Department of Biology, Queen's University, Kingston, ON, Canada.
BMC Microbiol. 2013 Jul 15;13:161. doi: 10.1186/1471-2180-13-161.
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.
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.
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.
镉是一种非必需的金属，由于其对铁、钙和锌等必需金属的干扰，具有毒性，会导致许多有害的代谢和细胞效应。自工业时代以来，由于采矿活动、农业中化肥和污水污泥的使用以及制造活动的排放，环境中这种金属的含量急剧增加。光养微生物的金属生物修复效用已经通过它们在有氧条件下将 Hg(II) 解毒为 HgS 的能力得到证明。金属硫化物通常非常不溶，因此在生物学上不可用。
当 Cd(II) 暴露于细胞时，绿藻莱茵衣藻、红藻 Cyanidioschyzon merolae 和蓝藻 Synechoccocus leopoliensis 将其生物转化为 CdS。向两种真核藻类补充额外的硫酸盐，但不是亚硫酸盐或半胱氨酸，可提高它们对镉的耐受性以及产生 CdS 的能力，表明硫酸盐同化参与了解毒过程。然而，用于监测硫酸盐同化的提取丝氨酸乙酰转移酶 (SAT) 和 O-乙酰丝氨酸(硫)裂合酶 (OASTL) 的联合活性在有利于硫化物生物合成的细胞处理过程中没有显著升高。可能是由于实验的长时间孵育（超过两天）可以弥补硫酸盐同化的低速率。这对于 S. leopoliensis 也是如此，其中亚硫酸盐和半胱氨酸以及硫酸盐的补充都增强了 CdS 的合成。一般来说，增加镉硫化物产量的条件也会导致半胱氨酸脱硫酶活性升高，这强烈表明半胱氨酸是 CdS 合成的直接硫源。
硫酸盐补充显著增强了 Cd(II) 的耐受性和 CdS 的形成，因此表明藻类和蓝藻可以以类似于 HgS 的方式产生 CdS。SAT-OASTL 活性测定的硫酸盐同化显著增加未被检测到。然而，半胱氨酸脱硫酶活性的增强表明它在有氧 CdS 生物合成中为 H2S 的提供具有重要作用。
