# Mercury Uptake and Transport by Plants in Aquatic Environments: A Meta-Analysis
## 3 Results and Discussion
### Influencing Factors
#### 3.4.3 pH  
Generally, the pH of aquatic environments ranges between 5 and 9, with acidity, neutrality, and alkalinity thresholds of \(<\)6.5, 6.5-7.5, and \(>\)7.5, respectively. The effect sizes (In BCF) of plants of the same type were combined according to acid-base classification. Thus, the mean and 95% CI values were calculated, and the corresponding forest plot (Figure 6, water pH) was drawn. Under the influence of different pH conditions, the total average effect size (mean = 7.259, CI: 6.223-8.220) of Hg in plant-enriched water was significantly higher than zero, whereas that in sediments was significantly lower than zero (mean = \(-\)0.886, CI: \(-\)1.189-0.523). In neutral environments, the average effect size of Hg in the plant-enriched water was the highest (mean = 14.144, CI: 11.837-16.070) compared with that under acidic or alkaline conditions. However, the Hg concentration in the plants was significantly lower than that in sediments. The average effect size of Hg uptake by plants in acidic environments (mean = 7.753, CI: 7.276-8.255) was higher than that in alkaline environments (mean = 5.179, CI: 4.014-6.345), suggesting that aquatic plants are better suited to absorb Hg in neutral or slightly acidic environments. Additionally, the Hg absorption capacity of floating plants was significantly higher than that of submerged plants and emergent plants in environments with pH \(<\) 7.5 (Figure 7b), but it was opposite in environments with pH \(>\) 7.5 (Figure 7c). Figure 7: Mean effect sizes (In R) of Hg concentrations in different life habit plants, and plant organs in terms of pH, and life habits. The error bars represent 95% bootstrapped CIs, and the numerals below the \(x\)-axis represent the number of observations in each category. “Mean” indicates the overall effect considering all observed objects in the same category. (**a**) Effect size of pH on Hg absorption by plant roots; (**b**) effect size of life habit on Hg absorption by plants in neutral and acidic water (pH \(<\) 7.5); (**c**) effect size of life habit on Hg absorption by plants in alkaline water (pH \(>\) 7.5); and (**d**) effect size of plant organs on Hg absorption by different life habit plants. pH is an important factor that influences the ability of plants to absorb heavy metals [79]. The results of this study showed that aquatic plants and sediments accumulated large amounts of Hg when the pH was neutral, with the plants exerting the strongest Hg absorption ability. When the pH was weakly acidic, the ability of the plants to absorb Hg was stronger than that observed when the water was weakly alkaline. The Hg absorption capacity of the roots also showed similar characteristics (Figure 7a). Strong acids and bases affect the normal growth and heavy metal accumulation characteristics of aquatic plants. Conversely, neutral water is beneficial for aquatic plant growth and heavy metal accumulation [80; 81; 82]. The absorption of Hg by sediments from water showed the same trend as that of plants. Under low pH conditions, the acidification and dissolution of heavy metals is positively affected by the competition between H\({}^{+}\) in water and organic acids secreted by the plant rhizosphere, which affect the ion exchange process between the metal ions and H in the functional groups on the cell wall, thus strengthening the absorption and enrichment of heavy metal ions by plants [81]. [PERSON] [83] and [PERSON] et al. [84] found that the efficiency of heavy metal absorption by _Sophora flavescens_ and aquatic organisms such as _Ceratophyllum demersum_ (goldfish algae) and _Mytiophyllum spacitatum_ (sargassum) increased with an increase in pH in acidic environments. However, when the pH reached 6, the absorption efficiency of some heavy metal ions peaked, but a continuous increase in pH led to a decrease in the absorption efficiency [83; 84].