel were performed in normoxic and deeply hypoxic conditions and in anoxic water containing H2S or H2S+FeSO4 on an equimolar basis. Active and quiescent eggs showed equivalent sensitivity to the metals, both suffering significant reductions in hatching success at 89µM Cd and 17µM Ni. As expected on the basis of the SEM/AVS model, Cd toxicity was almost completely suppressed in presence of sulfides. Dissolved Cd concentration drastically dropped and hatching success was generally >80%, as against values <6% observed in sulfide-free water, indicating that the applied experimental procedure can simulate metal-sulfide interaction. Ni toxicity was only slightly reduced by the presence of sulfides. High dissolved Ni concentrations were detected and mean hatching percentages were ≤32%, suggesting that Ni bioavailability/toxicity was only partially controlled by excess reactive sulfides. The results suggest that A. tonsa eggs could be a useful biomonitor to evaluate toxicity due persistent contaminants in anoxic conditions and the role of sulfides in reducing metal bioavailability/toxicity.

Standardized methods for acute and semichronic toxicity tests with the copepod Acartia tonsa.

The availability of standardized protocols for both organism culture and bioassay with ecologically relevant species is of great concern in ecotoxicology. Acartia tonsa represents an important, often dominant, member of zooplankton communities and meets all the practical criteria suggested for model species. New standardized procedures for laboratory culturing of the copepod A. tonsa and standardized methods for acute (24- and 48-h) and semichronic (7-d, static-renewal) toxicity tests with the nauplius stage are described. In both cases, eggs are the starting stage, and nauplius immobilization is the endpoint. The methods were the object of an intercomparison test involving nine laboratories, and nickel was the reference toxicant. Relative reproducibility was 24, 25, and 34% for 24-h, 48-h, and 7-d tests, respectively.

Bioavailability, bioaccumulation and tolerance of chromium: consequences in the food chain of freshwater ecosystems.

Many abiotic and biotic factors can influence the partitioning equilibrium of heavy metals, thus influencing metal impact on aquatic environments. Unicellular algal species release soluble organic substances able to complex metals. In our laboratory a Cr-tolerant strain was selected and isolated from a wild type strain of Scenedesmus acutus. The exudates released by the two strains counteracted the growth inhibition caused by Cr(VI) and the exudates of the Cr-tolerant strain were more effective. On the contrary, the exudates did not reduce chromium toxicity to the cladoceran Daphnia magna. The reduction of chromium effect on algae seems the consequence of an algae-specific interaction among Cr(VI), exudates and algal cells. Chromium uptake resulted to be energy-dependent since bioaccumulation rate in subdued light condition was lower than at high light intensity. The effect of Cr(VI) on algae changed depending on metabolism of the cells and in particular it seemed to be related to the bioaccumulation rate. Tolerance in the selected strain could not be ascribed to a lower uptake of chromium. The difference in sensitivity to chromium between the two strains was exploited to evaluate if tolerance acquired by algae could have consequences for Daphnia. After treatment with Cr(VI), the two strains of S. acutus were used as food source for D. magna. The results indicate that chromium is accumulated by algae in a form not available for daphnids and that Cr tolerance acquired by the algae can be of some advantage to the consumer organism.

Is Cr(VI) toxicity to Daphnia magna modified by food availability or algal exudates? The hypothesis of a specific chromium/algae/exudates interaction.

The long-term effects of Cr(VI) on life history traits of Daphnia magna Straus were studied in relation to different feeding conditions or to the presence of Scenedesmus acutus exudates, which had proved to reduce chromium toxicity to the alga. In high feeding conditions the concentration of 14 microg Cr(VI)/L had negative effects on survival, growth and fecundity. In low feeding conditions the effects on growth were not evident, but daphnids changed the reproductive strategy normally adopted in case of food shortage and showed a daily rate of newborn production higher than control; however, newborn were of a smaller size and less resistant to starvation. The most relevant effect of Cr was a shortening of the daphnid life span, which was independent of the feeding conditions and occurred also when growth and daily rate of neonate production were not affected. The algal exudates resulted ineffective in reducing Cr toxicity to daphnids, but caused a strong decrease in daphnid fecundity. Further experiments on S. acutus demonstrated that the detoxifying effect observed in the algae might involve a specific Cr/algae/exudates interaction, which occurs only when the algae are previously subjected to a short-time stress by chromium.