Increase of chromium tolerance in Scenedesmus acutus after sulfur starvation: Chromium uptake and compartmentalization in two strains with different sensitivities to Cr(VI).

In photosynthetic organisms sulfate constitutes the main sulfur source for the biosynthesis of GSH and its precursor Cys. Hence, sulfur availability can modulate the capacity to cope with environmental stresses, a phenomenon known as SIR/SED (Sulfur Induced Resistance or Sulfur Enhanced Defence). Since chromate may compete for sulfate transport into the cells, in this study chromium accumulation and tolerance were investigated in relation to sulfur availability in two strains of the unicellular green alga Scenedesmus acutus with different Cr-sensitivities. Paradoxically, sulfur deprivation has been demonstrated to induce a transient increase of Cr-tolerance in both strains. Sulfur deprivation is known to enhance the sulfate uptake/assimilation pathway leading to important consequences on Cr-tolerance: (i) reduced chromate uptake due to the induction of high affinity sulfate transporters (ii) higher production of cysteine and GSH which can play a role both through the formation of unsoluble complexes and their sequestration in inert compartments. To investigate the role of the above mentioned mechanisms, Cr accumulation in total cells and in different cell compartments (cell wall, membranes, soluble and miscellaneous fractions) was analyzed in both sulfur-starved and unstarved cells. Both strains mainly accumulated chromium in the soluble fraction, but the uptake was higher in the wild-type. In this type a short period of sulfur starvation before Cr(VI) treatment lowered chromium accumulation to the level observed in the unstarved Cr-tolerant strain, in which Cr uptake seems instead less influenced by S-starvation, since no significant decrease was observed. The increase in Cr-tolerance following S-starvation seems thus to rely on different mechanisms in the two strains, suggesting the induction of a mechanism constitutively active in the Cr-tolerant strain, maybe a high affinity sulfate transporter also in the wild-type. Changes observed in the cell wall and membrane fractions suggest a strong involvement of these compartments in Cr-tolerance increase following S-starvation.

Light intensity influences chromium bioaccumulation and toxicity in Scenedesmus acutus (Chlorophyceae).

The influence of light intensity on chromium uptake was studied in two strains of the freshwater unicellular alga Scenedesmus acutus (Chlorophyceae) having different sensitivity to Cr poisoning and light intensity. The two strains were subjected to different Cr treatments at 3000 and 80 lux. Cr toxicity was assessed by algal growth rate, recovery test, methylene blue staining, and determination of photosynthetic activity. After 2 and 4 days of treatment, bioaccumulated chromium, cell dry mass, and protein and carbohydrate contents were also assessed. When the algae were treated at 3000 lux, different bioaccumulation patterns were obtained when Cr content was related to dry mass, cell number, or protein content. A direct relationship between Cr content and cell mortality was observed only when the amount of Cr was related to protein content. In both strains Cr uptake was slower in subdued light, suggesting that it is linked to energy-dependent processes. The difference between the strains in sensitivity to Cr poisoning was also evident in subdued light.

Comparison between a normal and a Cr-tolerant strain of Scenedesmus acutus as a food source to Daphnia magna.

A normal (S2-N) and a Cr-tolerant (S2-T) strain of Scenedesmus acutus, grown in Cr-free medium or in the presence of 1, 5, or 10 mg/liter Cr(VI), were fed to Daphnia magna. An increase of growth and fecundity was observed in the daphnids when the algae had undergone an increase of dry mass following Cr treatment, which happened in S2-N in the presence of 1 mg/liter and in S2-T in the presence of 5 mg/liter. When the algae were pretreated with 10 mg/liter, growth and fecundity were reduced in the daphnids fed on S2-N, but remained normal in the daphnids fed on S2-T. It is concluded that the tolerant strain remains a good food source even if grown in the presence of a Cr concentration that reduces the nutritional value of the normal strain.

Chromium-induced sexual reproduction gives rise to a Cr-tolerant progeny in Scenedesmus acutus.

A clonal population of Scenedesmus acutus was treated for 3 months with 1 mg/liter Cr(VI) and then returned to Cr-free medium. After several months, the cells were treated with different Cr concentrations and then subjected to a series of morphological observations and metabolic tests. The results, compared with those obtained with a normal cell population treated in the same way, demonstrate that the progeny of algae subjected to the prolonged Cr treatment has acquired tolerance to the metal, as it can survive and grow in the presence of Cr concentrations that are lethal to normal cells. An ultrastructural description of gametes and zygotes is also given.

Hexavalent chromium induces gametogenesis in the freshwater alga Scenedesmus acutus.

The aggregates formed by Scenedesmus acutus cultivated in the presence of 1 mg/liter Cr(VI) persist even after 7 days of treatment. They were examined by fluorescence and electron microscopy and were found to be an early stage of sexual reproduction. They consist of four small cells deprived of a true cell wall and enclosed in a common envelope. When the envelope breaks up they are released in the culture medium as biflagellated mobile cells. These cells have been identified as gametes.

Chromium toxicity on two linked trophic levels. I. Effects of contaminated algae on Daphnia magna.

The effects of feeding Daphnia magna on algae (Scenedesmus acutus) pretreated with different concentrations of Cr(VI) were studied. A positive effect on growth and newborn production rate was observed in the daphnids fed on algae exposed to 1 mg/liter Cr(VI). Fecundity and growth were drastically reduced in daphnids fed on algae exposed to 10 mg/liter Cr(VI). Since the algae, cultured in the presence of these two Cr(VI) concentrations, supplied daphnids with similar amounts of chromium, the observed effects on the population dynamics of D. magna were attributed more to alterations of the nutritional value of the algal food, due to the Cr treatment, than to a toxic effect of the metal.

Chromium toxicity on two linked trophic levels. II. Morphophysiological effects on Scenedesmus acutus.

The morphophysiological effects of Cr(VI) on Scenedesmus acutus were studied by light and fluorescence microscopy. Axenic cultures of S. acutus were grown in the presence of 1, 5, and 10 mg/liter of Cr(VI) supplied as K2Cr2O7. It was found that Cr(VI) inhibits cell proliferation and the formation of coenobia. It also induces alterations in cell morphology; depending on the dose, it induces the formation of multicellular plurinucleated aggregates or the appearance of large uninucleated cells. By using methylene blue staining, it was possible to identify the algal cells that had undergone irreversible damage.