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.