These cultures were either the same as (Cyanidioschyzon and Synechococcus)
or only slightly lower in biomass (Chlamydomonas) over the 48 h growth period by comparison to the metal-free controls. Although cadmium stress has been shown to induce sulfur limiting conditions [7, 19], this was not entirely alleviated by the simultaneous provision of sulfate in any of the studied species, thus indicating that established metabolic reserves of sulfur other than sulfate itself, may be involved in cellular protection. Furthermore, it has been demonstrated that Cd exposure triggers a decline of photosynthetic apparatus thereby liberating sulfur as well as nitrogen and iron, which can be subsequently used for the synthesis of Cd detoxification enzymes . Assimilated sulfate appears Ro-3306 nmr to create an organic sulfur pool that can be readily employed to biotransform Cd(II) as it enters the cell in a similar
manner to that proposed for Hg(II) where chemical modification of thiols severely lessened HgS production [14, 15]. Why this cannot be provided by simultaneous sulfate provision is likely to be a product of the high energy demand (732 kJ mol-1) required to reduce sulfate to sulfide for thiol production, energy required for sulfate uptake, and the decline in sulfate uptake induced by cadmium itself . These organisms rely on photosynthesis to generate reducing power that is essential for carbon fixation. If this is shunted towards sulfate assimilation, it would inhibit cellular metabolism and growth. By temporally displacing Tucidinostat supplier energy requirements to a pretreatment period, this is overcome and the cells are able to adequately cope with any stress imposed by subsequent exposure to Cd(II). The simultaneous sulfate and metal treated cells grew marginally better than the cells treated Tangeritin with metal alone in Cyanidioschyzon and Synechococcus (Figures 1B & C), but not in Chlamydomonas. Metabolic differences
might ac-count for this; i.e. the former species may have relatively more efficient sulfate assimilation. Interestingly, in a separate study it was revealed that Synechococcus is able to utilize elemental sulfur as a sulfur source resulting in MK-8931 supplier enhanced metal tolerance (data not shown). These results point to the importance of sulfur nutrition in cadmium tolerance that has implications for other organisms [20, 21], including humans . Nevertheless, this has not been well documented in the literature. The other treatment in which Synechococcus grew better than in cadmium alone was that in which cysteine was supplied both prior to and during metal exposure. However, this cannot be accounted for by a relatively high cysteine desulfhydrase activity in Synechococcus (Figure 4). Both eukaryotic species were not as adept at coping with this form of sulfur supplementation.