Sulfite can be toxic to green algae [23] because of interactions with sulfide

bonds of glutathione and glutathione disulfide that severely affect anti-oxidation processes [24]. It can also lead to SO2 toxicity through sulfoxy-free radicals generated by the oxidation of SO3 2- by O2 −[23]. Furthermore, in membrane preparations of cyanobacteria, sulfite stimulates ATP hydrolysis and inhibits ATP synthesis [25]. GSK126 in vivo Exogenous cysteine is believed CB-839 supplier to have direct effects on transporters and enzymes that are sensitive to thiol/disulfide redox variations [26]. This could account for the deleterious effects on the eukaryotic organisms in this study as unfortunately, these treatments did not improve Cd(II) tolerance. However, cysteine did improve the growth of Synechococcus in the presence of cadmium. It is possible that this organism is not as susceptible to functional interference of its protein thiol

groups, or that it has a greater absorption and storage capacity for cysteine, thereby lowering its deleterious effects. Cellular sulfide production The measurement of acid labile sulfide is a convenient way to estimate amounts of metal sulfide within samples [27]. Our studies clearly indicated that the addition of Cd(II) caused PF-562271 de novo aerobic synthesis of metal sulfide, assumed to be predominantly CdS because there was no detected increase in metal sulfides when Cd(II) was not supplied to the cells under any conditions (data not shown). This production of metal sulfide TCL was generally comparable to that of HgS in our previous studies [13–15], and it was produced to a higher level in the more rapidly growing eukaryotic cell treatments (Figure 2A & B). The cyanobacterium, Synechococcus, was able to synthesize significantly higher amounts of metal sulfide over time under all investigated conditions, although it is much less tolerant to Cd(II) than the eukaryotic species. Heavy metals are known to bind with low molecular weight thiol compounds

such as glutathione and phytochelatins [28, 29]. The latter are low molecular weight metallothioneins synthesized from glutathione [17]. Like metal sulfides, per se, metals bound in this way are more stable and less likely to cause oxidative damage. Cytosolic fractions taken from species of cyanobacteria and algae after exposure to Cd(II) have shown that approximately 30% of these metals are bound with metallothioneins, including phytochelatins [30–32]. Metallothioneins can exist as low and high molecular weight variants. In low molecular weight forms the metal is bound to thiol groups, whereas in the high molecular weight forms, additional inorganic sulfur is incorporated into the complexes [33] which appear to stabilize and improve detoxification. Interestingly, it is this pool of inorganic sulfur that is probably associated with Cd to form CdS.