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Aerobic transformation of cadmium through metal sulfide biosynthesis in photosynthetic microorganisms.

Edwards CD, Beatty JC, Loiselle JB, Vlassov KA, Lefebvre DD - BMC Microbiol. (2013)

Bottom Line: In general, conditions that increased cadmium sulfide production also resulted in elevated cysteine desulfhydrase activities, strongly suggesting that cysteine is the direct source of sulfur for CdS synthesis.Cadmium(II) tolerance and CdS formation were significantly enhanced by sulfate supplementation, thus indicating that algae and cyanobacteria can produce CdS in a manner similar to that of HgS.However, the enhanced activity of cysteine desulfhydrase indicates that it is instrumental in the provision of H2S for aerobic CdS biosynthesis.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biology, Queen's University, Kingston, ON, Canada.

ABSTRACT

Background: Cadmium is a non-essential metal that is toxic because of its interference with essential metals such as iron, calcium and zinc causing numerous detrimental metabolic and cellular effects. The amount of this metal in the environment has increased dramatically since the advent of the industrial age as a result of mining activities, the use of fertilizers and sewage sludge in farming, and discharges from manufacturing activities. The metal bioremediation utility of phototrophic microbes has been demonstrated through their ability to detoxify Hg(II) into HgS under aerobic conditions. Metal sulfides are generally very insoluble and therefore, biologically unavailable.

Results: When Cd(II) was exposed to cells it was bioconverted into CdS by the green alga Chlamydomonas reinhardtii, the red alga Cyanidioschyzon merolae, and the cyanobacterium, Synechoccocus leopoliensis. Supplementation of the two eukaryotic algae with extra sulfate, but not sulfite or cysteine, increased their cadmium tolerances as well as their abilities to produce CdS, indicating an involvement of sulfate assimilation in the detoxification process. However, the combined activities of extracted serine acetyl-transferase (SAT) and O-acetylserine(thiol)lyase (OASTL) used to monitor sulfate assimilation, was not significantly elevated during cell treatments that favored sulfide biosynthesis. It is possible that the prolonged incubation of the experiments occurring over two days could have compensated for the low rates of sulfate assimilation. This was also the case for S. leopoliensis where sulfite and cysteine as well as sulfate supplementation enhanced CdS synthesis. In general, conditions that increased cadmium sulfide production also resulted in elevated cysteine desulfhydrase activities, strongly suggesting that cysteine is the direct source of sulfur for CdS synthesis.

Conclusions: Cadmium(II) tolerance and CdS formation were significantly enhanced by sulfate supplementation, thus indicating that algae and cyanobacteria can produce CdS in a manner similar to that of HgS. Significant increases in sulfate assimilation as measured by SAT-OASTL activity were not detected. However, the enhanced activity of cysteine desulfhydrase indicates that it is instrumental in the provision of H2S for aerobic CdS biosynthesis.

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Cadmium induced sulfide formation at 0 (grey), 24 (cross-hatched) and 48 h (black) for Chlamydomonas reinhardtii (A) and Cyanidioschyzon merolae (B) in 100 μM Cd(II), and Synechococcus leopoliensis (C) in 2 μM Cd(II). Means and SE (n = 4). An asterisk indicates significantly greater than the respective Cd(II) containing control (p < 0.05).
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Figure 2: Cadmium induced sulfide formation at 0 (grey), 24 (cross-hatched) and 48 h (black) for Chlamydomonas reinhardtii (A) and Cyanidioschyzon merolae (B) in 100 μM Cd(II), and Synechococcus leopoliensis (C) in 2 μM Cd(II). Means and SE (n = 4). An asterisk indicates significantly greater than the respective Cd(II) containing control (p < 0.05).

Mentions: Acid labile sulfide production was measured after 0, 1 and 2 days of metal exposure to assess the ability of Chlamydomonas and Cyanidioschyzon to bioconvert 100 μM of Cd(II) (Figure 2A, B). Similar measurements were applied to Synechococcus treated with 2 μM Cd(II) (Figure 2C). In all treatment conditions the highest amount of sulfide was produced by Cyanidioschyzon, especially when cells were supplemented with sulfate during metal exposure and even more when also pretreated with extra sulfate (Figure 2B; p < 0.05). Similar trends also occurred but not to the same degree in Chlamydomonas (Figure 2A; p < 0.05). The highest amounts of metal sulfide production were 3.5 (approx. 64 fold increase) and 1.2 μmol per mg protein (approx. 4 fold increase) for Cyanidioschyzon and Chlamydomonas, respectively. The cyanobacterium Synechococcus in the sulfate pretreated cells produced a much lower amount of metal sulfide at 0.48 μmol per mg protein (approx. 3.5 fold increase) and this required 48 h to become significantly different from the control. However, this species was exposed to only 2 μM Cd(II), one fiftieth that of the other species because it is not as tolerant to cadmium. In contrast to the two eukaryotic algal species, the cyanobacterium also made similar amounts of metal sulfides during sulfite treatments. No species made significantly more sulfide as a product of cysteine supplementation after 48 h, although Synechococcus did make significantly more after 24 h.


Aerobic transformation of cadmium through metal sulfide biosynthesis in photosynthetic microorganisms.

Edwards CD, Beatty JC, Loiselle JB, Vlassov KA, Lefebvre DD - BMC Microbiol. (2013)

Cadmium induced sulfide formation at 0 (grey), 24 (cross-hatched) and 48 h (black) for Chlamydomonas reinhardtii (A) and Cyanidioschyzon merolae (B) in 100 μM Cd(II), and Synechococcus leopoliensis (C) in 2 μM Cd(II). Means and SE (n = 4). An asterisk indicates significantly greater than the respective Cd(II) containing control (p < 0.05).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3750252&req=5

Figure 2: Cadmium induced sulfide formation at 0 (grey), 24 (cross-hatched) and 48 h (black) for Chlamydomonas reinhardtii (A) and Cyanidioschyzon merolae (B) in 100 μM Cd(II), and Synechococcus leopoliensis (C) in 2 μM Cd(II). Means and SE (n = 4). An asterisk indicates significantly greater than the respective Cd(II) containing control (p < 0.05).
Mentions: Acid labile sulfide production was measured after 0, 1 and 2 days of metal exposure to assess the ability of Chlamydomonas and Cyanidioschyzon to bioconvert 100 μM of Cd(II) (Figure 2A, B). Similar measurements were applied to Synechococcus treated with 2 μM Cd(II) (Figure 2C). In all treatment conditions the highest amount of sulfide was produced by Cyanidioschyzon, especially when cells were supplemented with sulfate during metal exposure and even more when also pretreated with extra sulfate (Figure 2B; p < 0.05). Similar trends also occurred but not to the same degree in Chlamydomonas (Figure 2A; p < 0.05). The highest amounts of metal sulfide production were 3.5 (approx. 64 fold increase) and 1.2 μmol per mg protein (approx. 4 fold increase) for Cyanidioschyzon and Chlamydomonas, respectively. The cyanobacterium Synechococcus in the sulfate pretreated cells produced a much lower amount of metal sulfide at 0.48 μmol per mg protein (approx. 3.5 fold increase) and this required 48 h to become significantly different from the control. However, this species was exposed to only 2 μM Cd(II), one fiftieth that of the other species because it is not as tolerant to cadmium. In contrast to the two eukaryotic algal species, the cyanobacterium also made similar amounts of metal sulfides during sulfite treatments. No species made significantly more sulfide as a product of cysteine supplementation after 48 h, although Synechococcus did make significantly more after 24 h.

Bottom Line: In general, conditions that increased cadmium sulfide production also resulted in elevated cysteine desulfhydrase activities, strongly suggesting that cysteine is the direct source of sulfur for CdS synthesis.Cadmium(II) tolerance and CdS formation were significantly enhanced by sulfate supplementation, thus indicating that algae and cyanobacteria can produce CdS in a manner similar to that of HgS.However, the enhanced activity of cysteine desulfhydrase indicates that it is instrumental in the provision of H2S for aerobic CdS biosynthesis.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biology, Queen's University, Kingston, ON, Canada.

ABSTRACT

Background: Cadmium is a non-essential metal that is toxic because of its interference with essential metals such as iron, calcium and zinc causing numerous detrimental metabolic and cellular effects. The amount of this metal in the environment has increased dramatically since the advent of the industrial age as a result of mining activities, the use of fertilizers and sewage sludge in farming, and discharges from manufacturing activities. The metal bioremediation utility of phototrophic microbes has been demonstrated through their ability to detoxify Hg(II) into HgS under aerobic conditions. Metal sulfides are generally very insoluble and therefore, biologically unavailable.

Results: When Cd(II) was exposed to cells it was bioconverted into CdS by the green alga Chlamydomonas reinhardtii, the red alga Cyanidioschyzon merolae, and the cyanobacterium, Synechoccocus leopoliensis. Supplementation of the two eukaryotic algae with extra sulfate, but not sulfite or cysteine, increased their cadmium tolerances as well as their abilities to produce CdS, indicating an involvement of sulfate assimilation in the detoxification process. However, the combined activities of extracted serine acetyl-transferase (SAT) and O-acetylserine(thiol)lyase (OASTL) used to monitor sulfate assimilation, was not significantly elevated during cell treatments that favored sulfide biosynthesis. It is possible that the prolonged incubation of the experiments occurring over two days could have compensated for the low rates of sulfate assimilation. This was also the case for S. leopoliensis where sulfite and cysteine as well as sulfate supplementation enhanced CdS synthesis. In general, conditions that increased cadmium sulfide production also resulted in elevated cysteine desulfhydrase activities, strongly suggesting that cysteine is the direct source of sulfur for CdS synthesis.

Conclusions: Cadmium(II) tolerance and CdS formation were significantly enhanced by sulfate supplementation, thus indicating that algae and cyanobacteria can produce CdS in a manner similar to that of HgS. Significant increases in sulfate assimilation as measured by SAT-OASTL activity were not detected. However, the enhanced activity of cysteine desulfhydrase indicates that it is instrumental in the provision of H2S for aerobic CdS biosynthesis.

Show MeSH
Related in: MedlinePlus