In fact, both types of cysteine treatments in all species had relatively high cysteine desulfhydrase activities at 6 h with no enhanced metal

sulfide production. Unfortunately, treatments with lower amounts of cysteine did not result in detectable increases in metal sulfide production (data not shown). This implies that the enzyme may not be involved in the supply of sulfide for CdS synthesis, or that excess cysteine is inhibitory. The latter is likely because supplementation with sulfate prior to and during Cd(II) exposure resulted in the highest desulfhydrase activities after 24 h in all three species as well as the VX-680 highest production scenarios for metal sulfide. In addition, the simultaneous addition of SB431542 in vivo extra sulfate with Cd(II) also resulted in relatively high extracted enzyme activity. This is consistent with the fact that Escherichia coli genetically engineered to contain unregulated cysteine desulfhydrase do produce elevated amounts of CdS [64, 65], and the formation of CdS nanoparticles appears to increase with extractable cysteine desulfhydrase activity in the photosynthetic bacterium Rhodopseudomonas palustris[66]. Although the accumulation of acid labile sulfide is high in the organisms presented

in this study, it remains to be seen if they comprise CdS nanoparticles. Conclusions The fact that cadmium tolerance was significantly enhanced by sulfate supplementation is supported by MRIP the discovery of the enhanced formation of metal sulfides under these conditions. Because Cd(II) was provided in the media in a much higher excess than other metal ions, the increase in acid labile sulfides can be attributed to CdS formation.

The cyanobacterium Synechococcus leopoliensis , the green alga Chlamydomonas reinhardtii, and especially the red alga Cyanidioschyzon merolae produce high quantities of CdS in a manner that appears to be similar to HgS biosynthesis ([13–15]. The addition of sulfate increased this production dramatically indicating the involvement of sulfate assimilation. Although SAT-OASTL was not shown to increase significantly under sulfate supplementation, the relatively long-term duration of this study could account for the accumulation of reserves used to make the sulfide moiety of CdS. The identity of these reserves could be glutathione or possibly sulfur SRT1720 concentration mobilized from the breakdown of photosynthetic apparatus [12]; however, this remains to be determined. Whereas the role of SAT-OASTL appears to be pedestrian, cysteine desulfhydrase can be implicated in the production of CdS because it does possess elevated activity during conditions conducive to metal sulfide production. Methods Culture sources and growth conditions The eukaryotic alga Chlamydomonas reinhardtii (UTEX 90) was obtained from the Culture Collection of Algae, University of Texas at Austin. Cultures were grown in high salt medium (HSM) [67] composed of 9.35 mM NH4Cl, 8.27 mM K2HPO4, 5.