# Mercury Uptake and Transport by Plants in Aquatic Environments: A Meta-Analysis
## 3 Results and Discussion
### Influencing Factors
#### 3.4.2 Life Habits  
Based on the field or laboratory environments of aquatic plant growth, aquatic plants can be categorized as floating, submerged, or emergent, depending on their life habits. The effect sizes (In BCF) of plants of the same family were combined, and the mean and 95% CI values were determined, after which a forest plot was drawn (Figure 6, life habit). The total average effect size (mean = 6.112, CI: 5.422-6.745) of Hg absorption from water enriched by different plant types was significantly higher than zero, whereas the total average effect size (mean = \(-\)0.373, CI: \(-\)0.761-0.389) corresponding to Hg absorbed from the sediments was less than zero. Additionally, the three types of aquatic plants had an Hg concentration that was significantly higher than that in water. Furthermore, the Hg concentration in floating and emergent plants was similar to that in sediments, whereas the Hg concentration in the submerged plants was significantly lower than that in sediments. The absorption effect sizes of different organs of these three types of plants on Hg in water were further analyzed (Figure 7d). The sample size of various organs of floating plants is small and the credibility is low, but the changing trend of Hg absorption capacity of different organs of submerged plants and emergent plants is similar to the conclusion in Section 3.4.1.  
Previous studies on heavy metal absorption by aquatic plants have shown that plants with different life habits have different heavy metal absorption capacities [73]; this is consistent with the results of this study. However, other studies have shown that the submerged plants have stronger heavy metal absorption capacities than both floating and emergent plants [74; 75], and the enrichment mechanism may be related to the plant surface area that is exposed to water. The larger this relative surface area (the surface area-to-volume ratio), the higher the heavy metal uptake rate [76]. As shown in Figure 6a, the Hg absorption capacities of these three plants with different life habits varied in the order: floating plants > submerged plants > emergent plants. This may be related to the special form of Hg pollution in nature. Reportedly, 80% of Hg in emissions from artificial sources enters the air in gaseous form and then enters water through atmospheric deposition [77]. Plant leaves are not only the receptors for the heavy metals introduced via atmospheric deposition, but they are also effective filters for gaseous heavy metals. Specifically, leaves absorb heavy metals from the atmosphere via stomatal osmosis. Thereafter, the heavy metals are adsorbed and internalized by the stratum corneum or trichomes, followed by storage in the leaf cell wall [78]. With Hg that has accumulated in sediments as a reference (Figure 6b), the concentration of Hg absorbed by submerged plants was lower than that by floating and emergent plants in this study, indicating that plant organs in contact with air can absorb Hg from the surrounding environment (water and air) better than they can from water only. Simultaneously, the analysis showed that floating plant leaves could accumulate substantial amounts of Hg (Figure 7d). Therefore, floating plants that absorb Hg from the aquatic environment have the advantage of a maximized surface area in contact with water, while also being able to use their large leaves to absorb gaseous Hg.