The relationship between sulfur metabolism and tolerance of hexavalent chromium in Scenedesmus acutus (Spheropleales): Role of ATP sulfurylase.

Sulfur availability and the end products of its metabolism, cysteine, glutathione and phytochelatins, play an important role in heavy metal tolerance, chromium included. Sulfate and chromate not only compete for the transporters but also for assimilation enzymes and chromium tolerance in various organisms has been associated to differences in this pathway. We investigated the mechanisms of Cr(VI)-tolerance increase induced by S-starvation focusing on the role of ATP sulfurylase (ATS) in two strains of Scenedesmus acutus with different chromium sensitivity.

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.

Identification of S2-T A63: a cDNA fragment corresponding to a gene differentially expressed in a Cr-tolerant strain of the unicellular green alga Scenedesmus acutus.

The gene expression of the wild type (S2-N) and a Cr-tolerant strain (S2-T) of the unicellular green alga Scenedesmus acutus has been compared in order to get more insight on their different chromium sensitivity. The RNA of the two strains was extracted after 4 days of culture in standard medium without chromium and analyzed by means of RNA differential display. The two strains showed differential gene transcription even in the absence of the heavy metal and six putatively differential amplicons were evidenced in the Cr-tolerant strain.

Sulfur starvation and chromium tolerance in Scenedesmus acutus: a possible link between metal tolerance and the regulation of sulfur uptake/assimilation processes.

In a laboratory-selected Cr-tolerant strain of the unicellular green alga Scenedesmus acutus, the capacity to synthesize higher amounts of cysteine (Cys) and reduced glutathione (GSH) than the wild-type was demonstrated to underlie tolerance to Cd and Cr(VI). In photosynthetic organisms sulfate constitutes the main sulfur source for the biosynthesis of GSH and its precursor Cys, hence it was hypothesized that the sensitivity of the two strains to Cr(VI) could be modified after culturing in sulfate-deprived medium. Both strains were grown in the presence of different concentrations or in the absence of sulfate (sulfur-starved) and then assayed for Cr(VI) tolerance in standard medium.