## 3 Results and Discussion
### Detection of Water Toxicity
#### 3.2.2 Zn as Water Toxicity  
Euglena and Paramecia were exposed to varying concentrations of zinc nitrate hexahydrate (Zn(NO\({}_{3}\))\({}_{2}\)-6H\({}_{2}\)O), another heavy metal water pollutant. The permissible level (PL) for Zn, as determined by the World Health Organization (WHO), is 5 mg/L. The Euglena and Paramecia were exposed to concentrations equivalent to 2, 10, and 20 times the PL. The cross-correlation function as a function of time was employed to assess the swimming ability of Paramecia under different Zn concentrations, as shown in Figure 6a. Additionally, Figure 6b illustrates the correlation time as a function of the concentration of Zn. A clear concentration-dependent response of Paramecia to Zn exposure could be seen. A significantly higher correlation time was observed as the Zn concentration exceeded the PL 10 times. Specifically, at 20 times the PL, the correlation time increased by approximately 30 times compared to that of the control. This significantly higher correlation time indicates excessive water toxicity conditions at 20\(\times\) PL or 100 mg/L Zn concentration. At this concentration, the swimming ability of Paramecia was completely hindered due to the toxic environment. In other words, no variation in biospeckle patterns was observed, indicating diminished activity. Surprisingly, contrasting results were observed at concentrations equivalent to the PL (5 mg/L) and 2\(\times\) PL (10 mg/L), where a significant reduction in correlation time compared to the control was observed in Paramecia. This reduction, approximately 18% less than the control, suggests the heightened activity and rapid movement of Paramecia at these lower concentrations. Similar behavior was observed in the case of Fe for Paramecia. This phenomenon could be attributed to the beneficial role of Zn as a micronutrient at lower concentrations, supporting the health and vitality of the Paramecia. In summary, the dual impact of Zn was seen on Paramecia, with concentrations exceeding 10 times the PL, proving high water toxicity. In contrast, lower concentrations demonstrate a stimulatory effect on microorganism activity, indicating relatively low water toxicity. This behavior might be due to the micronutrient properties of Zn. Figure 7a shows the cross-correlation function as a function of time, and Figure 7b shows the correlation time as a function of Zn concentrations for Euglena. The experimental results revealed a clear concentration-dependent response of Euglena to Zn exposure. As shown in Figure 7b, a significantly larger correlation time was observed compared to that of the control with increasing Zn concentrations beyond 2\(\times\) PL. More specifically, the  
Figure 5: (**a**) The cross-correlation function of Euglena per unit time under different Fe concentrations and (**b**) the correlation time as a function of varying Fe concentrations (mg/L) for Euglena and Paramecia, *, ** indicated the statistical significance of data \(p<0.05\) and \(p<0.01\) (N = 9). correlation time becomes approximately 2 to 4 times greater than that for the control. At 10 and 20\(\times\) PL Zn concentrations, the correlation time was significantly high, indicating the slower swimming ability of Euglena. This significant increase in correlation times suggests that concentrations of 50 and 100 mg/L of Zn were found to be toxic for Euglena, resulting in a complete hindrance of their swimming ability due to heightened water toxicity. Interestingly, at PL and 2\(\times\) PL, a significant reduction in the correlation time or cross-correlation function was observed compared to the control. This result indicates that within this range (5-10 mg/L) of Zn exposure, Euglena's swimming ability was increased compared to the control. This might be because of the micronutrient behavior of Zn. A similar kind of behavior was also observed for Paramecia in the case of lower Zn concentrations [19].