~ Pergamon Chemosphere, Vol. 37, Nos 14-15, pp. 3019-3025, 1998 © 1998 Elsevier Science Ltd. All rights reserved 0045-6535/98/$ - see front matter PII: S0045-6535(98)00343-9 EXUDATES FROM THE WILD TYPE AND A CR-TOLERANT STRAIN OF SCENEDESMUS ACUTUS INFLUENCE DIFFERENTLY CR(VI) TOXICITY TO ALGAE. M. G. Corradi , G. Gorbi °, H. Morsi Abd-EI-Monem #, A. Torelli and M. Bassi Department of Evolutive and Functional Biology, University of Parma, 1-43100 Parma, Italy; °Department of Environmental Sciences, University of Parma, 1-43100 Parma, Italy; #Department of Botany, University of Menoufia - Shebin E1 Koam, Egypt. ABSTRACT. Culture filtrates containing the exudates from the wild type (S2-N) and a Cr-tolerant strain (S2-T) of Scenedesmus acutus were tested for their capacity of reducing Cr toxicity to the algae and the filter-feeder Daphnia magna. The filtrates were supplemented either with 1 mg/l Cr(VI) and used as culture media for the wild type of S. acutus, or with 14 ktg/1 Cr(VI) and used as media to breed daphnids. In the experiments with algae both filtrates partially counteracted the growth-inhibiting activity of chromium, but the effect of the filtrate of S2-T was much greater. In the experiments with daphnids the algal filtrates had toxic effects, causing reduction of growth, fecundity and life-span. The same effects were obtained with the filtrates deprived of the organic matter. The filtrates of S2-T differed from those of S2-N because of their higher content in carbohydrates and because the electrophoretic pattern of the proteins extruded by S2-T showed three peaks not present in the proteins extruded by S2-N. INTRODUCTION The release of organic matter by algae has been widely described, both in cultures and in the natural habitats [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Such organic compounds can reduce the toxicity of heavy metals, such as Cu, Pb, Zn, Ag, Hg and Cd, either in culture experiments or in natural waters [11, 12, 13, 14, 15, 16, 17, 18]. We wondered if also chromium toxicity could be reduced by algal exudates and in such a case if the exudates from algal strains having a different sensitivity to the metal might show a different detoxifying capacity. In the course of studies on chromium toxicity to the freshwater green alga Scenedesmus acutus we obtained a strain which proved to be tolerant of Cr concentrations which were toxic to the original algal population [19]. We assumed that the exudates from the two different strains might have a different detoxification capacity. Therefore we used the culture filtrates as culture media for the wild type, after the addition of a sublethal Cr concentration which normally causes a marked inhibition of algal growth [20]. We also investigated if the exudates from the two algal strains differed in carbohydrate and protein content, since these are the substances mainly involved in detoxification processes [ 17, 21, 22]. The possibility that the algal exudates might lower Cr toxicity to the filter feeder Daphnia magna prompted a series of experiments, in which cohorts of daphnids were reared in presence of chromium after the addition of the algal filtrates. Preliminary results of these experiments are also reported here. 3019