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

tIn photosynthetic organisms sulfate constitutes the main sulfur source for the biosynthesis of GSH and itsprecursor Cys. Hence, sulfur availability can modulate the capacity to cope with environmental stresses, aphenomenon known as SIR/SED (Sulfur Induced Resistance or Sulfur Enhanced Defence). Since chromatemay compete for sulfate transport into the cells, in this study chromium accumulation and tolerancewere investigated in relation to sulfur availability in two strains of the unicellular green alga Scenedesmusacutus with different Cr-sensitivities. Paradoxically, sulfur deprivation has been demonstrated to inducea transient increase of Cr-tolerance in both strains. Sulfur deprivation is known to enhance the sulfateuptake/assimilation pathway leading to important consequences on Cr-tolerance: (i) reduced chromateuptake due to the induction of high affinity sulfate transporters (ii) higher production of cysteine andGSH which can play a role both through the formation of unsoluble complexes and their sequestration ininert compartments. To investigate the role of the above mentioned mechanisms, Cr accumulation in totalcells and in different cell compartments (cell wall, membranes, soluble and miscellaneous fractions) wasanalyzed in both sulfur-starved and unstarved cells. Both strains mainly accumulated chromium in thesoluble fraction, but the uptake was higher in the wild-type. In this type a short period of sulfur starvationbefore 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 significantdecrease was observed. The increase in Cr-tolerance following S-starvation seems thus to rely on differentmechanisms 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 thecell wall and membrane fractions suggest a strong involvement of these compartments in Cr-toleranceincrease following S-starvation.