# Dysprosium Absorption of Aluminum Tolerant- and Absorbing-Yeast
## 4 Discussion  
In this study, we successfully selected three Dy-absorbing yeast strains, one being the _S. cerevisiae_ laboratory strain, BY4741, and the other two the highly Al-tolerant and -absorbing strains Alt-OF2 and Alt-OF5. These may be used to recover Dy from the waste solutions produced following the metal manufacturing process. The concentration of REE absorption is from 0.5 to 1.0 mmol per 1 mg biomass, as reported in biosorption using various biomaterials [6; 7; 8; 9], and in recent research on the biosorption of REE using recombinant bacteria [15] and moss [16]. The recovery rate of REE absorption is reported to be 80-90% [6; 9; 15]. Our result shows that the Dy absorption of yeast is 1.0-2.0 pg per cell (in  
Figure 5: Dy content of protoplasts and solubilized cell wall fraction in the Alt-OF2 (**A**) and Alt-OF5 (**B**). Yeast cells were treated with 0.1 g/L of Dy acetate for 1 h, collected, and then digested with Zymolyase 100-T for 1 h. Closed diamonds and open circles represent the Dy concentration in protoplasts and the solubilizing cell wall fraction, respectively. Figure 2). When the yeast biomass was measured in our strains, we found 0.65-0.75 mg dry weight per 1 OD\({}_{600}\) unit (OD\({}_{600}\) = 1), similar to the reported value [17]. The calculated Dy concentration absorbed in 1 mg (dry weight) of our yeast biomass was 0.2-0.4 mmol. These values are by no means inferior to those when using other biomaterials, considering that the treatment in the Dy solution lasted 2 h. The recovery rate of Dy using our Al-tolerant and -absorbing yeast strains was more than 90%, similar to that using recombinant bacteria. Our yeast strains are more easily handled than bacteria containing recombinants in terms of biosafety. In particular, _S. cerevisiae_ is especially useful, if possible, because it is regarded as safe to use in the processing of food and beverages. It is strongly suggested that Dy absorption using the Al-tolerant and -absorbing yeast strains can establish a new recovery system. However, the mechanism of Dy absorption remains unknown in yeast. Our research results suggest that Dy is adsorbed in the cell surface region because Dy absorption was recognized using dead yeast cells. However, the Dy concentration using the dead yeast cells was 50% of that of the intact yeast cells. Our preliminary experiments suggest that Dy absorption is decreased to a similar level using dead cells, when the Dy treatment is carried out at a high temperature at which the yeast strains cannot grow (our unpublished data). These results suggest that bioactive factors are involved in Dy absorption. Additionally, this suggestion is supported by our results that the Dy absorption is decreased at a lower pH (in Figure 3), at which the biological activity is lower. The Dy concentration of the Dy-absorbing yeast cells when removing the cell wall was similar to that of the intact Dy-absorbing cells. These results may also show that Dy that is present in the cell surface region is located on the cell membrane and is transported into the intracellular region by bioactive factors. The Dy-absorption mechanisms of yeast have not yet been determined. However, the mechanisms involved in Al tolerance or absorption clearly contribute to the Dy absorption in yeast because the Al-tolerant and -absorbing strains, Alt-OF2 and Alt-OF5, show higher Dy absorption. Our results also support the belief that ionized Dy produces the same trivalent ion as Al. If the mechanism of Dy absorption in yeast is elucidated, our results may contribute not only to the study of the mechanism underlying the biosorption of Dy but also to elucidating the unknown mechanisms of the yeast response to rare earth metals because _S. cerevisiae_ is an ideal model organism [18]. In terms of the yeast response to Al, it is interesting to note that the deletion mutants of the Al transport-related genes that display an Al-tolerant phenotype harbor an Al transporter gene owing to the low probability of Al accumulation in the cell wall.