Eucalyptus Leaf Solution to Replace Metals in the Removal of Cyanobacteria in Wastewater from the Paper Mill Industry

The frequent occurrence of cyanobacterial blooms, caused by the eutrophication of water bodies, has triggered several ecological issues. Metal-controlled cyanobacteria are resulting in a series of secondary environmental problems and thus limiting environmental sustainability. Whether there is a more environmentally friendly way to replace metals in the removal of cyanobacteria is still unclear. To explore whether common heavy metals inhibit algal growth and whether Eucalyptus leaves (EL) can replace heavy metal ions in controlling algae outbreaks, here, we add Fe3+, Al3+, 3 mol/L of zinc (Zn3), 10 mol/L zinc (Zn10), and EL to a medium containing Cyanobacteria. We determine the medium’s color (456 nm), UV (254 nm), chlorophyll a, turbidity, temperature, pH, total dissolved solids, conductivity, and blue-green algae (BGA) at days 1, 4, 7, 11, 14, 19, and 21. We find that Fe3+, Al3+, Zn3, Zn10, and EL can inhibit chlorophyll synthesis, thereby impeding algae biomass growth due to metal ions’ disruption of the chlorophyll structure. The toxicity of Zn2+ may be higher than that of Fe3+ and Al3+ since it can completely destroy the structure of chlorophyll a. The damage of Zn (10) to chlorophyll a is stronger than that of Zn (3), indicating that high concentrations of metals have a stronger inhibitory effect on algae. The toxicity of EL to algae is lower than that of other metals, but it can significantly inhibit the growth of algae. We suggest the use of Eucalyptus leaves to inhibit algal growth in eutrophic water bodies. Our results provide a scientific basis for an environmentally friendly approach to controlling cyanobacteria outbreaks.


Introduction
Eutrophication is caused by excessive levels of nutrients, such as nitrogen or phosphorus, under the influence of natural factors and human activities [1][2][3][4]. The frequent occurrence of cyanobacterial blooms, caused by the eutrophication of water bodies, has triggered a series of environmental and ecological issues that have recently attracted increased interest from the scientific community [5]. The methods of controlling water eutrophication are mainly divided into exogenous and endogenous controls [2]. There are several physical, biological, and chemical approaches and technologies to tackling cyanobacterial outbreaks [6][7][8][9]. However, although the physical and biological methods are environmentally friendly, they require large amounts of labor and resources, making them impractical [10,11]. Chemical methods, such as the use of metals, are highly effective in controlling cyanobacterial outbreaks, but can also cause secondary environmental problems [12][13][14]. In addition, metals can impact algal growth and reproduction. Previous studies have shown that when metals enter the water environment, even low concentrations of Zn and other metals can promote the growth and reproduction of algae cells since metal ions are essential for cell synthesis [15][16][17][18]. However, at excessive concentrations, Table 1. Variations in chlorophyll a increment, respectively. The A, B, C, D, E, and F treatments underwent the addition of Fe 3+ , Al 3+ , 3 mol/L zinc (Zn 3 ), 10 mol/L zinc (Zn 10 ), Eucalyptus leaves (EL), and pure water, respectively. There were significant differences among treatments and culture days, respectively (p < 0.05). After transportation to the laboratory, the algae were cultured under a light intensity of about 3000 lx. The dark period of light illumination was t (bright): t (dark) = 12 h: 12 h, and the temperature was 25 • C. Artificial oscillation was carried out daily to ensure the normal growth of blue-green algae.

Cultivation Days
Subsequently, 650 mL of Fe 3+ , Al 3+ , 3 mol/L zinc (Zn 3 ), 10 mol/L zinc (Zn 10 ), and Eucalyptus leaves (EL) were added; as a control (CK), we used 650 mL of water without any metals ( Figure 1). The algae species were cultured for 21 days, and the color (456 nm), UV (254 nm), chlorophyll a, turbidity, temperature, pH, total dissolved solids, conductivity, and blue-green algae (BGA) in the sample bottles were determined at days 1,4,7,11,14,19, and 21.  After transportation to the laboratory, the algae were cultured under a light intensity of about 3000 lx. The dark period of light illumination was t (bright): t (dark) = 12 h: 12 h, and the temperature was 25 °C. Artificial oscillation was carried out daily to ensure the normal growth of blue-green algae.

Preparation of the Eucalyptus Leaf Solution
Fresh Eucalyptus leaves were washed with distilled water, cut into pieces, and placed into a 1-L flask drying, followed by the addition of three times the volume of methanol. Extraction was performed three times (2 h each) by ultrasonic waves at 25 °C. The three filtrates were combined and concentrated under reduced pressure to obtain the extract, which was stored at 4 °C until use.

Determination of the pH and Temperature of Water
Water temperature and pH were measured using a multifunctional water quality detector. We rinsed the probe with clean water and dried it with a paper towel. Then, we inserted the probe in the sample and recorded pH and temperature readings, repeating this three times for each sample. After measuring a sample, we rinsed the probe with clean water and dried it with a clean paper towel.

Preparation of the Eucalyptus Leaf Solution
Fresh Eucalyptus leaves were washed with distilled water, cut into pieces, and placed into a 1-L flask drying, followed by the addition of three times the volume of methanol. Extraction was performed three times (2 h each) by ultrasonic waves at 25 • C. The three filtrates were combined and concentrated under reduced pressure to obtain the extract, which was stored at 4 • C until use.

Determination of the pH and Temperature of Water
Water temperature and pH were measured using a multifunctional water quality detector. We rinsed the probe with clean water and dried it with a paper towel. Then, we inserted the probe in the sample and recorded pH and temperature readings, repeating this three times for each sample. After measuring a sample, we rinsed the probe with clean water and dried it with a clean paper towel.

Determination of Chlorophyll a Concentration
We used the hot ethanol method for the measurement of chlorophyll a concentrations. First, 25 mL of the shaken algal fluid was taken via a pipette and suction-filtered by 45-µm glass fiber filters. Second, the samples, together with the filter membrane sheared into fine pieces, were placed in a 10-mL colorimetric tube, followed by the addition of 10 mL of 95% ethanol and extraction in a water bath at 75 • C for 5 min. All samples were then uniformly placed in a 4 • C freezer for 12 h to extract chlorophyll a. Absorbance was measured at 649 and 665 nm. The chlorophyll a content was calculated as follows: X (ug/L) = 13.95A 665 − 6.88A 649 (1) where A 665 and A 649 are the absorbance values of the algae solution at 665 and 649 nm, respectively.

Determination of Turbidity
For turbidity analysis, we used the equation constructed by Púrtielje, based on measurements in shallow lakes in the Netherlands [11]. The minimum turbidity value (i.e., the maximum Z SD ) of 0.16/m in the dataset was set as TurbBck, assuming that there were no planktonic algae and no suspended algae particles at this time. In addition, we established the empirical relationship between chlorophyll a and Turb Tot (1/Z SD ). The chlorophyll a concentration had a corresponding minimum 1/Z SD value. The slope of this line was taken as 5% of (Turb Tot − Turb Bck )/chlorophyll-a, and the result was 0.01/m. Therefore, to calculate algae turbidity, we used the following equation:

Determination of BGA
We used the turbidimetric method and the dry weight method to measure algae biomass. After the cyanobacteria were continuously cultured for six days, they were diverted into nine samples with different concentrations, according to a certain proportion.
The absorbance values of each concentration were measured at a wavelength of 456.0 nm, and the samples were dried to a constant weight to obtain the dry cell weight (mg/L).

Data Analysis
All statistical analyses were performed using the software package SPSS 16.0. Descriptive statistics were applied to calculate the means and standard deviations for each set of replicates. A two-way ANOVA was used to analyze the differences in color, UV, chlorophyll a, turbidity, temperature, pH, total dissolved solids, conductivity, and BGA, using the treatment and culture days as independent factors. In addition, we determined the correlations between BGA, total dissolved solids (TDS), and temperature, as well as chlorophyll a, turbidity, and conductivity. All data met the requirements of a normal distribution and homogeneity of variance.

Variations in Water's Physicochemical Properties between Treatments
The pH and temperature of water can reflect algal growth [32]. Via photosynthesis, algae can increase the dissolved oxygen content in the water, resulting in a higher pH [33]. In addition, the energy released through algae cell reproduction can also increase the water temperature [34]. Here, the pH of the culture medium increased first and then decreased over time ( Figure 2). The pH of CK fluctuated greatly, with an average of 8.7, which was significantly higher than that of the other treatments (p < 0.05); in CK, it reached the maximum value of 10.3 on day 7. The pH fluctuation of Zn (10) was relatively low, with an average of 7.34, registering as 19.1% lower than that of CK. The control had the lowest pH of 6.9, which was significantly lower than that of the other treatments (p < 0.05). The pH of Zn (10) was significantly higher than that of Zn (3), but significantly lower than that of EL Water 2021, 13, 1014 5 of 10 (p < 0.05). In addition, the pH values of Fe 3+ and Al 3+ were significantly lower than those of the other treatments. Based on these results, we infer that algal growth was highest in CK and lowest in EL. This finding is in agreement with a previous study [35].
temperatures of the EL, Fe 3+ , and Al 3+ treatments decreased in the 11 days, maybe as the algae suspended growth during this period. This can also be confirmed by the biomass data of cyanobacteria. The temperatures of the treatments EL, Fe 3+ , and Al 3+ were significantly higher than those of the CK, Zn (3), and Zn (10) treatments. In EL treatments, the temperature reached a maximum value of 36.35 °C on day 7, indicating that the Eucalyptus leaves released more energy during this period. The lowest temperature of Zn (3) was 19.60 °C on day 3, and the maximum value was 85% higher than the minimum value. It was found that temperature had a significant effect on the BGA growth and chlorophylla synthesis of EL [18]. Most likely, variation in temperature affected the photosynthesis and respiration intensity of algae, with subsequent impacts on growth and development [17].

Variations in Color and UV
The color and UV of the culture medium could reflect algal growth and concentration [36]; the darker the color, the more pronounced was the growth of algae [37]. The color and UV of different treatments gradually decreased over time, except for the control (Figure 3). This indicated that metal ions and Eucalyptus leaves inhibited algae reproduction. We observed considerable differences between EL and the other treatments, most likely because Eucalyptus leaves contain more chloroplasts. The average color and UV values of EL were 755.50 and 1.51, respectively, which were higher than those of the other treatments. It is possible that the EL contained chlorophyll, resulting in higher color and UV values. In the early stage of the culture, the algae propagated in large quantities by using the nutrients in Eucalyptus leaves and the culture medium, and reached the maximum value of propagation. However, when the nutrients were exhausted, the number and growth of algae would slow down, resulting in decreases in color and UV. However, in other treatments, metal ions always affected the growth and reproduction of algae. The maximum color and UV values for EL treatment were 1025.70 and 2.01, respectively, on day 1, reaching 501.07 and 1.07, respectively, on day 21. The temperature of the culture medium showed two different trends with the addition of different inhibitors ( Figure 2). The temperature change trends of CK, Zn (3), and Zn (10) were similar, whereas those of the EL, Fe 3+ , and Al 3+ treatments were similar. The temperatures of the EL, Fe 3+ , and Al 3+ treatments decreased in the 11 days, maybe as the algae suspended growth during this period. This can also be confirmed by the biomass data of cyanobacteria. The temperatures of the treatments EL, Fe 3+ , and Al 3+ were significantly higher than those of the CK, Zn (3), and Zn (10) treatments. In EL treatments, the temperature reached a maximum value of 36.35 • C on day 7, indicating that the Eucalyptus leaves released more energy during this period. The lowest temperature of Zn (3) was 19.60 • C on day 3, and the maximum value was 85% higher than the minimum value. It was found that temperature had a significant effect on the BGA growth and chlorophyll-a synthesis of EL [18]. Most likely, variation in temperature affected the photosynthesis and respiration intensity of algae, with subsequent impacts on growth and development [17].

Variations in Color and UV
The color and UV of the culture medium could reflect algal growth and concentration [36]; the darker the color, the more pronounced was the growth of algae [37]. The color and UV of different treatments gradually decreased over time, except for the control (Figure 3). This indicated that metal ions and Eucalyptus leaves inhibited algae reproduction. We observed considerable differences between EL and the other treatments, most likely because Eucalyptus leaves contain more chloroplasts. The average color and UV values of EL were 755.50 and 1.51, respectively, which were higher than those of the other treatments. It is possible that the EL contained chlorophyll, resulting in higher color and UV values. In the early stage of the culture, the algae propagated in large quantities by using the nutrients in Eucalyptus leaves and the culture medium, and reached the maximum value of propagation. However, when the nutrients were exhausted, the number and growth of algae would slow down, resulting in decreases in color and UV. However, in other treatments, metal ions always affected the growth and reproduction of algae. The maximum color and UV values for EL treatment were 1025.70 and 2.01, respectively, on day 1, reaching 501.07 and 1.07, respectively, on day 21.

Variations in Water Conductivity
Conductivity can reflect the decomposition or destruction of algae [38]. The higher the degree of damage, the more ions are released, resulting in higher water conductivity [2]. The control showed an average conductivity of 393.28 us/cm, with no significant changes over time (p > 0.05). The conductivity of the metal ion and EL treatments decreased over time (Figure 4). In EL, the average conductivity was highest, with 954.87 us/cm, whereas in CK, it was 41% lower. In addition, the conductivity levels of the Fe 3+ and Al 3+ treatments were significantly higher than those of the CK, Zn (3), and Zn (10) treatments. This may be related to the enrichment efficiency of metal ions and algae cells and the toxicity of metals. When metal ions enter the water and come into contact with algae, those with positive charges will bind to the negatively charged functional groups on the cell walls, thereby accumulating on the cell surface [39]. The ability of different metal ions to accumulate on algal cell surfaces is related to electrostatic attraction and the radii of hydrated ions [40]. The EL treatment had the highest water conductivity of 1113.95 us/cm on day 1 and the lowest of 892.60 us/cm on day 21. The conductivity of EL was higher than that of the treatments with metals, most likely because Eucalyptus leaves contain more ions.

Variations in Water Conductivity
Conductivity can reflect the decomposition or destruction of algae [38]. The higher the degree of damage, the more ions are released, resulting in higher water conductivity [2]. The control showed an average conductivity of 393.28 us/cm, with no significant changes over time (p > 0.05). The conductivity of the metal ion and EL treatments decreased over time (Figure 4). In EL, the average conductivity was highest, with 954.87 us/cm, whereas in CK, it was 41% lower. In addition, the conductivity levels of the Fe 3+ and Al 3+ treatments were significantly higher than those of the CK, Zn (3), and Zn (10) treatments. This may be related to the enrichment efficiency of metal ions and algae cells and the toxicity of metals. When metal ions enter the water and come into contact with algae, those with positive charges will bind to the negatively charged functional groups on the cell walls, thereby accumulating on the cell surface [39]. The ability of different metal ions to accumulate on algal cell surfaces is related to electrostatic attraction and the radii of hydrated ions [40]. The EL treatment had the highest water conductivity of 1113.95 us/cm on day 1 and the lowest of 892.60 us/cm on day 21. The conductivity of EL was higher than that of the treatments with metals, most likely because Eucalyptus leaves contain more ions.

Variations in Water Conductivity
Conductivity can reflect the decomposition or destruction of algae [38]. The higher the degree of damage, the more ions are released, resulting in higher water conductivity [2]. The control showed an average conductivity of 393.28 us/cm, with no significant changes over time (p > 0.05). The conductivity of the metal ion and EL treatments decreased over time ( Figure 4). In EL, the average conductivity was highest, with 954.87 us/cm, whereas in CK, it was 41% lower. In addition, the conductivity levels of the Fe 3+ and Al 3+ treatments were significantly higher than those of the CK, Zn (3), and Zn (10) treatments. This may be related to the enrichment efficiency of metal ions and algae cells and the toxicity of metals. When metal ions enter the water and come into contact with algae, those with positive charges will bind to the negatively charged functional groups on the cell walls, thereby accumulating on the cell surface [39]. The ability of different metal ions to accumulate on algal cell surfaces is related to electrostatic attraction and the radii of hydrated ions [40]. The EL treatment had the highest water conductivity of 1113.95 us/cm on day 1 and the lowest of 892.60 us/cm on day 21. The conductivity of EL was higher than that of the treatments with metals, most likely because Eucalyptus leaves contain more ions.

Variations in Total Dissolved Solids and Turbidity
There were significant differences in TDS and Turb NTU among the six treatments ( Figure 5). For EL, CK, Zn (3), and Zn (10), we observed a downward trend in the number of cultivation days. Although there were no significant changes in the Fe 3+ and Al 3+ treatments in the first 11 days, the levels decreased in the last 10 days. The TDS of treatment EL was highest, with an average of 636.71mg/L, which was 1.2 times that of CK. The TDS value of the EL treatment was highest (752.50 mg/L) on day 1 and reached a minimum of 580.50 mg/L on day 21. The turbidity of the Zn (3) and Zn (10) treatments decreased over time, and other treatments showed a trend of increase, followed by decrease. The average turbidity of CK was 68.62 NTU (%), which was significantly higher than that of the other treatments. The turbidity levels of Zn (3), Zn (10), and EL tended to be consistent and close to zero, indicating that metals inhibited algal growth, although Eucalyptus leaves had a stronger effect. It was found that different concentrations of Fe 3+ , Al 3 +, and Zn 2+ could promote the growth and reproduction of Cyanobacteria in a certain concentration range [41]. Fe 3+ , Al 3 +, and Zn 2+ could inhibit algal growth at a high concentration [40]. However, in another study, the toxic effect of Al 3+ was greater than that of Zn 2+ [42,43]. The toxicity of metals may also be related to the tolerance of algae species to different metals. In our study, Zn 2+ was more toxic to Cyanobacteria cells under the long-term stress of Fe 3+ , Al 3+ , and Zn 2+ since the TDS and Turb of Zn 2+ were lower than those of Fe 3+ and Al 3+ . In addition, Eucalyptus leaves clearly inhibited algal growth, albeit only to a slightly lower extent than metal ions.

Variations in Total Dissolved Solids and Turbidity
There were significant differences in TDS and TurbNTU among the six treatments (Figure 5). For EL, CK, Zn (3), and Zn (10), we observed a downward trend in the number of cultivation days. Although there were no significant changes in the Fe 3+ and Al 3+ treatments in the first 11 days, the levels decreased in the last 10 days. The TDS of treatment EL was highest, with an average of 636.71mg/L, which was 1.2 times that of CK. The TDS value of the EL treatment was highest (752.50 mg/L) on day 1 and reached a minimum of 580.50 mg/L on day 21. The turbidity of the Zn (3) and Zn (10) treatments decreased over time, and other treatments showed a trend of increase, followed by decrease. The average turbidity of CK was 68.62 NTU (%), which was significantly higher than that of the other treatments. The turbidity levels of Zn (3), Zn (10), and EL tended to be consistent and close to zero, indicating that metals inhibited algal growth, although Eucalyptus leaves had a stronger effect. It was found that different concentrations of Fe 3+ , Al 3 +, and Zn 2+ could promote the growth and reproduction of Cyanobacteria in a certain concentration range [41]. Fe 3+ , Al 3 +, and Zn 2+ could inhibit algal growth at a high concentration [40]. However, in another study, the toxic effect of Al 3+ was greater than that of Zn 2+ [42,43]. The toxicity of metals may also be related to the tolerance of algae species to different metals. In our study, Zn 2+ was more toxic to Cyanobacteria cells under the long-term stress of Fe 3+ , Al 3+ , and Zn 2+ since the TDS and Turb of Zn 2+ were lower than those of Fe 3+ and Al 3+ . In addition, Eucalyptus leaves clearly inhibited algal growth, albeit only to a slightly lower extent than metal ions.

Variations in Chlorophyll a and Blue-Green Algae
There was a significant difference between the treatments and CK (p < 0.05) ( Table 2). Different concentrations of Fe 3+ , Al 3+ , Zn 2+ , and EL had different effects on the content of chlorophyll a in Cyanobacteria cells. However, the changing trend of chlorophyll a was similar to that of BGA. This was because high concentrations of chlorophyll a could effectively promote BGA. The levels of chlorophyll a and BGA in CK first increased and then stabilized, whereas chlorophyll a and BGA decreased over time in the treatments Fe 3+ , Al 3+ , Zn (3), and Zn (10). Especially in Zn (10), the chlorophyll a and BGA levels reached zero on day 14. The average chlorophyll a and BGA levels of EL were 34.10 and 13.70 ug/L, respectively, which were significantly higher than those in the treatments with metal ions. Compared with metal treatments, the chlorophyll a and BGA levels of EL decreased more slowly, most likely because of the slow decomposition of Eucalyptus leaves.

Variations in Chlorophyll a and Blue-Green Algae
There was a significant difference between the treatments and CK (p < 0.05) ( Table 2). Different concentrations of Fe 3+ , Al 3+ , Zn 2+ , and EL had different effects on the content of chlorophyll a in Cyanobacteria cells. However, the changing trend of chlorophyll a was similar to that of BGA. This was because high concentrations of chlorophyll a could effectively promote BGA. The levels of chlorophyll a and BGA in CK first increased and then stabilized, whereas chlorophyll a and BGA decreased over time in the treatments Fe 3+ , Al 3+ , Zn (3), and Zn (10). Especially in Zn (10), the chlorophyll a and BGA levels reached zero on day 14. The average chlorophyll a and BGA levels of EL were 34.10 and 13.70 ug/L, respectively, which were significantly higher than those in the treatments with metal ions. Compared with metal treatments, the chlorophyll a and BGA levels of EL decreased more slowly, most likely because of the slow decomposition of Eucalyptus leaves. Table 2. Variations in chlorophyll a increment, respectively. To the A, B, C, D, E, and F treatments we added Fe 3+ , Al 3+ , 3 mol/L zinc (Zn 3 ), 10 mol/L zinc (Zn 10 ), Eucalyptus leaves (EL), and pure water, respectively. There were significant differences among treatments and culture days, respectively (p < 0.05).

Cultivation (days)
A Previous studies have shown that photosystem II is the main target of Fe 3+ and Al 3+ , and high concentrations of these ions will have a strong and negative impact on the photosynthetic electron transport chain [44]. In addition, Fe 3+ and Al 3+ can seriously damage the chloroplast structure and form coordination compounds with chlorophyll, thereby impeding the formation and stability of chlorophyll and reducing the content of chlorophyll, which results in cell chlorosis and discoloration [45]. The content of chlorophyll a was significantly inhibited at the concentrations of 3 and 10 mol/L of Zn 2+ , most likely because Zn 2+ combines with the thiol groups of prochlorophyll ester reductase, δ-aminolevulinic acid synthetase, and bilinogen deaminase after entering the cell, changing the composition of enzyme molecules and reducing enzyme activity [46]. Similar to metals, high concentrations of Eucalyptus leaf solution in the cell will damage the function of the chloroplast and change the composition of chloroplast proteins, thus hindering the synthesis of chlorophyll a ( Figure 6).  Previous studies have shown that photosystem II is the main target of Fe 3+ and Al 3+ , and high concentrations of these ions will have a strong and negative impact on the photosynthetic electron transport chain [44]. In addition, Fe 3+ and Al 3+ can seriously damage the chloroplast structure and form coordination compounds with chlorophyll, thereby impeding the formation and stability of chlorophyll and reducing the content of chlorophyll, which results in cell chlorosis and discoloration [45]. The content of chlorophyll a was significantly inhibited at the concentrations of 3 and 10 mol/L of Zn 2+ , most likely because Zn 2+ combines with the thiol groups of prochlorophyll ester reductase, δ-aminolevulinic acid synthetase, and bilinogen deaminase after entering the cell, changing the composition of enzyme molecules and reducing enzyme activity [46]. Similar to metals, high concentrations of Eucalyptus leaf solution in the cell will damage the function of the chloroplast and change the composition of chloroplast proteins, thus hindering the synthesis of chlorophyll a ( Figure 6).  Figure 6. Effects of various inhibitors on algal growth. (a) represents the relationships among temperature, total dissolved nitrogen, and blue-green algae, and (b) the relationship among the turbidity NTU, chlorophyll a, and conductivity.

Conclusions
To determine whether common metals inhibit algal growth and whether EL can replace metal ions in controlling algae outbreaks, we added Fe 3+ , Al 3+ , Zn (3), Zn (10), and EL to an ASM medium, measuring changes in color, UV, chlorophyll a, turbidity, temperature, pH, total dissolved solids, conductivity, and BGA. The pH values of the treatments with Fe 3+ , Al 3+ , Zn (3), Zn (10), and EL increased significantly and were higher than that of CK. The average color and UV values of EL were 755.50 and 1.51, respectively, which were higher than those of the other treatments. The TDS of the EL treatment was highest, with an average of 636.71mg/L, which was 1.2 times that of CK. Fe 3+ , Al 3+ , Zn (3), Zn (10), and

Conclusions
To determine whether common metals inhibit algal growth and whether EL can replace metal ions in controlling algae outbreaks, we added Fe 3+ , Al 3+ , Zn (3), Zn (10), and EL to an ASM medium, measuring changes in color, UV, chlorophyll a, turbidity, temperature, pH, total dissolved solids, conductivity, and BGA. The pH values of the treatments with Fe 3+ , Al 3+ , Zn (3), Zn (10), and EL increased significantly and were higher than that of CK. The average color and UV values of EL were 755.50 and 1.51, respectively, Water 2021, 13, 1014 9 of 10 which were higher than those of the other treatments. The TDS of the EL treatment was highest, with an average of 636.71mg/L, which was 1.2 times that of CK. Fe 3+ , Al 3+ , Zn (3), Zn (10), and EL could inhibit the synthesis of chlorophyll and not affect the algae biomass. In addition, it was found that the toxicity of Zn 2+ may be higher than that of Fe 3+ and Al 3+ since it can completely destroy the structure of chlorophyll a. Based on our results, Eucalyptus leaves can be effectively used to inhibit algal growth in eutrophic water bodies.

Data Availability Statement:
The data that support the findings of this study are available upon request from the authors.

Conflicts of Interest:
The authors declare no conflict of interest.